Phineas Whiting extensively investigated the inheritance of colour, including tortie in cats in a series of breeding experiments starting in 1914 at the University of Pennsylvania. He also crossed his cats to Caffre cats owned by the Zoological Society of Philadelphia. He was very methodical in crossing different patterns and colours of cats and his conclusions represent an early stage in understanding cat colour inheritance. I’ve used Whiting’s papers as a basis and added commentary in square brackets as well as expanding on some of the papers he referenced. To avoid confusion, I’ve mostly avoided their individual methods of “factor” (gene/unit of inheritance) notation except where necessary to demonstrate their reasoning.

THE TORTOISESHELL CAT - Phineas W. Whiting 1st August 1915

In The Journal of Genetics (June, 1913), Doncaster summarized genetic data dealing with the tortoiseshell cat. His records were collected from fancy breeders and from the work of Dr. C. C. Little. Except for certain disputed points the inheritance is in accordance with simple sex-linkage.

If the factor for yellow be represented by Y and its allelomorph, the factor for black, by B, the lack of either by b, the sex factor by X, and the allelomorph of X by x, the normal zygotic possibilities are as follows : YX-bx = yellow male. BX- bx = black male. YX-YX = yellow female. BX-BX= black female. YX -BX = tortoiseshell female. [This method is confusing to modern readers as X and Y are now reserved for the sex chromosomes, while “yellow” is now represented by O (orange).] This means there can only be two classes of males, but three classes of females. Difficulties arise when it is attempted to explain the occurrence of black females produced either by the mating of a black female to a yellow male which should give only tortoiseshell females and black males, or by the mating of a tortoiseshell female to a yellow male, which should give only tortoiseshell and yellow females and black and yellow males. The occurrence of the rare tortoiseshell male is also the cause of considerable difficulty. In one mating, out of seventeen, of yellow females to yellow males there were produced 3 tortoiseshell females in addition to the expected yellow females (40) and yellow males (48).

To explain these discrepancies Doncaster suggested that the linkage of Y with X was not absolute. Yellow males might produce gametes bX and Yx in addition to the normal or more frequent gametes YX and bx. Gamete bX is female determining, while gamete Yx is male determining and yellow bearing [Doncaster was incorrect in suggesting that x (meaning the Y chromosome) carried colour]. The latter gamete [Yx] should produce a tortoiseshell male when it meets an egg BX. Based on this, he expected tortoiseshell males to be as frequent as anomalous black females from yellow fathers. From the matings recorded there were 18 anomalous black females and only 3 tortoiseshell males, and one of those tortoiseshell males had a black father. He could not explain how gamete bX differs from BX. Doncaster admitted those difficulties, stating that further work was necessary before a definite conclusion can be reached.

In a more recent paper Doncaster had suggested non-disjunction [failure of the chromatids to fully separate during cell division] of the sex-chromosomes in oogenesis [egg formation] as a possible explanation. That explained the matroclinous [inherited from the mother] black females, but not the lack of an equal number of patroclinous [inherited from the father] yellow males. It also didn’t account for the tortoiseshell male and the occurrence of tortoiseshell females among the offspring of yellow by yellow.

In a series of experiments begun upon cats at the University of Pennsylvania during 1914, Whiting investigated “the tortoiseshell problem”. He crossed a yellow Persian male with common cats — black, maltese [blue] and tabby. The results, although not at extensive by 1915, were sufficient to explain, at least in part, the anomalies observed, and to suggest a simple explanation for the occurrence of unexpected classes [unexpected colours]. When the yellow male was crossed with a maltese [blue] female, a maltese male and 2 blue and cream (maltese tortie/dilute tortie) females were produced. When mated to a black female the yellow male produced both dark and dilute kittens. This shows that the black female was heterozygous for dilution [carried the recessive dilution factor]. Two of the males were black and 2 were maltese. The 2 females were dark tortoiseshell. When the yellow male was crossed with a dark tabby, there were produced dark and light tabbies and maltese. Blacks were also to be expected from that mating. The mother was evidently hybrid between tabby and black and between black and maltese [i.e. carried non-agouti and dilution factors]. The female offspring showed yellow : the male offspring were without yellow except for tabby striping. The female offspring obtained from these matings could be arranged in a series, ranging from one that was predominantly yellow to one that was maltese except for a few cream-coloured hairs. The maltese with a few cream hairs occurred in the litter from yellow male x maltese female, which also included a maltese male and a maltese female with a small cream patch.

Whiting realised that a maltese cat with a few cream hairs or black cat with a few yellow hairs might be misidentified and recorded as maltese or black. He supposed that further segregation of distribution factors in the direction of black could produce a fully black female. This was comparable to the guinea-pig where yellow spotting was continuous with total black. The essential differences in cats were that the factors for yellow and for black were sex-linked, and that either alone was sufficient to produce its expected colour, but when one was balanced against the other, as in the tortoiseshell female, other factors governing the relative amounts of the 2 colours acted to produce a continuous variation from solid yellow to solid black [that other factor, unknown at the time, was random X-inactivation in females rather than a modifier gene].

The 3 tortoiseshell females from the mating of yellow x yellow were explained by supposing that the mother was gametically a tortoiseshell plus a sum of yellow extension factors and minus a sum of black extension factors. [Extension factors meant factors that increased the amount of a certain colour in the hair shaft – a high level of yellow extension factor would block black pigment and vice versa – much like Wide Band.]

The occurrence of the tabby factor restricted the black pigmentation producing yellow stripes [ground colour]. It was therefore harder to distinguish a tabby from a tabby-tortoiseshell than a black from a tortoiseshell. "We have had a few tabby-tortoiseshells that would have been recorded as tabbies if close examination had not been made. Another source of error in records involving the tortoiseshell pattern may be introduced by the occurrence of white spots". Doncaster made no mention of these in his paper, so that it is possible that they did not occur in the animals recorded.

In what is genetically a tortoiseshell and white cat the incidence of the white spotting might be at just those points which would otherwise be yellow.' Thus the occurrence of black and white daughters from yellow males could be explained. It was also possible that the yellow mother of the 3 tortoiseshell kittens recorded from a yellow x yellow mating might have been white at points which, if pigmented, would have been black. She would then have been gametically a tortoiseshell and white and some tortoiseshell kittens would have been expected.

Whiting suggested that the rare tortoiseshell male was genetically a yellow with an extreme of black extension factors or a black with an extreme of yellow extension factors. That hypothesis was rendered more probable by some slight evidence showing that male tortoiseshells breed like yellows [in modern research, male torties that breed like yellows are chimeras whose testes were formed from the yellow embryo]. There was then no need for assuming in the cat either breaks in sex-linkage or non-disjunction of the sex chromosomes in oogenesis.

INHERITANCE OF COAT COLOUR IN CATS - Phineas W. Whiting 1st August 1918

This is an annotated summary of his long and detailed paper “Inheritance of Coat Colour in Cats” P.W. Whiting, in The Journal Of Experimental Zoology, Vol. 25, No. 2, 1918. Whiting had far fewer colours to play with: black, blue (maltese), yellow (red), cream, black/brown tabbies, tortie, blue-cream tortie and tortie-tabbies, plus white, silver and “Siamese” pattern (seal point and blue point). Random occurrence of any other colour would have been overlooked as variations on the cat fancy’s accepted colours. Whiting quoted from papers of several other researchers, using their findings to add to his own data from experimental breeding.

Whiting’s Deductions:

• Ticking (agouti) was due to at least 2 and probably more different factors. Black/non-agouti/uniformity [solid/self] was recessive to agouti, A. An extreme amount of ticking was due to a third allelomorph in the series A^I.
• Banding [tabby] was due to at least 2 and probably 3 different allelomorphs (B series).
• Intense pigmentation, M, was a simple dominant over dilute or maltese, m. White, W, was a simple dominant over colour, w [in modern parlance it is epistatic].
• Yellow, Y, was allelomorphic with black, y and was at a sex linked locus, showing the male to be di-gametic [in this he was only partly correct: males are either yellow or non-yellow as yellow is on the X chromosome and there is no corresponding colour gene on the Y chromosome]. The heterozygous female was usually yellow-spotted or tortoiseshell, but ranged from solid black or solid yellow. [“Yellow spotting” meant tortoiseshell markings.]
• White-spotting was very irregular and probably depended upon many factors. It was not discussed in detail in this paper, except to note [correctly] that it was independent of the other colour factors mentioned above.
• Albinism, as found in rodents [pink-eyed], was not known cats, but that it was possible that Siamese dilution represented an approach toward albinism [he was correct!].
• Variations in the tone of colouration were extensive, but apparently not clearly segregating [i.e. there was a continuous range of variation]. Silvers represented a reduction of both yellow and black pigment, and smokes were very dark silvers [he did not realise that smokes were non-agouti]. The lighter bands of tabbies [background colour] were straw- or cream-coloured, varying to white in silver tabbies and brown in brown tabbies. Occasionally the brown varied to a rusty red. Silver creams were yellow cats in which the yellow pigment was reduced to a minimum so that the hair sometimes appeared almost white. Whiting noted that, according to fanciers, silver tabbies bred together occasionally throw brown tabbies.

Maltese Dilution.

He wrote that maltese dilution appeared to be a simple Mendelian recessive and not sex-linked. It existed in combination with all other factorial differences [i.e. it occurs in patterned cats as well as solid colour cats]. It was always sharply distinguishable from black, but varied in its own intensity. Cream or dull yellow was the corresponding dilution in the yellow series; blue and cream were dilute tortoiseshell.

White-spotting in cats was exceedingly irregular in amount and distribution, but appeared more commonly on the under parts. There was no regularity in dominance and probably many factors were involved. The degree of white spotting in the parents tended to appear in the offspring, although wide segregation occurred [distinct bicolour patterns rather than continuous variation].

Solid White and White Spotting.

Solid white appeared to be a complete dominant over colour whether the colour was self or was spotted with white [modern readers will know that it is epistatic and masks all other colours]. It was possible that it may be allelomorphic with one or more of the white spotting factors, but Whiting’s data was not conclusive on that point. He noted that other researchers considered solid white to be an extreme form of white spotting. When he crossed a male with low-grade white spotting to a blue-eyed deaf white female he obtained yellow-eyed non-deaf white kittens that had slight “smuttiness” on the tops of their heads; the smuttiness vanished later. When the male was mated to a yellow-eyed non-deaf white female he obtained both white and coloured kittens (a blue-cream and a cream) . From this, he deduced that the white mother carried factors which dominated the slight white marking of the father, and thus produced totally self-coloured kittens [this is the reverse of what happens – the white masking gene is at a different locus and dominates all other colours].

Adding to his evidence of a colour vs no-colour allelemorph, his mating of a blue-eyed white Angora male and a yellow-eyed short-haired white female produced solid white kittens and a black kitten with very low-grade white spotting. He reported a mating of a black Manx male and blue-eyed, deaf, white female, that resulted in two whites, one black, one tiger, and one maltese (blue) kitten.

Whiting considered the interesting correlation of blue eyes and deafness with white coat was not satisfactorily explained. Dominance of eye colour seemed irregular and he was informed by breeders of white cats that yellow-eyed by yellow-eyed may produce blue-eyed, and also that blue by blue may produce yellow. Odd-eyed cats also frequently appear in these crosses. He had also read that odd-eyed cats could be deaf on the blue-eyed side. Dr. C. C. Little showed Whiting a black and white cat with odd eyes. The hair surrounding the blue eye was white, while that about the yellow eye was black. Whiting noted that blue eyes in pigmented cats were rare, except in the case of the Siamese.

His working hypothesis was that white-spotting in connection with the dominant white factor produced the blue eye, or in other words a 'white spot' about the eye of a white cat made the eye blue, while a 'pigmented spot' about the eye of a solid white cat makes the eye yellow]. A 'white spot' in the ear of a white cat might make it deaf. This explained why it was difficult to get blue-eyed white cats with normal hearing: it was difficult to localize the 'white spot' upon the eye and to keep it away from the ear. This would also explain why odd-eyed cats often had defective in hearing on the blue-eyed side, as noted by Przibram.

Solid yellow and yellow-spotting.

Whiting referred to tortoiseshell as “yellow spotting” and wrote that the tortoiseshell cat was the subject of much interest and discussion in relation to sex-correlated phenomena. Doncaster (1905) had tried to explain the peculiarities of inheritance by variations in dominance. Little (1912) suggested the hypothesis of a single sex-linked pair of allelomorphs with the male di-gametic. He used the term 'sex-limited character,' which by 1918 had been restricted to simple Mendelian heredity in which sex reverses the dominance of the allelomorphs. Doncaster (1912) accepted Little's suggestion as generally satisfactory, but pointed out that occasionally black females were produced from black females x yellow male matings. According to Little, the females should always be tortoiseshell from the reciprocals of black by yellow and the males should be like the mother (disregarding dilution, tabby, etc). Doncaster suggested an occasional break in sex-linkage explained those anomalous blacks and rare tortoiseshell males. In 1913 and 1914 Doncaster suggested nondisjunction [incomplete separation] of the sex-chromosomes in oogenesis to explain the matroclinous black females [colour inherited from the mother]. Doncaster and Little both found those explanations unsatisfactory. The black-yellow allelomorphic pair in cats was of particular interest, being the only known case of sex-linkage known in non-human mammals.

