For a long time, researchers believed that black, rather than yellow (orange, red) was sex-linked. The problems in following the scientific literature was compounded by authors not using standard symbols, or worse, by authors Using the same symbols to mean completely opposite (and sometimes counter-intuitive) things. Because textbook authors tended to follow each other rather than keeping pace with scientific advances, the misunderstanding of sex-linkage of yellow and/or black was passed on to students.

I recently found a test answers paper from Union County Vocational-Technical Schools, New Jersey that still get it wrong. It stated that calico (tortoiseshell and white) is due to a sex-linked co-dominant allele and used the symbols B for black, R for orange and BR for calico (XB, XR and XBXR). Yellow is epistatic to black. The corresponding allele to yellow is “non-yellow” (or wild-colour). Black is a different gene, is not on the X chromosome and the alleles of black are “chocolate” and “cinnamon.”

1916. IBSEN, H.L. Tricolour Inheritance III. Tortoiseshell Cats. Ibsen posited 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. He denoted orange as b. (T is now used in notation of tabby patterns)

1922. SIRKS, M.J. “Handbook der algemeene Erfelijkheidsleer.” Martinus Nijhoff, The Hague. 1922. p. 256, also in later editions) considers black and yellow to be sex-linked but not allelic. He gives the symbols B and Y.

1927. BAMBER, R. “Genetics of Domestic Cats.” Bibliographia Genetica 3:1-86, 1927. P. 6: Bamber’s theory and symbols were Y for yellow and y for “extension of black.” (also BAMBER, R. C. and E. C. HERDMAN 1931. Jour. Genet. 24: 353-357; BAMBER, R. C. and E. C. HERDMAN 1932. Jour. Genet. 26: 115-128.)

1935. SNYDER, L.H. “The Principles of Heredity”. D.C. Heath, Boston. p. 68 “In cats, a factor for yellow is allelomorphic to one for black, and they are carried on the X-chromosome.”

1946. SNYDER, L.H. “The Principles of Heredity,” 2nd Edn., D.C. Heath, Boston. P. 91 assigns the symbols B to yellow, b to black, and Bb to tortoiseshell. (Fifth ed. with P. R. DAVID. 1957.)

1950. CROW, J. F. “Genetics Notes,” Burgess, Minneapolis. 1950. p. 17 says “In cats the genotype BB is black. Bb is tortoiseshell, and bb is yellow. The gene is on the X-chromosome,” but he revised this in the 7th (1976) Edition p. 34 “tortoiseshell cats [. . . ] are heterozygous for an orange colour gene. The orange spots contain cells where the dark colour gene is inactivated, and the dark parts have the orange gene inactivated.” (Lyon hypothesis).

1952. SRB, A.M. and R.D. OWEN. “General Genetics.” W.H. Freeman, San Francisco. P. 97 use the symbol Y for black and y for yellow, with Yy being the tortoiseshell female. They base their theory on Bamber (1927) but Bamber’s symbols were the other way round: Y for yellow and y for “extension of black.”

tortie tomcats

1956. DODSON, E.O. “Genetics.” W.B. Saunders and Co., Philadelphia, 1956. p.81.. “In cats, the gene B subserves yellow coat colour, while its allele b subserves black.” Bb resulted in tortoiseshell. This accords with SNYDER (1946) rather than with CROW (1950).

1957. KOMAI, T. “Supplementary Notes on the Genetics of Tortoiseshell Male Cat,” Jour. Fac. Sci, Hokkaido Univ. 'Ser. VI, Zool. 13, 1957. “The gene which is allelic to the sex-linked gene for orange (0) is the one for non-orange or wild-type of orange (0+), and not the gene for black (b) or tabby (b+) which is located on one of the autosomes. It is rather striking that this plain fact has escaped notice of some recent geneticists including Sprague and Stormont (1956).”

1959. SEARLE, A.G. “A Study of Variation in Singapore Cats.” “In Eastern Asia, the gene for yellow fur-colour is three to four times as common as in London; in Japan this has meant that the tortoiseshell male occurs often enough to be the object of certain superstitions.” Based on his work in 1949, he uses “+” for black, and y for sex-linked yellow. “Tortoiseshells are +/y and therefore normally female.” However, he also uses a for black in one table, having previously defined it as denoting non-agouti (agouti being +).

1962. KING, R.C. “Genetics.” Oxford Univ. Press, N.Y. 1962. p. 110. Black and yellow are sex-linked alleles. His second edition (1967), and his “A Dictionary of Genetics” (Oxford Univ. Press, N.Y. 1967 and 1974) both follow CROW in using BB for black. Bb for tortoiseshell, and bb for yellow.

