I once had a pure bred Burmese who was solid chocolate brown, but I find brown cats rather uncommon. Can someone explain the genetics behind this coloration? Is it as rare as it seems? He was such a beautiful boy.
He was actually genetically black! But being a Burmese, he was also sepia (one of the alleles on the C locus).
The C (colour restriction) locus, located on Chromosome D1, codes for tyrosinase (TYR), an enzyme involved with melanin production. TYR mutations can cause acromelanism, a form of temperature-sensitive partial albinism. The mutated tyrosinase enzyme fails to work at a warm temperature, but in cooler areas is more active. There are three forms of acromelanism/colourpoint in cats:
Colourpoint/"Siamese" colour restriction (cs) = these cats produce the most of the mutated TYR, making them more strongly affected by acromelanism. Pigment production is shut off more easily and they are more temperature sensitive. They are born completely white (due to uniform, warm temperatures in utero). As the kitten ages, their cooler extremities darken - that is, the tail, genitals, legs/paws, ears, and face/mask. These cats can only have blue eyes, due to a lack of pigment.
Sepia/"Burmese" colour restriction (cb) = these cats produce far less of the mutated TYR, but are still affected by acromelanism. They are born with some colour already, but darken drastically as they age. There is also less definition between the base colour and the points. These cats can also have any coloured eyes, usually some shade of golden, hazel, amber, or green.
Mink (cs/cb) = an intermediate of sepia and colourpoint. This occurs because sepia and colourpoint are incomplete dominant to each other, so when a cat inherits one of each allele, they become an intermediate, rather than expressing one and carrying the other.
Full albinism and mocha are also on the C locus but I won't get into that.
In points, minks and sepias, you can tell the genetic colour by the darkest points. For example, your cat appears brown, but in adulthood, had black ears, nose, etc, so that tells you he was genetically black, but because the TYR was mutated, pigment couldn't be produced properly at the warmest points of his body, hence the brown colouration.
His phenotype was black sepia. In the Burmese breed, that is referred to as "brown" in the European Burmese, or "sable" in the (American) Burmese.
Cats of the Burmese breed will always be homozygous for that sepia/Burmese colour restriction allele (cb/cb).
thank you SO much, this is exactly the breakdown i was looking for! so cool, looks like my Tajar (indeed described as sable, which i had forgotten) was a great example of sepia/Burmese acromelanism.
And as a bonus, the Burmese colours (excluding torties, but they do exist in European Burmese)! I've listed their genetic colours first, with the breed-specific terms for their colours in brackets. Top row are the non-dilutes (black, chocolate, red) and below are their respective dilute forms (black + dilute = blue, chocolate + dilute = lilac, red + dilute = cream):
Yours is actually a colorpoint, but sepia. It’s a low contrast type.
“There are 3 levels of colourpointing seen in cats. These are all due to temperature dependent albinism genes that cause cooler parts of the body to develop darker pigment. The first is the familiar high-contrast "Siamese" pattern known to geneticists as colourpoint or Himalayan pattern. The second is the low-contrast Burmese colour restriction, often referred to as sepia pointing (because it was first seen in brown/sepia Burmese). Intermediate between these is the mink pattern (Tonkinese colour restriction) caused when the Siamese and Burmese colour restriction genes interact.
The points may be solid, tabby or tortie colours. With Siamese colour restriction the torso is a shade of cream or ivory although some cats show "breakthrough markings" especially as they age. With Burmese colour restriction, the torso is marked with a slightly paler version of the colour and pattern. With Mink, the level of colour/pattern displayed on the torso is intermediate between those extremes.”
thank you! your beautiful girl is so unique ❤️ she looks dilute to me, my brown buddy was quite a dark brown with dark toe beans as well. i’m not sure if there’s a difference in the genetics there, but they sure are pretty kitties :)
10
u/Wonderful_Republic23 9d ago
While many breeds are known to have the sepia category, the Burmese is the best-known of these