Whiting had previously pointed out in 1915 that the hypothesis of simple sex linkage suggested by Little may be sufficient to account for the conditions if it be considered that the heterozygotes, which must be females, varied from black through various degrees of yellow-spotting to solid yellow. He presumed conditions were much more stable in the male, as it was impossible to have a heterozygote. Thus whiting suggested that a gametically yellow male may become tortoiseshell by extreme selection of black extension factors, while a gametically black male may become tortoiseshell by an extreme selection of yellow extension factors. He did not exclude the possibility of a single factor or particular combination of factors that produced yellow-spotting in the male. Ibsen (1916) suggested close coupling of two pairs of sex linked allelomorphs, and attempted to explain anomalies by crossing over, but admitted that this did not account for all the results.

Whiting’s noted an anomalous dilute yellow and white female reported by Little: When bred to 'yellow' males she produced three 'black' males, one 'yellow' male, four 'yellow' females, and one tortoiseshell female. Of these matings Dr. Little said: "The dilute yellow and white female is interesting because she forms gametes carrying black and breeds exactly like a dilute tortoiseshell and white animal, although there is no trace of black pigment anywhere on her." She was termed an anomalous yellow. [This could have been a rare extreme case of X-inactivation switching off the same X chromosome in all skin cells, or a case of black areas being masked by white patches.]

The sterility of the tortoiseshell male had frequently been remarked upon. Cutler and Doncaster (1915) showed drawings of sections of a testis of a sterile tortie male. Normal reproductive cells were absent. In summarizing the data on sterility of male tortoiseshells, they found that one was certainly fertile, 2 were completely sterile, one almost if not quite sterile, and 2 doubtful. Sterility seemed highly correlated with yellow-spotting in the male.

Siamese Dilution.

Bateson (1913) wrote of Siamese cats: ''These animals, which breed perfectly true, were introduced from Siam, where they have been kept for an indefinite period as pets of the royal household. Like the Himalayan rabbit, Siamese cats are born almost white, but the fur becomes a curious fawn with darker chocolate points on the ears and extremities." Crosses of Siamese by other cats were cited by Weir (1889) who quoting from a Mr. Young: ''They invariably showed the Siamese cross in the ground colour." But Lady Dorothy Nevill disagreed and wrote, ''None showed any trace of the Siamese, being all tabby." Two pregnant females of common cats brought into the laboratory produced kittens of a peculiar ashy colour with darker extremities. The kittens resembled [in colour distribution] very closely adult Siamese cats.

One of the pregnant females; a maltese produced 2 females which were ashy, with nose, ears, feet, and tail slightly darker, and two females and two males which were ashy with black extremities. A record taken fifty days after birth showed that the lighter kittens had become maltese, while the kittens with black extremities had become steel coloured and later became completely black [this typical of fever coat, caused by conditions in the womb and seemingly unknown to Whiting]. The kittens also had ghost markings. The other pregnant female was a black and white. She produced 4 kittens " two black females upon with no ghost markings, an ashy female with black extremities, and an ashy male with dark but not black extremities. Sixty-six days after birth the ashy female had developed, into a steel black which clearly showed ghost-blotched pattern, and 80 days after birth the ashy male was maltese with ghost-blotched pattern very evident [once again, typical of fever coat]. Unfortunately, the inheritance of this peculiar ashy colour could not be followed out at the time the kittens were on hand. Whiting could not determine whether it represented the heterozygote for Siamese dilution.

Ticking or Agouti.

Ticking or agouti in cats is characterized by yellow bands on the hairs. It increases with age so that kittens are relatively less ticked than cats. Whiting tentatively considered the agouti factors as a series of triple allelomorphs in the following order of dominance : A' (much ticked), A (little ticked) and a (non- ticked). Uniformity (lack of pattern) in yellow cats appeared due, as pointed out to Whiting by Dr. Sewall Wright, to some other condition than the lack of the agouti factor. As regards the existence of solid yellow cats, Mrs. 'Leslie Williams (1908) wrote: ''The self-orange Persian is more of an ideal than a reality, for it is actually a red tabby without the tabby markings, and at present it is a case of 'more or less,' the upshot being that the least marked cat in the class takes the prize."

Silver

Whiting wrote that silvering was a general reduction in the amount of yellow pigment so that the straw-colour bands of tabbies became white and black stripes alternated with white [background]. At the other extreme, some cats had a considerable amount of yellow pigment. He described a striped tabby that had lighter bands of a decidedly reddish colour, resulting in intense black stripes alternating with rusty red [background]. This cat was already pregnant and produced three male kittens " one striped with black and red; one blotched with black and red, and one striped with black and straw colour.” This indicated to Whiting that the extreme reddish tone was hereditary [this was probably “golden”].

Banding [Tabby Markings]

For an understanding of banding [tabby markings] he described the following patterns. Striped tabby had bands running longitudinally along the back. On the sides the bands were transverse [vertical] and tended to be broken into spots. He thought of this condition as having been produced by longitudinal and transverse waves of pigment-forming metabolic activity. The longitudinal waves formed the transverse bands. The areas of greatest activity formed the orange bands in yellow cats, and the black markings in tabbies. The areas of less activity formed the lighter bands [ground colour or background]. The transverse waves appeared to originate at the mid dorsal line and form longitudinal bands on the back. As the waves passed outward and down the sides, the areas of greater activity tended to thicken the transverse bands. In the areas of less activity the transverse bands were often evanescent [fleeting or faint]. It thus appeared to him that black or orange spots, in tabbies or yellows, respectively, were produced in the regions of greatest metabolic activity. The ticking and the banding factors appeared to act in the same regions, and thus the ticking revealed the straw-coloured rather than the orange bands.

He found agouti to be, in all probability uniform over the body surface in cats as in rodents. Some cats had a high degree of ticking, and thus showed the longitudinal bands clearly. Others were less ticked and the increased amount of black pigment on the back obscured the longitudinal bands. In a very dark-striped tabby the bands on the sides could be clearly seen, but the longitudinal bands were obliterated by the black pigment [black cape effect]. These patterns were distinct rather than continuous [i.e. striped was separate from blotched and separate from solid black] and could occur in the same litter. Whiting noted that ticking increased in cats with maturity. The same kitten could show different degrees of ticking at different ages. Within ticked cats, there were various degrees of ticking ranging from light to heavy.

Whiting noted that lined [ticked tabby or unpatterned tabby] cats occurred in Africa and to some extent in Europe. They were known as African, Caffre, or Abyssinian cats. He had not been able to breed a lined cat lacking agouti, but he hoped to do so by the proper crosses. Had it survived, an extremely ticked lined kitten would probably have grown to a sooty yellow adult. Its back was black, but well scattered with ticked hairs, The transverse bands were visible at edge of the skin at the sides and about the tail. The longitudinal bands were suggested by two ticked spots at the back of the neck, and just posterior of these spots were two parallel ticked lines. On the body, near the tail were the longitudinal bands. He noted that the lined, the striped, and the blotched patterns were fundamentally comparable, differing only in the width [and distribution] of the bands.

A pair of lined cats were owned by the Zoological Society of Philadelphia. The male was dark while the female was much lighter. Whiting comparison the degree of ticking of the two. The back of the male was black, the sides very dark showing narrow ticked bands. The back of the female was dark but ticked and graded into sooty yellow on the sides, showing no dark banding. The banding on the head and breast of the male was mostly black, while in the female it was brown shading to sooty. In the male the back and end of the tail were black, while ticked rings were seen only toward the base. In the female the entire tail was ringed with sooty yellow. In both animals the feet were sooty yellow and the soles black. In the male the black bands of the sides extended down the legs to the feet, while in the female the sooty yellow of the feet extended well up on the legs. When bred together they produced a dark-lined male, a dark-blotched male, a dark-blotched female, and a light-lined male.

Whiting bred together blotched and striped tabbies and self coloured cats and the results were consistent with his assumption of 2 loci, one for the banding [tabby marking] factors and one for the ticking factors. He found the 3 types of banding, lined, striped, and blotched to be entirely distinct with no intermediates. The natural assumption was that theyed form a triple allelomorphic series.

Wright's (1917) papers on colour inheritance in mammals classified colour factors according to their effects on either one of two enzymes. Enzyme 1, on its own, produced yellow. Enzyme 2 had no effect alone on yellow pigment, but when combined with enzyme 1 it oxidized chromogen [colourless precursor to pigment] to sepia. The agouti factors therefore determined the inhibiting of enzyme 2. “Factor A determined the production of an inhibitor with the same subtraction effect on enzyme 2 everywhere." This inhibitor acted in waves along the individual hairs. The regions of greatest concentration determined the yellow bands, while those of less concentration are black. In yellow cats, banding occurred over the whole surface of the body, straw-colour alternating with orange. Banding, therefore, affected enzyme 1. In black cats the bands were almost indiscernible from which Wright deduced there was enough of enzymes 1 and 2 generally distributed to produce a uniform black. In the presence of the agouti factor, however, yellow bands appeared in the individual hairs. Those bands were much wider in the areas corresponding to the straw-coloured bands [ground colour] of yellow cats. In fact, the black could be entirely obliterated in those regions. The hairs in the areas corresponding to the orange bands [marking colour] of yellow cats were much darker and could lack visible ticking. It therefore appeared that the banding factors affected enzyme 2, for if enzyme 2 were uniformly distributed, the agouti factor should have caused uniform ticking over the body surface, not an alternation of dark and light bands. The banding factors therefore determined waves of general metabolic activity affecting both enzyme 1 and enzyme 2. In the black cat the regions corresponding to the orange bands in the yellow cat would be a dense black, a sort of black dominant to agouti, comparable to Punnett's (1912 and 1915) dominant black in rabbits, while the regions corresponding to the straw-coloured bands would be comparable to ordinary black in being recessive to agouti.

The Origin Of Colour Varieties Of The Cat

It is generally assumed that the domestic cat is polyphyletic [derived from multiple wild ancestors] in origin. Darwin considered this to be the case. Keller (1902) agreed with Darwin on that point. Elliot (1883) believed the cat to be descended from a number of wild species and supposed that it had crossed at various times with small wild cats in different countries. He attempted to trace the well-known colour variations as well as conformation variations due to such hybridizing.

Rope (1881) and Pocock (1907) both recognized the blotched and striped forms and believed that all cats, whatever their colour, fell into one or the other of these two classes. Pocock stated that the “decided difference in the 'pattern' of Domestic Cats” provided a surer basis for their classification than the length of their hair, the colour of their coat, or the stunting of their tail. Pattern seemed a more important clue to its ancestry and he noted that the so-called 'blotched' pattern could be detected in certain lights even in 'White’ and 'Black' Domestic cat. Pocock referred to the lined variation as Abyssinian. 'Abyssinian' breed cats were thought to be descended from Felis ocreata exported from Abyssinia as the pattern resembled that [sub] species. However it was hard to distinguish them from fulvescent torquata-type (striped) 'Ticked' Cats in which the pattern was broken up and evanescent. Most authorities considered the naming of African wildcat types to be very confused [they were dealing with variations within a single species, not – as they thought – with separate species.]

Dr. A. Nehring (1887) believed the cat to be descended from a domestic Chinese cat [which would have been derived from F s bieti] and from the Egyptian cat, Felis maniculata. The origin of the striped pattern was easily traced to the European or African wildcat. Lydekker suggested the blotched type represented Dr. Nehring's presumed Chinese element in the cat's parentage, and that the missing wild stock might be one of the numerous phases of the leopard-cat (F. bengalensis), some of which had an incipient spiral arrangement of markings on the shoulder. Pocock thought that the blotched tabby was derived from an extinct, probably Pleistocene cat, of western Europe.

Whiting considered it unnecessary to attempt to trace all variations in morphology and colour to one or other wild ancestor since the variations could occur under domestication. He considered the strictly domestic colour variations to be maltese, white, white-spotting, yellow, and Siamese dilution. Such variations were not characteristic of any wild species, but occurred in numerous domestic animals and were variations by which domestic species 'mimic' each other.

Whiting wished to name genetic factors rather than cat species so he discarded Latin terms and adopted the English words blotched and striped to avoid confusion with species names (e.g. torquata, ornata or catus). He used the term 'mimic' to denote similar patterns occurring in different domestic animals e.g. white spotting and believed that the mimicry was due to the limited number of ways in which the mammalian coat could vary in dilution and distribution of the pigments black, brown, and yellow since “no other pigments can be developed.” He cited Metz (1916) that numerous cases of resemblance were probably due to homologous factors, even in widely separated species.

On the other hand, wrote Whiting, variations in the ticking factors and in the banding or pattern factors occurred in both wild and domestic mammals. Such variations in the domestic cat produced colour patterns closely similar to numerous wild species. Variations from red to silver occurred in wild cats. The tiger had a high degree of red with a moderate amount of ticking. Thus the pattern was very well marked. In the lion, the puma, the jungle cat and others, the red was reduced to yellow while the ticking was very intense. Hence the pattern appeared only in young animals and was obliterated by the increase of ticking at maturity. Other cats like the ounce or snow-leopard and Pallas' cat represented an extreme reduction of yellow pigment comparable with silvering in domestic tabbies. Loss of agouti producing black varieties of leopards and others were well known. Small species of African and Asiatic cats varied so much in colour that their taxonomy was confusing. Whiting considered all of this diversity to be due to variations in ticking, in banding [tabby marking], and in the red-silver series. He believed spots to be produced by crossing of longitudinal and transverse weaves of pigment-forming metabolic activity. In those respects he considered the domestic cat tended to 'mimic' its wild relatives, but left the question open as to whether those variations originated by crossing of wildcat species or by mutation in domestic cats.