1967. SINGLETON, W.R. “Elementary genetics,” Van Nostrand, Princeton, N.J., 1962 and 1967. p.221 Cats have a sex-linked “gene pair causing black or yellow hairs.” P. 558 “The gene for black and yellow is sex-linked.” He uses B for black, Y for Yellow. A yellow male is shown as Y- (i.e. no gene on the Y-chromosome). Using Y in this way is confusing as it is both a chromosome and a gene symbol.

1964. HUTT, F. B. “Animal Genetics,” Ronald Press, NY. 1964. p. 143 follows CROW in using BB for black. Bb for tortoiseshell, and bb for yellow. “Black and yellow coats in cats are determined by the sex-linked alleles B and b.” Second Edition (HUTT, F. B. and B. A. RASMUSEN, “Animal genetics” 2nd Edition, John Wiley, NY. 1982) refers to ROBINSON and changes the symbols to o and O (non-orange, orange respectively).

1968. STRICKBERGER, M. W. “Genetics,” Macmillan, NY. 1968. p. 478 followed SRB and OWEN using Y for the black allele and y for the yellow allele. On p. 707 Strickberger reviews a gene-frequency report by TODD and says “In the heterogametic XY males. The presence of the yellow gene produced yellow fur, while its normal allele (y+ or simple +) produces a darker colour whose particular shade and pattern depends on the presence of other genes.” This is repeated in later editions (3rd Ed. 1985). It causes confusion because Strickberger refers to Y, y and y+ i.e. 3 alleles!

1968. COMMITTEE ON STANDARDIZED GENETIC NOMENCLATURE FOR CATS. Standardized genetic nomenclature for the domestic cat. J. Hered. 59:39-40. 1968. This gave a standard set of symbols, thus removing the confusion of B, b, Y, y and y+ in papers and text books. However many authors continued to use the older symbols copied from pervious authors.

1975. MERRELL. D. J. “An Introduction to Genetics,” W. W. Norton. NY. 1975. p.110 follows Plate in designating b for black and B for yellow.

1980. WAGNER, R. P., B. H. JUDD, B. G. SANDERS, and R. H. RICHARDSON. “introduction to Modern Genetics,” John Wiley, NY. 1980. p. 112 refer back to CROW’s earlier edition and say: “a pair of alleles, B and b, appear to be sex-linked in cats. In females BB gives black coat colour, homozygous bb gives yellow, while the ‘hybrid’ animal (Bb) is the familiar ‘tortoiseshell’.”

1985. SNYDER, L. A., D. FREIFELDER. and D. L. HARTL. General Genetics. Jones and Bartlett, Boston. 1985. p. 59 says: “The black and yellow pigments in the coats of cats are determined by an X-linked pair of alleles. Males are either black or yellow." They use novel gene symbols: cb and cy. (cb is the recognised symbol for Burmese colour dilution).

1986. MILLER, W.J. and W.F. HOLLANDER, “The Sex Linked Black Cat Fallacy: A Textbook Case,” Journ. Heredity 77: 463-464, 1986. This highlighted the confusing use of symbols and noted that authors of teaching textbooks tended to follow each other rather than referring to technical literature. They believed that the source of the fallacy that black (not yellow) was sex-linked to a misunderstanding in BAMBER’s (1927) study. Bamber used the same terms and symbols as LITTLE (1912) and DONCASTER (1913). Little did not mention “wild type.” Doncaster wrote that black “corresponds with ‘normality’." Bamber recognized that self-black i.e. “non-ticked" was a simple autosomal (non-sex-linked) recessive to tabby (wild-type colour). This was designated “a”, the generally used symbol for recessive black (nonagouti) in mammalian genetics. “Tabby” or “tiger-striped” meant the phenotype of small wild cats i.e. the “wild type.” Both black and yellow (orange) were distinct and independent mutations. Combining non-agouti “aa” with yellow, hemizygous or homozygous, looked the same as yellow on its own due to masking or epistasis (ROBINSON, SEARLE).

They suggested that textbook authors could have avoided misunderstanding by using the term “mutant” in describing sex-linkage and the tortoiseshell coat of the heterozygote e.g. “The yellow or orange mutant coat-colour type in cats is sex-linked. The modern symbol is O (Committee on standardized genetic nomenclature for cats, (Standardized genetic nomenclature for the domestic cat. J. Hered. 59:39-40. 1968). The heterozygous female, O/o+, shows the tortoiseshell effect, a yellow (O) and non-yellow (o+) patchwork.” There was a failure to identify which genes were mutants and which were standard/normal/wild type.

They noted that most textbooks paid only lip-service to the usefulness of the “+” symbol to denote wild type and that because textbook publishing is a lucrative business, deviating from dogma was unwelcome (MILLER, W. J. “Appropriate Gene Symbols in Teaching Genetics,” Proc. Iowa Acad. Sci. 92:115-118, 1985) hence fallacies are passed to yet another generation.

Sadly, as the test answers paper from Union County Vocational-Technical Schools, New Jersey demonstrates, schools genetics classes are still not teaching the relationship between black and yellow correctly!