AMBER AND RUSSET - LATE COLOUR CHANGE GENES
The ancestors of the domestic cat were nondescript black/brown striped tabbies. Over the centuries, mutation produced a wide array of colours based on 2 different pigments. Eumelanin gives the blacks, browns and blues while phaeomelanin gives the reds, fawns and creams. A few other genes give further variations on those colours such silvers, colourpoints and solids/selfs. Mutations continue to occur and unexpected colours also turn up due to inbreeding where recessive genes, hidden for generations, start showing up.
AMBER AND LIGHT AMBER
A "late colour change" mutation, again causing an end result of golden, has been observed in Norwegian Forest Cats. The Black Modifier gene, found in Norwegian Forest Cats, brightens black or blue areas of the coat to Amber (apricot-to-cinnamon colour) and Light Amber (pale beige). At birth, kittens appeared to be black or blue, and lightened to Amber or Light Amber respectively. Amber has also occurred in conjunction with silver: the kittens were born as poorly coloured black-silver or blue-silver tabbies whose tabby ghost-markings faded as they matured and their colour became a bright apricot to cinnamon colour with dark brown paw pads and nose leather with no black rim (the black rim is characteristic of silvers). Their original birth colour could be seen only on the back and tail.
During the 1990s, some purebred Norwegian Forest Cats in Sweden produced chocolate/lilac and cinnamon/fawn offspring. However, those colours are not found in the purebred Norwegian Forest Cat gene pool. Had the gene pool become polluted by someone, perhaps generations ago, breeding their Norwegian Forest Cat to another breed? Was it a spontaneous mutation? Crossing of those cats with known chocolate and cinnamon colour cats of other breeds ruled out chocolate/lilac and cinnamon/fawn genes. These cats were a totally new colour, peculiar to the Norwegian Forest Cat gene pool and dubbed the "X Colours". They are now called Amber and Light Amber. The Amber effect is due to the extension gene (also called red factor) which controls the production of red and black pigment. The dominant version of the gene produces normal black pigment in the coat while the recessive version produces red pigment. The name comes from the effect of black or brown pigment not being extended throughout the whole coat, but being restricted to the skin of the extremities and to the eyes (for example in bay horses).
This Norwegian Forest Cat was bred by Yve Hamilton Bruce from a silver mackerel tabby female (imported from Denmark) and a classic red tabby and white male. The result was 1 silver tabbies and 2 silver tabbies with white. At just over 3 months old, this silver and white tabby male developed a large patch of bright red hair on his back which continued to spread. Eventually the whole fur will become amber. The effect of amber during the colour-change stage depends on the original colour - solid black or blue, bicolour or tabby. The cat pictured is not a typical amber as it has the silver gene so the amber effect is overlaid on silver.
A non-agouti amber Norwegian forest Cat resembles a silver tabby, but has a distinctive black nose and black paw-pads instead of the pink/reddish nose with black outlining found on conventional silver tabbies. The photo below shows a non-agouti amber. According to Rui Pacheco, although this cat is clearly non-agouti because of the black nose, he has the phenotype of a silver shaded. Rui's theory is that the amber gene invalidates the action on the non-agouti gene; wide-banding will further lighten the base colour giving a very pale cat with black nose and paws. Amber cats are basically black cats with a colour modifier that affects the deposition of pigment on the hair shaft, but not the skin colour. That is why in non-agouti cats the nose remains black and in agouti cats it remains pink.