Literature Cited By Whiting (some with relevant quotes included)

Bateson, W. 1913 Mendel's Principles of Heredity. Cambridge University Press.

• Red is dominant to black in males. Tortoiseshell is the corresponding form of the heterozygote in females. Doncaster (109). Dilution-types, blue, and cream, recessive to saturated colours.
• The abbreviated tail of the Manx Cat is a dominant (more or less imperfect) to the normal tail (see Anthony, 2 ; Hind, 151; Davenport, 98; Kennel, 166,0). Godron (Mdm. Ac. Stanislas, 1865) records a similar observation in the Dog.
• Polydactylism is almost certainly dominant; but, as in other types, irregularities doubtless occur.
• Nageli failed altogether to realise the importance of Mendel's work. Nageli was of course especially devoted to the study of heredity, and even made it the subject of elaborate mathematical treatment. [. . . ] when in 1884, the year of Mendel's death, he published his great treatise on heredity, no reference was made to Mendel or his work. That this neglect was due to want of comprehension is evident from a passage where he describes an experiment or observation on cats, which as it happens, gave a simple Mendelian result. The Angora character (recessive) disappeared in a cross with a certain common cat whose hair-character is, as we now know, dominant. The cross-breds were mated together and the Angora character reappeared in one individual among a litter of common cats. This typically Mendelian fact was thus actually under Nageli's own observation, but from the discussion which he devotes to the occurrence it is clear that Mendel's work must have wholly passed from his memory. - C. Nageli, Mechanisch-physiolvgische Theorie der Abstammungslehre, 1884, p. 199.
• [Here’s a very rough summary from Nageli ‘s book: An Angora (the term for longhair) cat bred to an ordinary shorthair cat (first generation) produced only shorthair cats (second generation). Despite their appearance, the kittens contained much Angora blood and a mating between them produced a third generation containing a female angora cat among ordinary shorthair cats. In a fourth generation, the Angora blood became “even cheaper” (more common?). Based on this, it proves that we cannot rely on the visible features for how else could two shorthair cats produce a longhair cat? In sexual fertilisation the idioplasm of the two parents combine into a seed. How can the child have characteristics that are not present in the parents? Nageli spoke of dominating and latent characteristics and theorised that the character of organisms in various generations were solely determined by potentialities latent in the germ cells which give rise to them, and that the character of their parents and grandparents had nothing to do with it.]
• The only other example I know of a race which combines an albino eye with external colour is the breed of cats called " Siamese." These animals, which breed perfectly true, were introduced from Siam, where they have been kept for an indefinite period as pets of the royal household. Like the Himalayan rabbit, Siamese cats are born almost white, but the fur becomes a curious fawn with darker chocolate points on the ears and extremities. The eyes are more or less deficient in pigment throughout life. Some pigment however is formed in the eye, as is evidenced by the fact that the iris has a blue colour and is partially opaque [Bateson noted that eye colour was linked to fur colour in mammals and birds].
• In cats we meet a new complication in the inheritance of yellow, namely that it is disturbed by sexual dimorphism. The rarity of tortoise-shell males is well known, and Doncaster has produced evidence which makes it practically certain that in the immense majority of instances the female heterozygote of orange x black is tortoise-shell, namely, patched with orange and black (like the guinea-pig), but that the male heterozygote similarly produced is orange (see Chap, x, dealing with the heredity of Sex). The dilute types cream and blue are similarly related to blue tortoise-shell.

Castle, W. E. 1916 Genetics and Eugenics. Harvard University Press.

• Mutilations. It is now all but universally admitted that somatic modifications due to mutilation are not inherited. Nevertheless " cases " are from time to time reported, in which a man or a domesticated animal which by accident had lost a limb has produced offspring similarly defective. One of the most frequently recurring of these stories has come to me at first hand. A cat which had accidentally lost her tail gave birth to kittens part of which were short-tailed. It is not necessary to suppose that the report is inaccurate. Certain races of cats are naturally short-tailed, and a cat might produce offspring short-tailed by inheritance quite irrespective of any injury to either parent. On the other hand where docking of the tail has been followed up systematically for many generations and on a large scale, as is the case in sheep, no racial shortening of the tail is observable.
• In some rodents not only the colour factor, but also the extension factor is subject to locally inhibited action. Local inhibition of the extension factor produces yellow spots in an otherwise black, brown, or agouti coat. This colour variation, which follows Mendel's law in crosses, may be called yellow spotting. It behaves as a third allelomorph (eO alternative both to full extension (E) and to full restriction (e). When yellow spotting coexists with white spotting, a tri-colour condition of the coat results, spots of yellow, white, and black (or brown) being found on the same individual. Familiar examples are found among guinea-pigs, cats and dogs.
• Cats, Domestic cats are descended from a wild species (Felis maniculata) still found in northern Africa. The domestication was accomplished in ancient Egypt and the domestic cat was introduced into Europe in the middle ages, since Roman times. The wild species is similar in size and colour to the common tabby or tiger cat. This has a coat consisting of agouti-like hairs, which contain both black and yellow pigments, but the body is marked with stripes in which black pigment predominates, and it is these black stripes that produce the tiger pattern, which is a dominant unit-character. Different forms of the tiger pattern, distinguished as lined, striped, blotched, etc., are probably multiple allelomorphs. In the self -black variety the tiger pattern and agouti marking of the hairs have been covered up by a greatly increased amount of black. The black variety probably originated as a sport and it behaves as a recessive to tabby. An all yellow variety represents another unit-character variation imperfectly dominant over black. Homozygous individuals are all yellow but heterozygous females usually show both yellow and black (tortoise shell) though occasionally they may be all yellow. The inheritance of yellow is sex-linked and of the Drosophila type. (See Chapter XVIIl.) Yellow cats usually, if not always, show the tiger pattern, which leads to the question whether this pattern is ever lost even in the black variety. It may be only covered up with black pigment. Darwin notes the fact that black kittens often show the tiger pattern which is not visible in them later in life. All-white varieties of cats exist having coloured eyes (either "yellow" or blue). The relation of this variation to coloured forms, as regards dominance, is uncertain, but it probably represents an extreme form of white spotting. Blue (or Maltese) is a dilute form of black, recessive to the latter. The dilution factor probably affects the appearance of tabby and yellow also, but definite information on the point is not available. White spotting is a character the behavior of which as regards dominance is unknown. Yellow spotting occurs only as a heterozygous character in the cross between yellow and black and then chiefly in the female sex. Long (angora) hair is a recessive variation from normal coat in cats as in rabbits and guinea-pigs. A short stumpy tail, seen in the ''Manx" cat, represents an imperfectly dominant unit-character variation. Homozygous dominants are tailless; heterozygotes are short-tailed; normal (long) tail is recessive. Polydactylism (the possession of extra toes) is an imperfectly dominant variation. - Castle, W. E., 1919c. Siamese an albinistic colour variation in cats. Am. Nat., 53.

Cutler, D. W., and Doncaster, L. 1915 On the sterility of the tortoiseshell torn cat. Journal of Genetics, December.
Davenport, C. B. 1905 Details in regard to cats. Report on the work of the Station for Experimental Evolution, Cold Spring Harbor.
Doncaster, L. 1905 On the inheritance of tortoiseshell and related colours in cats. Proceedings of the Cambridge Philosophical Society, 13, pt. 1, p. 35.
Doncaster, L. 1912 Sex-limited inheritance in cats. Science, N. S., vol. 36, no. 918, August 2.
Doncaster, L. 1913 On sex-limited inheritance in cats, and its bearing on the sex limited transmission of certain human abnormalities. Journal of Genetics, June.
Doncaster, L. 1914 Chromosomes, heredity, and sex. Quarterly Journal of Microscopical Science. February.

Ibsen, Heman L. 1916 Tricolour inheritance. III. Tortoiseshell cats. Genetics, vol. 1.

• Johannsen (1913) believes Doncaster is not justified in postulating sex chromosomes as bearers of the factors for black and orange. He therefore makes some modifications of Doncaster’s interpretation in order that the factorial treatment may be more “purely Mendelian.” Male cats are represented factorially as Mm and females as mm, and the factors for black and orange are assumed to be rather closely linked to m. Even with these modifications the hypothesis is essentially the same as Doncaster’s and is open to practically the same objections. - Johannsen, W., 1913 Elemente der exakten Erblichkeitslehre. 2. Aufl.,
• The rare tortoiseshell male, Whiting (1915) suggests, is “genetically a yellow with an extreme of black extension factors or a black with an extreme of yellow extension factors.” This hypothesis lacks definiteness, and is practically impossible to prove or disprove.

Little, C. C. 1912 Preliminary note on the occurrence of a sex-limited character in cats. Science, N. S., vol. 35, no. 907, May 17.
Nehring, a. 1887 Ueber die Sohlenfiirbung am Hinterfusse von Felis catus, F. caligata, F. maniculata und F. domestica. Sitzungs-Berichte der Gesellschaft naturforschender Freunde zu Berlin (On the markings of the soles and hind feet of Felis catus, F. caligata, F. and F. maniculata domestica. Session Reports of Berlin Society of Friends of Natural Science).
PococK, R. I. 1907 On English domestic cats. Proceedings of the Zoological Society of London, Februaiy 5.
Przibram, H. 1908 Vererbungsversuche uber asymmetrische Augenfarbung bei Angorakatzen. Archiv fur Entwicklungsmechanikder Organismen, XXV, S. 260.
Rope, G. T. 1881 On the colour and disposition of markings in the domestic cat. Zoologist, vol. 5, no. 57.
Whiting, P. W. 1915 The tortoiseshell cat. The American Naturalist, vol. 49, August.
Whiting, P. W., 1919 Inheritance of white spotting and other colour characters in cats. Am. Nat., 53 (published after the paper I have summarised).
Williams, Mrs. Leslie. 1908 The cat. Henry Altemus Company, Philadelphia
Wright, Sewall. 1917 Colour inheritance in mammals. The Journal of Heredity, vol. 8.

Chase, Herman B, 1939 Studies On The Tricolour Pattern Of The Guinea P1g. The Distribution Of Black And Yellow As Affected By White Spotting And By Imperfect Dominance In The Tortoise Shell Series Of Alleles

• One modifier, however, has been a principal object of the present paper. This is white spotting. Wright (1917, 1920) observed the relation between these two types of spotting, the presence of white tending to decrease the amount of brindle, to separate the pigmentation into clear-cut areas of black and yellow, and where brindling persists to organize the spots so that black tends to be central and yellow peripheral. The effect of white spotting on tortoise shell has also been discussed by Iljin (1 928). Wright found no change in the ratio of yellow to black areas in relation to increased amounts of white in tricolours of the same stock. In animals of low grade spotting, much brindling is generally still present as well as the segregated colours; in animals of high grade spotting, brindling may occur but is comparatively rare. Frequently in the case of the latter, only one colour is present in the spots. These occasional bicolours (black-white, yellow-white) breed, however, as normal tricolours. Whether these effects depend on white spotting as such or on the specific factor s is a question which will be considered later. [. . . ] Of tortoise shell patterns in other mammals, that of the cat is of particular interest here. The heterozygous condition of the sex-linked black and yellow factors produces a mixture of black and yellow. Whiting (1919) found that the presence of white tended to decrease the amount of brindle and to separate the coat into clear areas of black and yellow. He was unable to discover any increase in the amount of yellow relative to black.

TRICOLOUR INHERITANCE.III. TORTOISESHELL CATS - Heman L. Ibsen (1916)
This as it gives a commentary on Doncaster’s papers.

Both Doncaster (1904 and later) and Little (1912) established that orange (yellow) and black are sex-linked. According to them the female is homozygous and the male heterozygous for sex determination. On their interpretation a female bearing the orange factor on one chromosome and the black factor on the other [in the same chromosome pair] is a tortoiseshell, i.e. spotted with black and orange. Therefore black and orange are allelomorphs but neither is dominant to the other. For the normally expected colour types Doncaster and Little had the same interpretation, but different explanations for the unexpected colours from a mating. Little considered the rare tortoiseshell male a mutation, while Doncaster thought it due to crossing over in the male; crossing over would also account for the unexpected class of black females from a black female X yellow male mating.

Doncaster admitted to some difficulties connected with his hypothesis: some black females should produce orange as well as tortoiseshell females when mated to a black male. Using his notation “B” = presence of black and “b” = absence of black. His hypopthesis showed that b-b males would be produced. Since b means the absence of both black ( B ) and orange (Y] those b-b males would be neither orange, nor black, nor tortoiseshell! Doncaster also stated that tortoiseshell or black females mated to tortoiseshell males do not get tortoiseshell male off spring as one would expect by his hypothesis [his fertile tortie males must have been chimeras, breeding as either black or as yellow males]. Ibsen criticised Doncaster for assuming that the crossing over of a sex-linked factor takes place in the male, which is assumed to be heterozygous for sex. This was contrary to all known facts, for in those cases even in which a Y [i.e. male-making] chromosome was known to be present it has never been demonstrated that it carries any hereditary factors. [The Y chromosome was not always visible]

Whiting (1915) thought that black females born from black or tortoiseshell females mated to an orange male were really tortoiseshells where the black pigmentation extended to such a degree that little or no orange was visible. Ibsen admitted that this was possible as he had encountered this in his guinea pig breeding experiments, but he found it improbable that such high percentages (20%) of female offspring would be accidental blacks, as would happen with Whiting’s hypothesis.

