CORIN GENE: "LIGHT GOLD" ("COPPER") PHENOTYPE IN BRITISH TIPPED SHORTHAIRS/LONGHAIRS
Copyright 2016-2022, Sarah Hartwell

In the early 21st century a new type of “golden” appeared in Tipped and Shaded Golden (Golden Chinchilla and Golden Shell) British Shorthairs. It looked like an "ultra wide-band" effect where the colour was pushed so far to the tips of the hairs that the genetic colour (black, blue, chocolate etc) was barely discernible and the paler areas were almost white. The term "copper" (now known as Light Gold by WCF) was adopted by Linda Petersson Wahlqvist to describe Golden Shell cats with white toes, white undersides, golden colour with the dark tips (e.g. black or blue) restricted to the tail tip and an absence of dark nose-liner. These phenotype deviates from the standard for tipped cats but became widespread in Russia and in some German lines of British Golden Shorthairs. Breeders sought recognition for this colour as "Light Gold".

The new goldens (and related new silvers) British Shorthairs/Longhairs that appeared in Europe were not ny11 (tabby-based goldens), but were ny25 (ticked-based goldens) and attracted a lot of interest in Russia. In adulthood the ticked-based goldens and silvers had extremely uniform tipping with no hint of a tabby pattern. Breeders, particularly in Russia, continued to select the brightest gold chinchillas to breed with and the brightest, most orange, golden cats had pale toes and pale markings around the eyes as well as an ivory belly. Some cats were pale yellowish gold with nearly-white toes. The bleached (or “excessive”) version was eventually termed "light gold" and fans of the colour saw it as a natural evolution of colour to create the perfect golden cat. Confusingly, the light gold (colour of set honey), which is different from the counter-shaded reddish copper phenotype, is also called British Sunshine but is not the same as the Siberian sunshine mutation.

From 1st March 2021, associations affiliated with WCF had to segregate the standard golden kittens from the light gold (copper, extreme golden) kittens which have pale bellies and white feet. Different coding on the pedigree keeps the new colour separate from the original golden colour so breeders can decide whether or not to use a light gold cat in their golden programmes. Until “light gold” is officially recognised with its own code, these cats can’t compete in the golden class against classic goldens.

NEW CORIN MUTATION IDENTIFIED IN BRITISH SHORTHAIR/LONGHAIR “LIGHT GOLD/COPPER” CATS

This section is a summary of:
Abitbol, M., Dargar, T. & Gache, V. (2022) Golden cats: A never-ending story!. Animal Genetics, 00, 1–4.

The light-gold, akita or copper phenotype was reported by breeders during the 2010s. This modification restricted eumelanin to the tip of the tail and hairs showed a wideband modification. Pedigree analyses showed an autosomal recessive inheritance pattern. The CORIN gene (Corin, serine peptidase) was identified as the strongest candidate, since two CORIN variants had already been identified in Siberian cats with a golden phenotype. A homozygous CORIN:c.2425C>T was identified in copper British cats. All 30 copper cats tested were homozygous for the variant. 20 British control cats were found to be carriers. 340 cats from the 99 Lives dataset did not carry the mutation. 218 cats from 12 breeds also did not have the mutation. CORIN:c.2425C>T is a third variant and represents the wbBSH (British recessive wideband) mutation. A fourth variant has been identified in Bengals, but not published at the time of the article.

The currently known variants of the CORIN gene are:
CORIN:C.839G>A - extreme-sunshine Siberian cats.
CORIN:C.2383C>T - sunshine Siberian cats.
CORIN:C.2425C>T – copper/light gold/akita British Shorthair/Longhair cats.
CORIN:C.1759C>T - golden tiger.

BACKGROUND TO THE COPPER/LIGHT GOLD CATS

Tipped and Shaded Golden British Shorthairs (Golden Chinchilla and Golden Shell) are related to Silver series cats. The term "copper" (called Light Gold by WCF) was adopted by Linda Petersson Wahlqvist to describe an extreme phenotype of Tipped Golden with white toes, ivory undersides, golden colour with the dark tips (e.g. black or blue) restricted to the tail tip, and an absence of dark nose-liner. This deviates from the standard for classic tipped golden cats, but became prevalent in Russia and in some German lines of British Golden Shorthairs. Silver chinchillas have the same colour distribution, but we do not see it because "silver" is the absence of pigment! Gold chinchillas from completely different lines had the same effect of "whitening" the breast and other parts. To further split hairs, copper phenotype is reddish with ivory undersides, and there is also a light gold colour that is the colour of set honey, also with paler underside.

Shaded and tipped (shell/chinchilla) are due to the dominant Inhibitor gene (silver) + Wide Band + Ticked Tabby. Although "Wb" is treated as a single gene for simplicity, there now appear to be different genes in different breeds and there may be other modifiers at play. In ticked tabby cats without silver, Wide Band restricts the pigment to the end of the hair leaving a pale golden band between the skin and the pigment. Through selective breeding, this effect has been maximised. One opinion is that "classic Golden Chinchilla" without the "milky-white" chest is really a shaded colour (not a tipped colour) where black pigment at the hair tips is maximally transformed into red-brown. Unlike the classic golden phenotype, the copper and light gold phenotypes have a harp dividing line between the reddish colour and the milky-white underparts. This is why Light Gold is believed to be the same mechanism as extreme Chinchilla (where reddish pigment is completely suppressed).

Alongside the Copper and Light Gold phenotypes there are chinchillas where the cats looked white except for the eye colour and paw pads. This shows that the Ultra Wide-Banding effect operates on cats with and without silver. Silver cats without Wide Band are cool-toned ticked tabbies. Golden (ticked) kittens can be born very dark, but become lighter as they mature.

In tipped cats there has been selection for an increase in depigmentation, thus increased/longer depigmented bands. In Light Gold the increased depigmentation is milky or peach-gold. The golden zone "overlaps" unnecessary ticking on the hair. Tipping with ultra wide-band is "hidden ticking" - all the colour zones, except for the hair-tip, are masked, although the genes determining them are still present. To get a Copper/Light Gold also requires full activity of theoretical recessive polygenes to remove pattern i.e. "chaos" and "band frequency" genes. For convenience, we'll consider "chaos" + "band frequency" to be a single trait designated "U". Copper and Light Gold would the theoretical genetic formula A-UUWbBSHWbBSH (Agouti + homozygous U + homozygous CORIN mutation).

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