The platinum effect is found in some lines of Silver Persian/Chinchilla Longhair. In 1986/7, Cheryl Bennett reported in The Silver and Golden Persian Newsletter (ACFA) that one of her shaded silver females (Kelley Lane Contessa of WeANDE) had changed from pure silver to pale golden. Contessa’s parents were a shaded golden and a shaded silver; these were full siblings. At 10 – 12 months of age, Contessa began to “tarnish” i.e. show cream/reddish patches and by 3 years of age she was entirely pale golden. She produced a number of Silver Persians without tarnish, but she failed to produce any offspring when mated to a golden sire. One of Contessa’s male offspring turned from silver to golden as an adult. An adult Silver Persian from England (Lynchard Silver Shadow) was exported to Australia and also turned golden. Shadow had a few golden hairs on his paw, but did not turn golden until he was a year old when his coat turned to pale beige. By 3 years old he was entirely pale golden. Shadow was bred to a genetically golden female, but the pairing only produced silver offspring. However, at least one of his silver offspring later turned pale golden. Other descendents of Contessa also went through the late colour change. During the 1980s, several other breeders of Shaded Silver and Chinchilla Persians came forward to report that their cats had developed reddish, brownish or golden-coloured fur along their spines as they aged. Many of the cats had no golden in their ancestry. At first it was dismissed as an unavoidable genetic fault where silver was incompletely dominant and did not hide the recessive golden colour.
A common ancestor of all the colour-changing Silver Persians was a stud cat called Kelly Lane Andromeda (in the UK) whose descendents were exported to the USA in the 1950s and 1960s. Some of those descendents were influential stud cats and would have spread the mutant gene far and wide. Controlled inbreeding (linebreeding) helped establish the gene, which would later double up to produce colour-changing Silver Persians. The effect of this gene on Golden Persians, if indeed it has any effect, is not known.
Except for red-silvers and cream-silvers, silver cats should entirely lack phaeomelanin (red or cream pigment) and only have eumelanin (black, brown, blue etc) present. Perhaps some other gene causes the eumelanin structure to change so that it is perceived as a golden colour. Chemical analysis proved that the late colour change Silver Persians did not have phaeomelanin pigment present.
AMBER-TYPE EFFECT IN MANX
An amber-type effect has also been seen in a Manx cat. Janet McArthur bred a male Manx (a longy i.e. with tail) that was born an ordinary black/brown mackerel tabby without much rufousing, as was his sister. In addition, the male had white ticking at the tips of his hair. As the male grew, his stripes faded out and became spots while the black markings also changed from black to a burnt red-brown colour and his white ticking became smaller. The change from stripe to spots was probably an optical effect caused by the change from black to red-brown. His dorsal line became a burnt golden brown, and the background colour was similar to a washed out sorrel colour (sorrel is a colour found in Abyssinians).
There are a few oddities in his ancestry. His mother, Pearl, was a black Manx and his sire was a normal black/brown tabby from the same black Manx mother. The inbreeding was accidental, and may have brought out a recessive gene. Pearl was born to a black/red tortie (carrying dilute and colourpoint) and a red van-pattern sire; this pairing should not produce solid black females so, barring an anomaly in her sire's germ line, Pearl may be a tortie where the red has not been expressed in the coat. Pearl has also produced a kitten that changed from black with white ticking at the hair-tips, to black smoke and then to solid black without ticking.
BIMETALLIC / SUNSHINE
The informal term "bimetallic" (referring to a mix of silver and golden or silver-gilt) describes some Siberian silver tabbies that turn golden in a rather patchy fashion. Unlike the amber gene, the colour change does not start on the back and work downwards, but seems distributed throughout the coat. The formal name "sunshine" has been proposed for this emerging gene.
Over the years, there have been accounts of pure silver Persian kittens turning to a pale golden colour as they mature. This often began with yellowish "tarnish" or cream spotting appearing in the coat of a shaded silver or chinchilla cat. Reddish hairs first appeared on the spine, face or paws, and progressed through a yellowing of the undercoat. Chinchillas derived from chinchilla-to-chinchilla matings sometimes exhibited reddish or golden hair along their spines which was attributed to incomplete dominance of the golden gene "breaking through" in the coat. Some silver kittens were, when 2 or 3 years old, very definitely pale golden adults. Offspring born to such cats were born as silvers, showing the "golden" cat was genetically a silver, but some of those offspring also had a tendency to turn golden at about 2 or 3 years old.