Johannsen (1913) believed Doncaster unjustified in postulating sex chromosomes as bearers of the factors for black and orange. He modified Doncaster’s interpretation to make it more “purely Mendelian.” He represented male cats factorially as Mm and females as mm, and assumed the factors for black and orange to be rather closely linked to m. Ibsen objected that this hypothesis was essentially the same as Doncaster’s and had the saw flaws.

Whiting noted the possibility of white spotting making a tortoiseshell cat look like a bicolour, but only if there was a high degree of white spotting. Whiting’s hypothesis that a tortoiseshell male was genetically a yellow male with an extreme of black extension factors or a black male with an extreme of yellow extension factors could not be adequately tested.

Ibsen was attracted to the problem of inheritance in the tortoiseshell cat because he had been studying tortoise[shell] guinea-pigs where self black was dominant to tortoise. He was intrigued that this was evidently not the case in cats. Tortoise in guinea-pigs was due to a definite single factor, while in cats, it was assumed to be due to the interaction of the black and orange factors or of extension factors. Ibsen could explain many of the apparent anomalies of colour inheritance in cats by assuming that the tortoiseshell coat was due to one definite factor, which he called “T”, and which could act only in the presence of black ( B ) , causing the black to be restricted to spots and leaving orange areas between. Two other assumptions were necessary in order to explain all the facts; first that black ( B ) was dominant to orange ( b ), and second, that under ordinary conditions T was closely linked to b (orange) [i.e. adjacent on the sex chromosome]. If T acted only in the presence of B (black) it would have no visible effect on males carrying the b (orange) factor or on females homozygous for this factor. So long as T and b remain on the same chromosome ibsen’s hypothesis was as efficient as Doncaster’s or Little’s in explaining the normally expected classes in the different matings. The unexpected classes could be explained by the occasional crossing over of the factors. Ibsen’s hypothesis explained all outcomes except for Black female x Orange male producing Black female offspring in which Whiting’s explanation stands. Also Orange male x Orange female producing tortoiseshell females could be explained by Whiting’s hypothesis.

Since, in Ibsen’s scenario, T and b are closely linked, he assumed that crossing over rarely occurred and could only occur in the heterozygous female i.e. tortoiseshell female that normally forms gametes Bt and bT. When crossing over takes place, gametes BT and bt would be produced. If this female were mated to an orange male, bT, the following would occur: both orange males and females, both tortoiseshell male and females. If this female were instead mated to a black male, Bt, the following would occur: black females, orange males and both tortoiseshell male and females.

Doncaster (1913) recorded several tortoiseshell males, all of which came from tortoiseshell females by unknown sires. It seemed that tortoiseshell males, when they occur, come almost invariably from tortoiseshell females. This matched with Ibsens’s hypothesis. The black female x Orange male producing a Black female was an apparent exception to his theory and he had no explanation for that case, unless the black mother was really a tortoiseshell with no visible orange hairs.

[Doncaster should have been aware of the following fertile tortoiseshell male show cats: 'Champion Ballochmyle Samson,' owned by Lady Alexander, and 'Champion King Saul,' owned by Mrs Herring.

BALLOCHMYLE SAMSON - Short-haired tortoiseshell male. Born May, 1894
Dam: Mr. Shaw's Topsy. Sire not recorded. Parents’ colours not recorded.

BALLOCHMYLE MERMAID - Short-haired tortoiseshell-and-white female. Born March, 1897
BALLOCHMYLE BOUNTIFUL BERTIE - Short-haired tortoiseshell female. Born March, 1897
Sire: Ballochmyle Samson (tortie male). Dam: Mrs. Heslop's Topsy (colour not recorded)
Topsy’s colour is unrecorded so we can’t tell if Samson bred as an orange male or a black male.

KING SAUL - Short-haired tortoiseshell male
Sire: Darkie. Dam: Flossie (common names, cannot be traced further back) (colours not recorded)

KING ORION - Short-haired orange male. Born May, 1899
NORA - Short-haired black-and-brown (tabby?) female. Born July, 1900
Sire, King Saul (tortie male). Dam. Lady Jasmine (short-hair tortie)
By modern analysis, King Saul bred as a black male.]

Doncaster had recorded the mating of a tortoiseshell male with a black female. The female was not confined after copulation was observed, and he ad mitted that there could be some doubt as to the paternity of the offspring! The only offspring recorded were a black male and tortoiseshell female, which was exactly what would be expected by Ibsen’s hypothesis . When tortoiseshell females were mated to tortoiseshell males, Doncaster obtained tortoiseshell females, orange females, orange males, and black males. According to Ibsen’s hypothesis no orange females should result [Males that bred as orange males may have had black spotting due to lentigo – black freckles.]

Ibsen explained what offspring could be expected from tortoiseshell males based on his T and B/b factors. A tortoiseshell female could be either Bt/bT or bT/BT. The second type of tortoiseshell female would have a double dose of T and a single dose of B and Ibsen thought it possible that the extra T restricted the black to such an extent that the animal appeared to be an orange instead of a tortoiseshell. Ibsen suggested that a further test of this hypothesis would be to determine what colour offspring were obtained from the F1 females resulting from the tortoiseshell male x tortoiseshell female cross. By hypothesis one-half of their male offspring should be tortoiseshells no matter what the colour of the father. He found no record of this type.

The anomalous black females reported by Doncaster were attributed to crossing over. When crossing over occurs in the tortoiseshell female, it produced BT and bt gametes. No matter what colour the male parent is (blaci or orange), the male offspring are BT/- tortoiseshell, and bt/- orange. The bt/- orange male differs from other orange males in that b is not linked with the T factor. By the mating of this sort of orange male with either a black female or a tortoiseshell female there should be black female offspring. This neatly explains the anomalous black females. Doncaster did not report the offspring from individual matings. If he had done so, Ibsen believed it would have been possible to better test his hypothesis regarding unexpected black females. Ibsen noted that when black females were mated to bt/- orange males all the offspring should be black, but Doncaster did not provide sufficiently detailed information to investigate this. When tortoiseshell females were mated to bt/- orange males, none of the female offspring should be tortoiseshells.

Doncaster obtained part of his breeding data from Bonhote (1915) where the individual matings were presented and it was possible to trace the offspring of three orange males. Two of the males had the usual tortoiseshell and orange daughters when mated to either black or tortoiseshell females, while the third had I orange, 3 black, 6 blue and 7 tortoiseshell female offspring when similarly mated. This last mating does not fit in at all with the theory that the orange male parent was bt/-. Ibsen noted that Bonhote always selected tortoiseshell mothers carrying a large amount of black, which would facilitate the production of tortoiseshell daughters with large amounts of black also. Some of these daughters might therefore have the appearance of blacks, while being tortoiseshells. Many of the blacks were dilute and hence classified as blues. Ibsen had found that in dilute tortoise guineapigs it was much more difficult to detect the small yellow (dilute red or orange) spots surrounded by dilute black hair, than it is to detect the small red spots surrounded by deep black hair. It was therefore possible that tortoiseshell offspring were mis-classified as blacks (blues).

The crossover bt/- orange male should be as rare as the tortoiseshell male. The latter is often sterile and it is possible the bt/- orange male is sometimes sterile also. Taking all this into consideration it is probable that matings between either black or tortoiseshell females and bt/- orange males are comparatively infrequent; thus it is quite possible that Doncaster had no record of this type of mating. If such matings occur, however, the black female offspring would be of the formula Bt/bt. These bred to either orange or black males should have orange and black sons in equal proportions. The orange sons would be bt/-. On Whiting’s hypothesis the unexpected black females should have orange and black sons also, but the orange sons should be bT/-. Doncaster knew of “no satisfactory record of a yellow male mated to a black female having yellow sons.” From this, one may infer that there have been cases reported in which a black female had orange sons, but none of them so far have been thought reliable. A larger number of records may furnish some reliable cases. He had tried so far to account for the tortoiseshell males and the unexpected black males. The three tortoiseshell females from Doncaster’s orange x orange mating remain to be explained.

Barton (1908), wrote for the general cat fancier, “If there is no white [in a tortoiseshell], then the amount of black hair should be small, compared with the red [orange] and yellow [dilute orange] markings.” According to both Doncaster and Whiting, the three tortoiseshell females resulting from an orange x orange mating were littermates, making it possible that the apparently orange mother was really a tortoiseshell with no discernible black hairs.

Ibsen admitted that his hypothesis was not entirely satisfactory, and that carefully controlled experiments were necessary for its substantiation. However, his hypothesis had two distinct advantages: it was quite definite which made it comparatively easy to prove or disprove; and it did not violate any of the accepted tenets of [Mendelian] genetics.

Ibsen’s General Comparison Of Tricolour In Guinea-Pigs, Basset Hounds And Cats

Considering their visible appearance, Ibsen noted that the tricolour coats of guinea-pigs, and tortoiseshell cats resembled each other, but were both different from the tricolour coat of Basset hounds. He first considered black and red alone in their relation to each other, and then considered the effects of white spotting. In both cats and guinea-pigs the black spotting was variable in amount and irregularly distributed. In both, black may be so far extended that the animal appeared self black or so little extended that it appeared self red. In tortoiseshell cats, as bred by the fanciers, any white spotting was small in amount and insufficient to blot out all of the orange or all of the black [fancy cats were bred to have minimal white spotting]. In tricolour guinea-pigs, white varied greatly in amount and distribution and could blot out one or other colour to produce red-and-whites or black-and-whites. In guineapigs, the variability in amount and distribution of both the black and the white spotting helped produce the unusual colour types, while in cats this was affected only by the black spotting. Because of this, cats had fewer of the unusual types than did guinea-pigs.

Basset hounds differed in that black was localized on the back [saddle] so that if there were no white present the head, legs and belly would be tan in colour, making the animal a black-and-tan. There was no chance for the black to be so far extended that the animal appears to be a self black or so little extended that it appeared to be a self red.

The distribution of white was also somewhat different. In both guinea-pigs and dogs, as well as in mammals in general, pigmentation tends to recede toward definite centers, and each of those centers could be devoid of pigmentation. The order in which the centers become pigmentless seemed to be quite irregular in guinea-pigs, while in dogs there was greater regularity and the ear patches were the last from which pigmentation entirely receded. In Basset hounds, the ear patches were always tan so a dog could be entirely white except for tan ear patches (tan-and-white). Black-and-whites could not occur because black pigmentation was never found on the head or ears of Bassets.

Ibsen summarizing by saying that, in cats, white spotting played a very unimportant role in the production of red-and-whites and black-and-whites ; the extension of black spotting could be occasionally responsible for those colour types. In guinea-pigs black spotting and white spotting were co-equal in their effects, while in Basset hounds white spotting was instrumental in the production of tan-and-whites, the black spotting merely being passive and restricted to the saddle area.

Considering their factorial basis, he said that the white spotting factors in the three animals may have some resemblances so far as was known, but since white spotting had not been adequately treated in a factorial manner, he was not able to discuss this. The black spotting factor, on the other hand, could be more definitely discussed. In guinea-pigs black spotting was called the partial-extension factor (e^p) and was the middle term of a triple allelomorphic series of which entire extension ( E ) and non-extension ( e ) were the two extremes. It was therefore recessive to entire extension. In cats on the other hand the factor for black spotting was assumed to be a dominant partial-restricting factor ( T ) , sex-linked, and also closely linked to the orange factor ( b ) . It was thought dominant to entire extension of black. In Basset hounds there were two factors to be considered. E was present as in guinea-pigs, but black spotting was not due to a modification of E, but to a new factor ( T ) not found in either guineapigs or cats. The T in cats and the T in dogs are two entirely different factors. In dogs, T is the factor for the self-coloured condition. In its absence ( t ) the animal was bi-coloured, either red-and-lemon or black-and-tan, hence t differed from e^p in guinea-pigs and T in cats in that it influences red spotting as well as black/chocolate spotting. To get dogs spotted with black but not with red, E and T must both be present. Ibsen concluded that though characters in different animals showed some resemblances [in modern terms, phenotype] , they may differ entirely in their factorial analysis [in modern terms, genotype].

References included:

Bonhote J,. L., (1915) Vigour and heredity. London: West, Newman and Co.

INHERITANCE OF WHITE-SPOTTING AND OTHER COLOR CHARACTEES IN CATS - Dr. P. W. Whiting
The American Naturalist , Vol. LIII. November-December, 1919 No. 629

Whiting had previously presented data relating to the inheritance of coat-color in cats. The experiments at the University of Pennsylvania were still ongoing, but eczema infected the stock and the investigations were ended due to the death of several animals. Whiting summarized the remaining results in this paper . I have omitted his gametic (genetic) notations.

A cream male x his solid-white yellow-eyed half-sister (derived from a white male x "anomalous" cream female mentioned below) produced 1 orange male and 1 cream female , both with extreme white-spotting. The same male x his "anomalous" cream mother gave 2 cream females. The same male x black female gave 1 maltese male and 4 tortoiseshell females. The total progeny from the cream male x his "anomalous" cream female were 4 litters containing 2 maltese males, 1 cream male and 4 cream females . A total of 8 matings of "yellow" male x "black" female gave 16 "black" males and 17 "tortoiseshell" females .

Dr. Charles Penrose, of Philadelphia, loaned his Caffer cat for crossing, a much-ticked lined male mentioned in Whiting’s previous paper. Caffer cat x orange striped female gave 3 tortoiseshell females: 1 much-ticked lined , 1 lined with ticking present but with so much yellow that the exact degree was uncertain, 1 ticked of uncertain degree in which the banding was also uncertain on account of admixture of black and yellow.

Caffer male x orange striped female gave 5 orange lined males. Caffer male x tortoiseshell female gave 4 lined non-yellow kittens, 2 little-ticked males and 2 much-ticked females . Caffer male x blotched maltese tortoiseshell gaved 4 lined orange males and 2 lined tortoiseshell females Caffer male x dilute tortoiseshell female gave 1 lined orange male and 1 lined tortoiseshell female. Caffer male x tortoiseshell female gave 2 orange lined males , 1 little-ticked lined male and 2 lined females - one being tortoiseshell with so much yellow that the degree of ticking could not be made out - and 1 non-yellow with so much white that the degree of ticking could not be made out.

The crosses of this Caffer cat were reciprocal to those summarized above, crosses of "yellow" males with "black" and "tortoiseshell" females. Here Whiting had a "black" male x "yellow" females giving 5 "yellow" males and 3 "tortoiseshell" females , and a "black" male x "tortoiseshell" females giving 3 "black" males, 7 "yellow" males, 4 "tortoiseshell" females and 3 "black" females. He observed that the principle of sex-linkage applied in all cases.

The progeny of the Caffer cat also illustrated ticking and banding [tabby]. Caffer cats had the narrow banded or "lined" condition. Banding of intermediate width, "striping," appeared to be recessive to lined, and the widest bands, "blotches," were recessive to both "lined" and "striped." The parents of the Caffer cat were both lined, but produced both lined and blotched offspring. Whiting deduced that the cat was the homozygous segregate, for 24 of his 25 offspring were lined and in the other there was so much white-spotting and so much intermixture of yellow and black in the pigmented areas that the condition of banding is uncertain. It was probable, however, that even in that case a wider type of bands - stripes or blotches - would have been discernible. Of the females crossed with the Caffer cat, 2 were blotched, and 4 were striped but known to carry blotched. Lined was therefore dominant to both striped and blotched. The results did not demonstrate the allelomorphism of the 3 types of banding. In order to do that Whiting would have needed to cross one of the offspring carrying striped to blotched cats. All kittens should be lined or striped, but if blotched occurred it would demonstrate that two loci were involved and Whiting’s nomenclature would have to be changed.

The production of orange and tortoiseshell lined cats was as expected in every way comparable to other oranges and tortoiseshells except for the narrower bands.

Results in regard to ticking were considered. In the previous paper “a” was used to denote lack of ticking ; A, little-ticked or dark tabby; and A1, much-ticked or light tabby. Whiting had since found 2 hereditary grades of ticking previously grouped together under A1, but which were quite different. Comparison of kittens at birth or of adult cats made the distinction clear. During growth, intermediates appeared because ticking increased with age. A1 was therefore be divided into Ae, extreme-ticking, and Am , much-ticking. There was as much difference between Ae and Am as between Am and A.

A blotched male x black female gave 4 blotched kittens. A blotched female x lined little-ticked male gave 1 lined and 3 blotched offspring. The 2 blotched cats in these crosses were extremely-ticked, as were the 8 kittens. A much-ticked Caffer female x little-ticked Caffer male gave 1 much-ticked and 3 little-ticked kittens. The much-ticked mother and son were very similar and contrasted strongly with the extremely-ticked cats as well as with little-ticked cats. Except for 5 kittens, the offspring of the much-ticked male were useless for determining degree of ticking due to amount of yellow present. Three kittens were little-ticked like their grandfather. Two kittens were much-ticked like their father and grandmother. The mothers of all of these kittens were non-ticked. The same degrees of ticking were possessed by 3 generations.

The crosses summarized in previous papers and above were then considered in terms of white-spotting. Solid-white was a complete dominant to other colors. White-spotting in cats graded all the way from solid-white to self colour. In individual fraternities, however, it showed wide and clean segregation as Whiting’s crosses demonstrated. A "self" cat might have a minute white spot on the breast of belly or a few sparsely scattered white hairs and be called near-self. Restricted spotting denoted white on nose, breast, belly, or feet. It segregated widely from near-self in the crosses he made, but graded into moderate spotting, which denoted the extension of white to the sides of the bodyl. Moderate spotting in turn graded into considerable spotting, which denoted more white than color. Extreme spotting denoted that pigment was limited to small spots on head, back, or tail.

Crosses involving only self, restricted and moderate spotting, and solid-white were considered first. A self male x 2 self females sired 8 self offspring. One of those self offspring x self female gave 5 self offspring. Self therefore bred true.

The first mentioned self male x 2 restricted spot females sired 3 self and 4 restricted spotted offspring. The other self male x 2 restricted spot females sired 5 self and 5 restricted spot offspring. A restricted spot male crossed twice to the same self female sired 3 self and 7 restricted spot. A restricted spot female crossed to a self male produced 6 self. Self x restricted spot therefore produced 17 self and 16 restricted spot, the expectation if restricted spot was heterozygous.

The restricted spot male x two restricted spot females gave 1 self, 3 restricted spotted, and 1 moderate spotted. This was in line with expectation if spotting was dominant, the moderate spotted possibly represented the homozygote. The same restricted spot male x solid- white female gave 2 solid-white and 2 completely self colour. This was in line with the assumption that the white female was homozygous for self and heterozygous for color, or that spotting and white were both allelomorphic with self and that she carried self. The male would then be restricted spotted or moderate spotted.

Crosses involving greater amounts of spotting were then considered.

The self male x considerable spotted female gave 1 self and 2 considerable spotted offspring. When crossed to a moderate spot female he sired 3 self and 2 considerable spotted. These results showed that considerable spotting segregates from self and that a greater degree of spotting may be produced by less spotted x self. Modifiers were suspected.

The same male x a considerable spotted female gave 2 restricted spotted. In this case modifiers may have been assorted to produce restriction, but the female was derived from a cross of a considerable spotted x a restricted spotted, each of which was known to carry self. She may therefore have been heterozygous for much spotting (derived from her considerable spotted parent) and little spotting (from her restricted spotted parent). She would then produce restricted offspring when crossed to self. Offspring of self by spotted-carrying-self were therefore 21 self and 20 spotted.

The considerable spot male was crossed to various spotted females that were known to produce self kittens. With a restricted spot female he sired 2 self, 1 restricted, 3 moderate, and 3 considerable spot. With a moderate spot female he sired 2 self, 1 restricted, 1 moderate and 1 extreme. With a restricted spot he sired 1 self, 1 restricted, 1 moderate, and 1 considerable spot. With a moderate spot female he sired 2 considerable and 1 extreme spot. With a restricted spot he sired 2 self, 2 considerable spot (one of which graded toward extreme), and 1 very extreme. That last cross was interesting for the offspring varied far in both directions from the parental types.

Crosses of spotted x spotted, when both carried self, produced 23 spotted to 8 self which was very close to the Mendelian 3:1 ratio expected.

The considerable spot male was crossed to his considerable spot mother and sired 5 considerable and 2 restricted spot offspring, the segregation being striking through the absence of any moderately spotted offspring. This was in line with the supposition that the mother carried little spotting. The same male was crossed to a solid-white half sister from the same mother and sired
by a white male. This produced 2 extreme spotted offspring.

The failure of anything higher than restricted spotting to occur among the offspring of restricted spotting x self (although cats with considerable spot may carry self) indicated that allelomorphic factors determined the different degrees of spotting. It appeared that self was recessive to spotting and that color was recessive to solid-white. Whiting suggested that there was a quadruple allelomorphic series: W (solid-white); wm (much spotted); wl (little spotted); and w (non-spotted/self) in that order of dominance. Crosses of white x self and of spotted x self would check this principle. Any one white cat might throw white either much spotted, little spotted, or self kittens; a much spotted cat might throw much spotted, and either little spotted or self. A little spotted should throw little spotted, or little spotted and self. If 3 distinct types were produced from any one white or spotted cat crossed to numerous self cats, this would demonstrate modifiers to be very important or could disprove Whiting’s hypothesis ofquadruple allelomorphism.

Attention was called to the interesting but unexplained relationship between yellow-spotting and white-spotting. "Self" tortoiseshells have yellow hairs closely intermixed with non-yellow. This made it very difficult to determine degree of ticking in such animals. Tortoiseshells with restricted white-spotting tended to have yellow separated into patches, while further extension of white separated yellow and non-yellow areas still more. Separation of yellow into patches appeared not to be correlated with the amount of yellow.

General Summary of Inheritance of Coat-Color in Cats

Ratios were not significant since fraternities from homozygous dominants and heterozygotes were included together.

Maltese dilution was presumably a simple recessive to intensity. Intense x intense produced 41 intense. Intense x dilute produced 37 intense and 23 dilute. Dilute x dilute produced 18 dilute.

Solid-white, W, acted as a simple dominant over color, w. It was true-breeding in the hands of fanciers. White x color (amount of white-spotting undetermined) produced 3 white and 4 colored (one near-self).

Table I showed summaries for white and white-spotting of determined degree.

It was obvious to Whiting that although extensively pigmented animals appear among the offspring of cats showing much white there was little tendency for a kitten to show more white than appears in either parent.

Table II gave a summary of the results collected in reference to the inheritance of yellow. Doncaster's 2 summaries from fancy breeders and from Little 's data were given, kittens of undetermined sex being omitted. The 3 tortoiseshell females from one pair of Doncaster 's yellow x yellow could be readily explained if it was supposed that the mother was an extreme yellow variant of tortoiseshell, comparable with Whiting’s cream female number 23. Anomalous black females might be similarly heterozygous. Anomalous blacks and tortoiseshells were to be expected from anomalous yellow females. Anomalous offspring were recorded in italics in Table II.

As regarded banding, certain creams and blacks could not be classified and were omitted from the summaries. Lined x lined gave 2 lined and 2 blotched. Lined x striped gave 17 lined and 2 striped. Lined x blotched gave 12 lined and 4 blotched. Striped x blotched gave 19 striped and 8 blotched. Blotched x blotched gave 4 blotched.

As regarded ticking, it was necessary to omit all yellows and many tortoiseshells, as well as some with much white. Extremely-ticked x little-ticked gave 4 extremely-ticked. Extremely-ticked x black gave 4 extremely-ticked. Much-ticked x little-ticked gave 1 much-ticked and 3 little-ticked. Much-ticked x black gave 2 much-ticked and 3 little-ticked. Little-ticked x little-ticked gave 5 little-ticked and 1 black. Little-ticked x black gave 7 little-ticked.

COLOUR INHERITANCE IN CATS, WITH SPECIAL REFERENCE TO THE COLOURS BLACK, YELLOW AND TORTOISE-SHELL. BY C. C. Little 1919.

Little’s paper had objects : (I) the critical examination of experimental data on, and of current hypotheses concerning the inheritance of black, yellow, and tortoise-shell coat colours in cats; (2) the suggestion of possible explanations for the occurrence of (a) unexpected colour classes in ordinary crosses between blacks, yellows, and tortoise-shells, and of (b) both sterile and fertile tortoise-shell males which appear extremely rarely.

The Facts Requiring Explanation.

The critical and apparently contradictory facts which have been brought out by breeding experiments with cats, and which must be satisfactorily accounted for and explained, were:

• (1) In yellow males x black females, where the expectation on the basis of complete sex-linkage is black males and tortoise-shell females, black females were sometimes produced. (Doncaster, 1918.)
• (2) In yellow males x tortoise-shell females, where yellow males, black males, yellow females and tortoise-shell females were the only classes expected on the basis of complete sex-linkage, black females were sometimes produced (Doncaster, 1913.)
• (3) In yellow x yellow, although only yellow young were expected, 2 aberrant results have been noted: (a) A mating of this type has produced tortoise-shell females besides yellows of both sexes. (Doncaster, 1913.) (b) A mating of this type has produced tortoise-shell females and black males besides yellows of both sexes. (Whiting, 1918.)
• (4) There is no record of black x black producing yellow or tortoise-shell young.
• (5) Tortoise-shell males were produced much more rarely than any of the aberrant classes recorded under headings 1, 2, and 3 above. (Doncaster, 1913; Wright, 1918.)
• (6) Such tortoise-shell males were usually sterile. (Cutler and Doncaster, 1915.)
• (7)Where they were not sterile they did not give tortoise-shell sons, but bred as yellows. (Doneaster, 1913.)

Investigators usually tried to explain all of the facts by a single hypothesis. (Doncaster, 1913; Whiting, 1918.) This was difficult and unsatisfactory (Ibsen, 1916 ; Wright 1918.) Little believed that the experimental evidence favoured the existence of 2 genetically independent agents at work in the production of these aberrances, for (a) The appearance of the unexpected individuals noted under headings 1, 2, and 3 above, was relatively frequent, and produced regular results involving neither sterility nor the formation of new colour types and (b) the occurrence of tortoise-shell males was very infrequent, not regular, and mostly connected with sterility.

Little tried to explain the appearance of the unexpected individuals noted under headings 1, 2, and 3 (unexpected individuals of normal colour types) by one hypothesis and the occurrence of tortoise-shell males by a different one.