Silver and chinchilla cats have been selectively bred to eliminate (as far as possible) polygenes that would modify the colour's purity. The silver should, therefore, be devoid of genes that cause cream, yellow and red pigment. Yellow denotes the technical term for the pigment granule which produces all of these colors. Polygenic complexes have plus and minus polygenes that influence the trait. Diligent breeding resulted in accumulations of either plus or minus polygenes in a specific breed or colour. However, these polygenes are carried on different chromosomes and inherited independently of each other.
In silvers, the inhibitor gene means the round granules of eumelanin (black) are absent from most of the hair shaft and are clumped near the tip. Being incompletely dominant, the effect ranges from shaded silver through to tipped (chinchilla). In normal goldens, the granules of the yellow pigment, phaeomelanin, are possibly influenced by the recessive form of the inhibitor gene and/or a wide band factor. However, the silvers that turn into goldens don't have any phaeomelanin. Instead, these "pseudo-goldens" turned out to have an additional mutation that caused their eumelanin granules to be elliptical rather than round and to show as a beige or pale golden colour. These mutant granules are not completely eliminated from the hair shaft, but are smeared along it, causing a pale golden undercoat. They are also clumped at the tip to give the shaded or tipped effect.
Eumelanin is usually described as "black", but is actually a deep brown (sepia) that appears black to our eyes. As the eumelanin granule becomes elongated, it appears paler and more reddish-brown or dark-yellowish-brown. The concentration of the mutant eumelanin granule on the hair shaft gives colours ranging from copper-brown through apricot to reddish honey, all with darker tips. The mutated eumelanin gene has been described as a "late colour change" gene for obvious reasons. Its mode of inheritance not fully understood. It is also possible that some apparently golden-from-birth Persians may be pseudo-goldens and can have silver kittens - generally considered an impossibility as silver is dominant over normal golden and cannot be carried as a recessive (though silver can reoccur as a spontaneous mutation).
Similar to amber is russet, which turned up in a line of seal (brown) European-style Burmese in New Zealand in 2007. It has subsequently occurred found in the related Mandalay (similar to the Asian in Europe) and appears to be a mutation of the extension gene. The first known russet was a pure-bred Burmese called “Molly” in 2007. There is now an experimental programme in NZ to breed Russet Burmese and to investigate dilute russet, russet tabby and solid russet (as opposed to the Burmese sepia form of russet).
The first russet kitten “Molly” was born an "odd-coloured lilac (lavender)" which gradually lightened as she grew, progressing through lilac-caramel and chocolate ticked tabby and then dramatically changing to red. Chocolate ticked tabby and red were both impossible from her pedigree, and in any case, reds are not born as chocolate tabbies! Several more unusually coloured kittens were born in different litters and all went through the same colour/pattern changes. The ancestors of these kittens were seal Burmese and had no silver, tabby or Mandalay (Asian) blood. The pedigree had both dilute and chocolate, there were no reds, creams or torties until 4 generations back. DNA testing showed some kittens to be genetically seal and other genetically chocolate. Hence the new colour is due to a different mutation currently known as russet. Russet appears to be due to a recessive mutation that causes black pigment (eumelanin) to gradually fade to a minimal amount while leaving red pigment (phaeomelanin)unaffected.
Russet kittens resemble tabbies at birth, but have pink noses and paw-pads, pale fur around the pads and genitalia and a pale tail-tip - all of which would be dark in tabbies. The muzzle and fur around the eye is ivory. The back is solidly dark rather than ticked, becoming pale ivory halfway down the flanks. The back becomes more ticked appearance, almost a saddle, as kittens undergo the colour change and the face becomes reddish. By age two, they may resemble a red Burmese. It differs from amber as ambers have dark noses and paw-pads. Non-agouti (solid) amber kittens are very dark with a dark face that is last to go red while russet kittens have off-white faces (possible due to Burmese sepia gene in the mix), which are the first part to go red (rather than the last as in ambers), and pale undersides. The russet colour change appears to be slower than the amber colour change. Russet kittens to date have been larger at birth than their siblings and somewhat on the large side as adults.