The Relation Between Yellow And Black,

First he considered the nature of the genetic relation between yellow and black coat colours. Ibsen, 1916, and Wright, 1918, believed black or extension of black pigment to the coat, to be epistatic to yellow or the restriction of black pigment from the coat. Doncaster, 1913, and Whiting, 1918, consider the two coat colours allelomorphic, the heterozygote being commonly tortoise-shell. The terminology used by them is as follows:

• Ibsen,, 1916 : Black B is dominant to orange b which is borne in the X chromosome. Under ordinary conditions the factor for orange b is closely linked to T, a factor for tortoise-shell which acts only in the presence of B-black. The female is XX, the male X0 in formula [the Y chromosome was hard to detect].
• Wright, 1918: Black is due to the action of a factor A, while tortoiseshell is produced by heterozygosis of an " extension" factor E. Tortoiseshell females are thus Ee, yellow males e-, and black males E-, in formula. The factor E is borne in the X chromosome. The female is XX, the mate X0 in formula.
• Doncaster, 1913: yellow and black are allelomorphic, and expresses yellow by Y, and black by B. Where both are present, a YB or tortoise-shell animal is produced. The female is XX, and the male X0 in formula.
• Whiting, 1918: yellow Y is allelomorphic with black y, and supposes Y to be borne in the X chromosome. The female is homozygous, the male heterozygous for X.

In 1912, Little modified Doncaster’s terminology due to the production of blacks and tortoise-shells by 2 yellows and the failure of black x black to produce anything except black offspring. The relationship between those two colours might be more accurately expressed: B = factor for the production of black pigment and found in all X gametes. Y = factor for the restriction of black pigment from the coat allelomorphic to y. y = factor for the extension of black pigment to the coat. One "dose" of Y was normally completely epistatic to one "dose " of B, thus producing yellow individuals; but two "doses" of B to one of Y produced tortoise-shell. The factor Y and its allelomorph y are also borne in the X chromosome. Thus :

YBX YBX = Yellow female
YBX 0 = Yellow male
yBX yBX = Black female
yBX 0 = Black male
YBX yBX = Tortoise-shell female

An Attempt To Explain The Appearance Of Unexpected Individuals Of Normal Colour Types.

It was assumed that at some time(s) in the past, there must have been a genetic change, ridding certain gametes of the epistatic colour factor, whether it be the Y of Whiting, the E of Wright, or the T of Ibsen, otherwise neither the hypostatic form nor the tortoise-shell heterozygote could have appeared. Little accepted the set of symbols given above, and assumed that a change from Y to y must have occurred at some point. There was no experimental evidence to show when or how often that change took place, but he assumed it was probably still taking place in a portion of the gametes of certain individuals which would account for all the results obtained under headings 1, 2, and 3. Such a change from an epistatic to a hypostatic condition would be directly comparable to the appearance of the recessive pink-eyed mutation in a stock of dilute brown mice recorded by Little in 1916 [summary: mutations happen!]

Animals whose gametes had newly occurring mutations would show no trace of it in their own somatic characteristics, but their offspring would give results in agreement with the actual aberrant classes obtained. An occasional yellow female would form gametes yBX in addition to normal YBX gametes. Similarly, certain yellow males would be found which showed by their progeny that they were forming among their X gametes some which were yBX instead of the normal YBX type. Yellow males of that unusual kind would, wen crossed with black females, produce some black females, the number depending upon the frequency with which the unusual yBX sperm was formed. This would explain the aberrances listed above.

Similarly, such unusual yellow males x normal tortoise-shell females would give a certain number of black females in addition to the other classes normally expected. This would cover category two of exceptions mentioned above. Finally, a yellow forming yBX gametes, when crossed with a normal yellow or with one of its own type, would give rise to unexpected black or tortoise-shell young, the proportion depending upon whether the yellow male or the female or both formed yBX gametes.

If the male was alone concerned, he would produce tortoise-shell females, but no black males in his progeny. This appears to be the case in the mating recorded by Doncaster (1913) in which 2 yellows gave among their progeny 3 blue females with a cream coloured patch (tortoise-shells). If, on the other hand, the female parent was the unusual mutative individual, black males would occur in addition to tortoise-shell females and yellows of both sexes. This condition was realised in the case of a female dilute yellow (formerly owned by Little) whose breeding record was reported by Whiting, 1918. An explanation of this sort would account for the aberrances noted above.

From the number of tortoise-shell and black young obtained in the 2 cases referred to, and from the numerical relation of the black females under headings I and 2 to the expected colour classes (Doncaster 1913), it seems probable that yellow animals forming yBX gametes do so in approximately 50% of the gametes they form, as would a normal heterozygote.

In addition to yellow animals, certain tortoise-shell females might theoretically be expected to show the same phenomenon. Such animals would form an excess of, or possibly exclusively, yBX gametes, and would breed as blacks. Such an occurrence would, however, give rise to no unexpected classes of young in crosses, but might result in the absence of some of those normally expected from certain matings. Quite naturally this fact might, in a small number of progeny, escape notice.

There is no evidence to show that the appearance of any of the classes above referred to is in any way connected with a break in sex-linkage or with the occurrence of tortoise-shell males, and we may therefore, until such evidence is presented, fairly consider them as independently produced.

Criticism Of Existing Hypotheses To Explain The Appearance Of Unexpected Individuals Of Normal Colour Types.

Attempts to explain the appearance of the aberrant colour classes referred to, have involved either (a) the breaking of sex-linkage with "crossing over" in the male, or (b) the occurrence of a series of modifying factors determining the relative degree of black and yellow pigmentation. They may be separately considered as follows:

• (a) Doncaster's hypothesis of a break in sex-linkage : this hypothesis, which in a modified form is a basis for Ibsen's later explanation of the appearance of unusual colour types, involves, if it is to explain the exceptional black females, the existence of "crossing over" in the male between the X and the O chromosomes. Such crossing over has not, in so far as I am aware, been demonstrated in any forms in which the male is X0 in formula as in cats. It further would suppose that, as tortoiseshell males were formed by the same process, they would be expected to occur with as great frequency as the exceptional black females. It further leaves entirely unexplained the appearance of blacks or tortoiseshells from a cross between yellow animals. These objections seem to be of sufficient weight to throw the chances against Doncaster's or Ibsen's hypotheses.
• (b) Whiting's hypothesis of modifying factors which at one end of the series-would serve to make tortoise-shell animals yellow, and at the other end of the series make them black, remains as a possibility though seriously invalidated by certain points as follows:
  • (1) There should be records of black females (genotypically tortoise-shell) which if crossed with other blacks should give yellow males and tortoise-shell females, or if crossed with yellow males should give unexpected yellow females. Neither of these results has been recorded.
  • (2) Doncaster, 1913, reports that the three tortoise-shell females produced from a single cross between two yellows were "blue with a cream patch" thus showing that they were near the black end of Whiting's modifier series. Inasmuch as under his hypothesis one of their parents must have been at the opposite or yellow end of the series, it is difficult to explain how and why many of its progeny should show the condition characterising nearly the other end of a graded series.
  • (3) The occurrence of these young in a single mating makes it seem likely that the particular animal was forming ordinary yBX gametes in a considerable number.
  • (4) The tortoise-shell young produced by dilute yellow female [previously owned by Little] before she was sent to Dr Whiting, were normal tortoise-shell in colour ; if anything, more nearly on the black end of the graded series, than on the yellow. This case serves to support that reported by Doncaster, and tends to show that the yellow animal transmitted to its progeny no peculiar set of modifiers.

An Attempt To Explain The Occurrence Of (a) Sterile, And (b) Fertile Tortoise-Shell Males.

(a) The production of sterile tortoise-shell males.

It is agreed by all those who have reported on breeding experiments with cats that the female appears to be homozygous, the male heterozygous, for sex. The former is therefore XX, the latter X0 in formula. This places cats in the same category with Drosophila, and this in turn means that one may refer to the work of Morgan et al when explaining a peculiar result which shows exceptional conditions of sex-linkage. If one considers the phenomena of non-disjunction of the X chromosome in Drosophila, reported by Bridges in 1913 and 1916 and further established by him after an extensive series of breeding experiments, one cannot fail to be impressed by the similarity between the results of that process in Drosophila, and the observed experimental facts in cats.

For example, non-disjunction is neither frequent in its occurrence nor is it clear enough in its hereditary behaviour to give striking numerical results in as slow breeding an animal as a cat, unless it were watched for deliberately. In Drosophila it gives rise to two very significant exceptions to the normal sex-linked inheritance. First, it produces animals apparently male but sterile, and second, mosaic forms are apt to arise in non-disjunctional stocks. If one considers that the majority of tortoise-shell cats which are apparently males are sterile, and second that they are also a mosaic form in a sex where commonly none is found, the comparison becomes interesting and extremely suggestive.

We may now consider what the probable results of non-disjunction would be, did this phenomenon exist in cats. The characteristic of primary non-disjunction is that in oogenesis the two X chromosomes go together into a single egg, leaving another egg without even the normal single X. This may be shown as follows :

Non-disjunctional female XX forms gametes XX and “-“ If the eggs of such a female are fertilized by sperm of a normal male we have four possible types of zygotes:

XX egg + X sperm = XXX zygote - Dies
“-“ egg + X sperm = X”-“ zygote - "Near male" always sterile
XX egg + 0 sperm = XX0 zygote- Female with peculiar gametic condition
“-“ egg + 0 sperm = 0”-“ zygote – Dies

Bridges demonstrated that the XXX and 0- forms die, and that the X- form although appearing like a male is always sterile. If now we imagine a cross to be made between a tortoise-shell female cat showing non-disjunction and a normal yellow male, we should have the following condition :

Non-disjunctional Tortoise-shell female YBX yBX forming gametes YBX yBX and “-“
Normal Yellow male YBX 0 forming gametes YBX and 0

Zygotes are:
(a) YBX yBX YBX - Dies
(b) YBX yBX 0 Tortoise-shell with peculiar gametic conditions
(c) YBX- - "Near male" always sterile
(d) -0 - Dies

If now one assumes that absence of the 0 chromosome allows the "near male" class (c) to develop into a tortoise-shell, disturbing the normal relation of yellow to black to produce a somatic mosaic, we could account for the appearance at rare intervals of tortoise-shell "near males" which were not fertile. It seems not unlikely that the absence of the 0 chromosome might well upset the somatic relationships of certain of the characters whose factors are carried by the X chromosome. This would account for the appearance of a tortoise-shell "male" from a mating of yellow male x tortoise-shell female. (Doncaster, 1913.)

Another mating .which, according to Doncaster , has produced a tortoise-shell male is that of yellow male by black female. Here, if the black female showed non-disjunction, the following condition would be found :

Black non-disjunctional female yBX yBX forming gametes yBX yBX and “-“
Normal Yellow male YBX 0 forming gametes YBX and 0

Zygotes
(a) yBX yBX YBX - Dies
(b) yBX yBX 0 - Black female with peculiar gametic conditions
(c) -YBX - Tortoiseshell? "near male" always sterile (as in previoius mating)
(d) -0 - Dies

The third type of mating reported by Doncaster as having produced a tortoise-shell male is that of black male with tortoise-shell female. Here everyone is in difficulty. If, as Doncaster suggests, the occasional crossing over of Y, the factor for yellow, to a 0 gamete is responsible for the production of a tortoise-shell male, nothing that could happen in either the gametes of the black male or of the tortoise-shell female would produce a tortoise-shell male. On Whiting's hypothesis we should have to suppose that the tortoise-shell female, although she herself showed no marked modifiers (or she would have been black), transmits unusually heavy modifiers to her sons. These gametes would in turn have to be met by an equally heavy set of modifiers from the black male, or a yellow would result.

Further than this, by Whiting's hypothesis the yellow male is YX0 in constitution, and this makes the source of the black that he must produce somatically under the influence of modifiers in order to become a tortoiseshell, uncertain. This condition is, of course, not impossible but is highly improbable. Finally, the phenomenon of non-disjunction meets with distinct difficulties. Unless the black male forms gametes with neither the X nor 0 chromosomes present it would be hard to see how the tortoise-shell male could be produced by this mating. Formation of sperm without X or 0 would not be likely. Yet the possibility exists and may therefore be considered. What seems to me altogether more likely is that the breeder's records on which Doncaster based his observation were in this case uncertain or incorrect, a circumstance quite possible in cats even with the best possible intentions.

(b) The production of fertile tortoise-shell males.

We have seen that peculiar tortoise-shell females of formula YBXyBX0 may possibly be produced by primary non-disjunction. If now one of these females is crossed with a black or a yellow male peculiar yellow males of the constitution YBX00 would be formed as follows:

Non-disjunctional Tortoise-shell female YBXyBX0 forming gametes YBXyBX, YBX0, YBX, 0, yBX0 a,d yBX
Crossed with
Yellow male YBX0 forming gametes YBX and 0

Zygotes
(a) YBXyBXYBX - Dies
(b) YBX0YBX - Peculiar yellow female
(c) YBXYBX - Yellow female
(d) 0 YBX - Yellow male
(e) YBXyBX0 - Peculiar tortoise-shell female
(f) YBX 00 - Peculiar yellow male
(g) YBX 0 - Yellow male
(h) 00 - Dies
(i) yBX0YBX - Peculiar tortoise-shell female
(j) yBX00 - Peculiar black male
(k) yBXYBX - Tortoise-shell female
(l) yBX 0 Black male

If now such a peculiar yellow, YBX00, is mated with any female showing primary non-disjunction - an animal which might well prove to be a fertile tortoise-shell male would be produced. Thus:

Black non-disjunctional female yBX yBX forming gametes yBX yBX and--
Crossed with
Non-disjunctional yellow male YBX00 forming gametes YBX0, YBX, 0 and 00

(a) yBXyBX YBX0 - Dies
(b) yBXyBX YBX - Dies
(c ) yBXyBX 0 - Peculiar black female
(d) yBXyBX 00 - Peculiar black female?
(e) -YBX0 - "Tortoise-shell male "fertile?
(f) -YBX - "Tortoise-shell ' near male' sterile?
(g) -00 - Dies
(h) -00 - Dies

Here the assumption is made that an animal formed from the combination of gametes, YBX0 and -, may be somatically a tortoise-shell, and that the 0 chromosome which is brought into the zygote by an X-bearing gamete does not in all cases exert its full influence until gametogenesis. The YBX0-male would then be supposed to develop somatically just as does the YBX-animal, but upon gametogenesis the 0 chromosome of the YBX0 male is able to prevent the sterility which exists in its absence. This seems quite possible for it appears that in Drosophila the 0 chromosome is not needed for the development of the normal male somatic characters, but that it is necessary, however, for successful gametogenesis in the male.

A fertile tortoise-shell male would, when he formed gametes, behave exactly like a normal yellow male. That is to say, although he was himself the product of a combination of X0 and - gametes, he would in gametogenesis form only X and 0 gametes, just as would a normal male. This has been the breeding behaviour of the one recorded certainly fertile tortoise-shell male (Doncaster, 1913) which acted in crosses with tortoise-shell females apparently exactly as a yellow male would have done.

It will be seen that the above hypothesis, although somewhat complicated, is nevertheless in accordance with experimental facts and accounts for sterile and fertile types of tortoise-shell males ; it explains their frequency of appearance, and possibly their failure to transmit their own color pattern to their descendants ; it is supported by the work of Bridges with Drosophila--the most completely investigated form showing a similar type of sex-linkage; it is further capable of experimental tests.

>Criticisms Of Existing Hypotheses To Explain The Occurrence Of Tortoise-Shell Males.

• (1) Doncaster's hypothesis, as already pointed out, requires "crossing over" in the male between X and 0 chromosomes--a condition not shown to exist in gametogenesis of any X0 male form. It further fails to account for (a) the comparative infrequency of tortoise-shell males as compared with aberrant black females, (b) the sterility of the majority of tortoise-shell males, and (c) their peculiar behaviour in breeding.
• (2) Ibsen's hypothesis does away with the need of crossing over in the male, but fails, as does Doncaster's hypothesis, to meet or explain points (a), (b), or ( c ) stated above.
• (3) Whiting's hypothesis of modifiers would not be able to give a tortoise-shell male which according to his formula would be XY0 without adding a factor for black to the formula given by him to be carried in the X gamete. It further would suppose that by selection (which undoubtedly has occurred) it would be possible to transmit the necessary modifiers to a considerable number of his male progeny, thus forming tortoise-shell males--and this, though great efforts have been made, has proved impossible. Whiting's hypothesis, like those of Doncaster and of Ibsen, takes no account of the sterility of the majority of tortoise-shell males.
• (4) Wright's hypothesis is that tortoise-shell males are really XX individuals in which the abnormality lies not in the colour but in the sex. He likens them to certain sex intergrades already described in some forms by other investigators. This hypothesis meets trouble when a fertile tortoise-shell male is encountered. It also is contrary to the evidence obtained by Bridges who shows that in Drosophila XX forms are females, even though they contain other abnormalities of chromosome distribution.

Summary And Conclusions.

• (I) The genetic constitution of the normal colour varieties of cats as regards yellow and black pigmentation appears to be as follows : B = a factor producing black pigmentation, Y= a factor which restricts black from the coat, y=a factor allelomorphic to Y and hypostatic to it, allowing black pigment to extend to the coat.
  YBX YBX - Yellow female
  YBX 0 - Yellow male
  yBX yBX - Black female
  yBX 0 - Black male
  YBX yBX - Tortoise-shell female
• (2) The unexpected but normally pigmented individuals appearing in certain matings can be accounted for by supposing that Y becomes y in a certain proportion of the gametes of exceptional individuals.
• (3) Sterile tortoise-shell males may possibly be" near males" formed as a result of non-disjunction of the X chromosome and therefore YBX in constitution.
• (4) Fertile tortoise-shell males may also be the product of nondisjunction (secondary) and would be zygotes formed from the fusion of gametes YBX0 and “-“. These males in gametogenesis would behave as ordinary yellows.

CROSSES WITH SIAMESE CATS. By K. Tjebbes 1924.
DOMINANT BLACK IN CATS AND ITS BEARING ON THE QUESTION OF THE TORTOISESHELL MALES. By K. Tjebbes and Chr. Wreidt. 1926.

Existence of two different kinds of black colour in cats had been indicated in Tjebbes' experiments on Siamese cats, published in 1924. The occurrence of this dominant black in cats was of special interest because it might explain some tortoiseshell males. A mating between Siamese and tabby (wild type) apparently gave 3 blacks. Three other Siamese x tabby matings gave F1 black heterozygotes only, totalling 10 uniformly black coloured offspring.

The F2 generation produced from those black cats produced 4 blacks, 1 tabby and 2 Siamese [refers to pattern, not breed]. The Siamese were genetically blacks; Tjebbes had previously shown that [black] tabby may be combined with the recessive factor that causes the Siamese pattern. An F2 Siamese x an F1 black male gave 1 black, 1 Siamese and 1 tabby. Together, those matings gave 8 blacks (uniform blacks plus Siamese which are genetically black) against 2 tabbies. The data was admittedly scanty, but Tjebbes was aware of dominant black in rabbits, dogs and swine so he contended that cats had both dominant and recessive black mutations. Punnett (1912) proved the existence of dominant black in an acromelanistic Himalayan rabbit, considered the nearest parallel to the Siamese cats.

Tjebbes then mated a Siamese female with a striped yellow male resulting in 1 female tortoiseshell and 2 male tortoiseshells. All earlier workers on tortoiseshell cats had agreed on the extreme rarity of tortoiseshell males. Doncaster encountered only 3 such males amongst 225 males from mating black x yellow, tortoiseshell x yellow, and tortoiseshell x black.

It was believed that all tortoiseshell males were infertile, which led to a variety of hypotheses. It was supposed that they were due to either non-disjunction or to a process equivalent to freemartins in cattle. However, some tortoiseshell males had proven fertile. In the light of the Tjebbes 2 tortoiseshell males produced by a mating of [supposedly] dominant black x yellow, he thought that the co-operation of dominant black with yellow could produce tortoiseshell tom-cats.

Regarding chromosomes, it wasn’t possible to give a full explanation, but Tjebbes mentioned that studies by other researchers indicated that genes were found on the Y-chromosomes of vertebrates. Sex-linked inheritance in mammals had been found in only three cases, making it likely that sex-linked inheritance was often masked by the presence of genes on the Y-chromosome and also that crossing-over between X and Y chromosomes might occur.

DOMINANT BLACK IN CATS AND ITS BEARING ON THE QUESTION OF THE TORTOISESHELL MALES--A CRITICISM. By Ruth C. Bamber (Mrs Bisbee) and E. Catherine Herdman. 1927

Bamber and Herdman summarised Tjebbes findings and noted that ordinary black in cats was recessive to the ticking that produced tabby. Tjebbes had postulated the existence of dominant black based on a Siamese female x striped yellow male producing 1 female and 2 male tortoiseshells. Bamber and Herdman had been studying both inheritance and sex-determination in cats. They found that the suggestion of Tjebbes and Wriedt were not in harmony with the facts of the case. From Tjebbes' own experiments it was clear that his dominant black was not affected by the ticking factor that produced tabby. If tortoiseshell males were the result of the combination of yellow and dominant black it would be impossible to produce get a tabby-tortoiseshells male. Bamber and Herdman possessed a tabby-tortoiseshell male; he was a mixture of yellow and tabby arranged in irregular patches, with white chest and feet. The pattern was common in females, but extremely rare in males. While cat fanciers referred to them casually, Bamber and Herdman knew of only one other confirmed case.

According to them, the only difference between a black and a tabby lay in the presence of the ticking factor (also mentioned by Doncaster, 1913, Whiting, 1918 and 1919, Bamber, 1927) so their male was tortoiseshell + ticking. They admitted the possibility that 2 doses of the ticking factor [gene] might give tabby with dominant black, although 1 dose (as in Tjebbes' results) had no visible effect. They thought it likely that a "tabby" produced by dominant black + 2 ticking factors would be distinguishable from a normal tabby. The fact that the large patches of tabby on their male were perfectly normal suggested that dominant black was not responsible for his coat colour. He was fertile but their breeding experiments lacked sufficient offspring for them to draw firm conclusions about him.

Tjebbes' and Wriedt's recorded 2 tortoiseshell males and 1 tortoiseshell female from the cross of striped yellow x Siamese. Bamber and Herdman believed a yellow male would not normally transmit yellow to his sons, yellow being apparently sex-linked (Little 1912, 1919; Doncaster 1912, 1913, Bamber 1927, Bamber and Herdman 1927). This made them dubious of Tjebbes’ claim of 2 tortoiseshell males in one litter. They suggested he had not properly identified the sex of the newborns (this often being difficult) and hoped the kittens would either be allowed to grow up, or would be dissected to confirm their sex. Even if the kittens had been correctly sexed, the occurrence of 2 tortoiseshell males did not justify the assumption that dominant black was the key to the tortoiseshell tom-cat problem. These males got their yeliow from their father and that, according to Bamber and Herdman, was abnormal.

Crossing- over between the X and Y chromosome in the yellow male would be sufficient to explain the results, and the idea of the dominance black was superfluous. If Tjebbes and Wreidt insisted on dominant black, then they had overlooked the fact that it was the presence of the yellow in their male kittens that was unexpected, not the presence of the black. Bamber and Herdman looked forward to further studies involving “dominant black” and yellow.

DOMINANT BLACK IN CATS AND TORTO1SESHELL MALES. A REPLY. By K. Tjebbes and Chr. Wreidt. 1927.

Tjebbes and Wreidt welcomed the criticism as giving them an opportunity to publish more details on their tortoiseshell males. Their suggestion that the occurrence of dominant black may have something to do with the occurrence of tortoiseshell had not been intended to explain every such male. The thought the occurrence of 2 tortoiseshell males in a single litter of 3 tortoiseshells from a cross with a dominant black was distinctly suggestive, and that dominant black was a more cause than non-disjunction or freemartinism.

It was beyond all doubt [to Tjebbes and Wreidt] that dominant black had occurred in their Siamese Material. It was also beyond doubt that the black did not lie in the sex chromosome [at the time, the role of Y was poorly understood]. They admitted the possibility that some Y chromosomes might contain a factor that co-operated with dominant black, causing tortoiseshell. They admitted that the question of tortoiseshell males remained unsolved and needed more breeding studies.

Regarding their 2 tortoiseshell males, one had died at 2 days old. Dissection showed him to be male beyond any doubt. The other was alive at the age of 17 months, but had produced no offspring with any of the oestrus females offered to him. Tjebbes and Wreidt believed this cat had never copulated and was either abnormal or infertile. They were trying, among other experiments, to repeat the same mating that had produced the 2 tortoiseshell males.

THE INCIDENCE OF STERILITY AMONGST TORTOISESHELL MALE CATS
Ruth C Bamber (Mrs Bisbee) and E Catherine Herdman.
Journal of Genetics , Volume 24, Issue 3 , pp 355-357, 1931

A REPORT ON THE PROGENY OF A TORTOISESHELL MALE CAT, TOGETHER WITH A DISCUSSION OF HIS GAMETIC CONSTITUTION.
Ruth C Bamber (Mrs Bisbee) and E Catherine Herdman.
Journal of Genetics , Volume 26, Issue 1 , pp 115-128, 1932

Bamber and Herdman noted that they included tortoiseshell with and without white and also tabby-tortoiseshell with and without white. They summarised previous papers on the subject noting the expected and anomalous offspring from matings between black, orange and tortoiseshell cats. The anomalous cases include appearance of tortoiseshell males, yellow male siring a black daughter, a yellow female that bred as a tortoiseshell and also a yellow x yellow mating that gave 3 tortoiseshell females. There was also a record of an “anomalous” yellow female from a yellow male x yellow female cross – the “anomalous” female had a small black spot that she transmitted, along with yellow, to her own male offspring.

Bamber and Herdman noted that tortoiseshell males were most often sterile. Samson, owned by Sir Claud Alexander, was fertile, but the cat fancy records of his progeny were too incomplete [only gave his registered offspring] to deduce his gametic constitution. [Claud Alexander had something rather odd going on in the Ballochmyle shorthair cats – a number of tortie males, some very well brindled, occurred so they can’t all be dismissed as misidentified blotched golden tabbies. He and his wife were experienced cat breeders well able to distinguish between different colours. Perhaps he had an unusual mutation in his shorthair breeding line.]

Another fertile tortoiseshell male, Lucifer, was owned by Bamber and Herdman since 1926 and was the only male available for purely scientific breeding experiments. He died in 1931 before those breeding experiments were complete. He was previously owned by Mrs Langdale and had sired 2 kittens “Eve Lucie” and “Rose Lucie” - in a controlled mating. All subsequent offspring were sired in experimental matings.

Lucifer was tabby-tortoiseshell and white, tabby [black-brown tabby] being gametically black. He was mated to black, yellow and tortoiseshell females and sired 56 offspring. Lucifer bred exactly like a yellow male and his daughters, mated to unrelated cats, bred normally as did their progeny [Lucifer appears to have been a chimera and his testes had formed from a yellow male embryo.] Some of his male offspring were chloroformed at birth [no doubt superfluous to the experiment apart from recording their colours] and others died before maturity. One of his surviving sons was chloroformed at 2 years old; he was externally male with undescended testes and showed no sexual behaviour. Other tom cats treated him simply as a kitten and did not resent his presence even when females were in season. On dissection his testes were found to be represented by two rounded bodies, very unlike testes in appearance, connected to the vasa deferentia and situated in the abdominal cavity. [Indicates he was intersex with ovotestes]

In 1932, Bamber suggested that "in the formation of gametes of this tortoiseshell female [Lucifer's mother], part of an X-Chromosome, carrying black, failed to separate from the yellow-carrying X-Chromosome" - i.e. Lucifer arose by partial non-disjunction in his mother. Lucifer's mother had, in their hypothesis, inherited part of the factor for black on the same X chromosome tht carried yellow and had transmitted this aberrant gamete to her son.

Bamber’s list of Male Tortoiseshells

• Samson. Owned by Sir Claud Alexander. Mated repeatedly and sired many kittens. Fertile. Recorded by Doncaster 1904, 1913, Cutler and Doncaster 1915.
• Unnamed. Owned by Sir Claud Alexander. At time of study had mated only once and sired 2 kittens. Fertile.
• Lucifer. Owned by Bamber and Herdman. Mated repeatedly and sired many kittens. Fertile [bred as an orange male]. Recorded by Bamber and Herdman 1927, 1928.
• Bachelor. Owned by Sir Claud Alexander. Mated repeatedly with no result. On one occasion one kitten was produced but since the female had not been kept in confinement the paternity was doubtful. Certainly almost sterile. Reported by Cutler and Doncaster 1915.
• Benedict. Owned by Sir Claud Alexander. Mated repeatedly without result. Sterile. Reported by Cutler and Doncaster 1915.
• Tom Noddy. Owned by Sir Claud Alexander. Owned by Sir Claud Alexander.
• William. Owned by Doncaster. Mated repeatedly without result. Sterile. Recorded by Doncaster 1914, Cutler and Doncaster 1915, Bamber 1922.
• Torchlight. Owned by Mrs Langdale. Mated repeatedly without result. Sterile.
• King Saul. Owned by Mrs Herring. Bamber reported “Was reported to have sired kittens, but no definite records” [Two of his progeny are recorded in studbook records] [Fertile, breed as a black male] Reported by Cutler and Doncaster 1915.
• Solomon. Owned by Sir Claud Alexander. Was reported to have sired kittens in his youth, but no definite records. As an old cat was too savage to mate. [Fertile?] Reported by Cutler and Doncaster 1915.
• Unnamed. Owned by Sir Claud Alexander. Savage with females and will not mate.
• Tom. Owned by Bamber and Herdman. Died unmated at about 10 months old.
• Neptune. Owned by Sir Claud Alexander. Died unmated. Reported by Cutler and Doncaster 1915.
• Unnamed. Owned by Sir Claud Alexander. Died unmated at about 9 months old.

The origin of the tortoiseshell male cat is an old puzzle of genetics. Bamber (1927) stated: "The normal mode of inheritance of black, yellow and tortoiseshell is only very imperfectly understood. - That black is sex-linked is widely accepted, but has never actually been proved. Either yellow or black or both are certainly sex-linked.” Komai in papers in 1952 and 1956, and Komai and Ishihara in 1956, had shown that the gene for orange (yellow) color was sex-linked, whereas the gene for black or tabby color was autosomal. Searle (1949) had made a similar census in London, and had reached the same conclusion. The effect of the gene for orange, in the homozygous or hemizygous state, was perfectly epistatic to that of the gene for black or tabby, and the orange color completely covered the black or tabby color. However, it is incompletely epistatic in the heterozygous state, resulting in the tortoiseshell coat.

[Hemizygous means a gene where only one copy is present instead of a pair. Genes on the X chromosome in male cats are expressed regardless of whether they are dominant or recessive because there is no corresponding gene on the Y chromosome.]

This was upheld by the results of censuses of cat coat colors and sexes (Searle 1949, Komai 1952, Suzuki 1953). In orange cats there were always many more males than females among these cats, but no such disparity in black or tabby cats. Darwin (1868) had stated, "The peculiar colour called tortoiseshell is very rarely seen in a male cat, the males of this variety being of a rusty tint." Doncaster (1904) had said "Orange females are very rare, although males are common." [orange females are less common than orange males, but are not rare]

Komai stated that gene which is allelic to the sex-linked gene for orange (O) is the one for non-orange or wild-type of orange (o+), and not the gene for black (b) or for tabby (b+) which is located on one of the autosomes. Komai found it striking that this plain fact had escaped notice of recent geneticists including Sprague and Stormont (1956) as that basic fact was essential for understanding the origin of tortoiseshell males.

Origin Of Tortoiseshell Males

Komai had previously stated that the mothers of all tortoiseshell male cats were tortoiseshells. Records of the births of 65 tortoiseshell males from various parts of Japan proved this untrue. They clearly showed that the mother of a tortoiseshell male could be of any color. That implied that she carried either O or o+ in her X-chromosome and either b or b+ in one of her autosomes. But, the sex-linked gene possessed by her and that possessed by her mate should be in a heterozygous relation 0/o+. Komai deduced that the tortoiseshell male was heterozygous for O and o+, much like the female tortoiseshell.

Tortoiseshell males should be hemizygotic, but heterozygous for O. He assumed that the Y-chromosome in such exceptional males carried O or o+ due to crossing-over between X and Y chromosomes in a germ cell of the father of the cat which would transfer the O or o+ from X to Y. By 1957, the X and Y chromosomes were known to be differentiated into pairing and non-differential segments, and chiasma [joins] could be formed between the pairing segments of the two chromosomes. Komai considered it likely that crossing over took place between X and Y. During that process, the sex-linked gene O or o+ might be transferred from X to Y and a resulting male offspring would get the gene in question. Komai considered this accounted for the occasional heterozygous male.

Regarding sterility in tortoiseshell males, Komai assumed the presence of a gene(s) governing male fertility was on the Y chromosome. If it transferred from X to X chromosome during crossing over then it would be absent from the male offspring getting that Y chromosome. He admitted that this might be revealed to be wrong by future studies. Komai found three specimens of fertile tortoiseshell males. Two were examined by Ishihara and their testes were perfectly normal and apparently fertile. No breeding data was been available for these cats. The third specimen had proven to be fertile. He assumed that the crossing-over between X and Y had failed to remove from Y the gene complex that governed male fertility, even though it did transfer O from Y to X. If so, a fertile tortoiseshell son should produce further tortoiseshell males. Scientific literature recorded three fertile tortoiseshell males. One had no breeding record. Another, reported by Cutler and Doncaster (1915, 1932) might have originated through Komai’s crossing-over hypothesis. The third was “Lucifer” (Bamber and Herdman, 1932) who was a genetic orange despite of its tortoiseshell appearance, but the covering effect of the orange gene had apparently become incomplete by the presence of a modifying gene or by some other cause.

It was difficult to present any cytological evidence for the crossing-over hypothesis because the sterile testes contained few mitotic or meiotic figures. Komai hoped that a tissue culture of somatic tissue would provide mitotic figures. In the 1950s it was hard to detect any structural change in the Y-chromosme. Komai hoped to identify and study tortoiseshell males born of the same mother, but with different fathers.

Of previous hypotheses, Komai noted that the freemartin hypothesis had been revived by Sprague and Stormont (1956). This postulated that female embryos could render male embryos intersex or infertile in utero [seen in cattle where a female twin causes feminisation in her male twin]. The gonads of sterile tortoiseshell males showed no sign of intersex/feminisation.

Komai concluded:

• Clear evidence for the sex-linkage of the gene for orange color and the non-sex-linkage of the gene for black or tabby color.
• The mother of a tortoiseshell male could be of any color and such males were heterozygous for orange.
• Crossing-over between the pairing segments of X- and Y-chromosomes in the father ws probably responsible for the production of a tortoiseshell male.
• There was probably caused by a transfer of gene(s) for male fertility of the male from Y to X by the same crossing-over process.
• Tortoiseshell males were not intersex.
• Some fertile tortoiseshell males differ in origin from sterile tortoiseshell males by being genetic oranges in which the covering effect of the gene for orange is imperfect and allows the black to appear partially.

References included:

Hayes, F. A. 1923. The tortoiseshell cat. J. Hered. 14, 369-376.
Komai, T. 1946. On the inheritance of black, yellow and tortoiseshell colour in cats, and the problem of the tortoiseshell male. Proc. Jap. Acad. 22, 265-268.
Komai, T. 1946. On the inheritance of black, yellow and tortoiseshell colour in cats, with special reference to the problem of the tortoiseshell male. (in Japanese). Seibutu 1, 1-7.
Komai, T. 1947. A new hypothesis on the origin of tortoiseshell male cat. Mem. Coll. Sci. Kyoto Univ. ser. B, 19, 17-21.
Ohshima, H. and Oyama, J. 1925. On a tortoiseshell male cat with odd eyes. (in Japanese). Toyogakugei-zasshi 507, 9-18.
Searle, A. G. 1949. Gene frequencies in London's cats. J. Genet. 49, 214-220.
Tjebbes, K. and Wriedt, C. 1926. Dominant black in cats and its bearing on the question of the tortoiseshell males. J. genet. 17, 207-209.

Jude and Searle wrote that in many animals, sex was determined by a difference in one chromosome pair. The females having 2 similar X-chromosomes while the males one X- and one Y- chromosome. Whereas similar chromo¬somes paired along the whole of their length, the Y- pairs only partially with the X-chromosome. Sex-linked genes were those carried by the X-chromosome, e.g. the gene for yellow coat colour in the domestic cat. The yellow gene had a black counterpart (alle¬lomorph) meaning that at a given point on an X-chromosome there may be found either the yellow coat colour gene or the black coat colour gene. A female could inherit one black and one yellow gene in which case it would be tortoise¬shell colour.
According to these simple ideas of sex determination in mammals, it seemed that nature prohibited tortoiseshell males, for the male cat with its single X-chromosome could carry black or yellow, but not both. In defiance of theory, male tortoise¬shell cats turn up on rare occasions, but are usually sterile. So far no one had been able to account satisfactorily for the occurrence or the sterility of those tomcats.
An even rarer phenomenon, the fertile tortoiseshell tomcat, was described by them in Nature magazine.

A FERTILE TORTOISESHELL TOMCAT – A.C. Jude and A.G. Searle 1957 (Nature)

Jude and Searle wrote that the gene for yellow coat-colour in the domestic cat was sex-linked; tortoiseshell cats were heterozygous for yellow and therefore normally female ; the rare tortoiseshell male is usually being sterile. However, a tortoiseshell male of known parentage had produced in the space of two years at least sixty-five offspring. 26 were recorded as female, 17 as male and 22 were unsexed. The kittens were the result of systematic matings carried out in confinement, so their paternity was not in doubt. The male concerned was “Blue Boy”, a dilute tortoiseshell English rex ; his father was ‘Kalli’, dilute yellow and the first English rex cat. His mother was ‘Serena’, a tortoiseshell heterozygous for rex and for dilute. Because the cats concerned were being used for establishing the rex breed, the results of matings were carefully recorded even before Blue Boy’s true nature was realized.

Eleven further tortoiseshell males were recorded among Blue Boy’s forty-three sexed offspring. Three of theoe males had reached matur¬ity by 1957, but none had yet been proved fertile. Jude and Searle knew of no previous records of tortoiseshell males siring further tortoiseshell males ; one of the tortoiseshell males studied by Doncaster had produced tortoiseshell off¬spring, but their sex was apparently unrecorded.

Blue Boy was mated to 6 different tortoiseshell females, all related to him. The offspring included in each sex all three possible genotypes at the yellow locus ; that is, males and females which were phenotypically (i) black, blue or tabby, (ii) tor¬toiseshell or dilute tortoiseshell (blue-cream), (iii) yellow or cream. They thought most likely hypothesis was that in Blue Boy the yellow gene was only partially sex-linked instead of fully sex-linked. It would have to be present in the pairing segment of the Y-chromosome, instead of being absent from Y.

Using the notation Y= Ychromosome, X= X chromosome, y= yellow and yb=black, they thought Blue Boy was ybX/yY.

The fact that Blue Boy’s father was a yellow male made it probable that the y gene would be on the Y-chromosome rather than on the X. Mated to a tortoiseshell female, ybX/yX, a fertile ybX/yY tortoiseshell male could produce yellow males, tortoiseshell males and females, and black females as non-recom¬binant types ; and black males, tortoiseshell males and females, and yellow females as recombinant types. The production of all three genotypes in both sexes would be theoretically possible, which agreed with observation. About half the offspring from that type of mating would be tortoiseshells ; in fact, 14 out of 25, in fully sexed litters were tortoiseshell. The results of mating Blue Boy with yellow and with black females also agreed with their hypothesis of partial sex-linkage of yellow. Taken as a whole, the results suggested that linkage between the yellow locus and the sex-determining gene complex was fairly loose.

Various predictions could be made on the basis of their hypothesis ; for example, that those of Blue Boy’s sons which are yellow or black with regard to the y-locus should also be capable of producing tortoiseshell males.