Cat colour genetics: the key genes and how to predict kitten colours

Dominant and recessive genes in brief

A kitten inherits one copy (allele) of each gene from each parent. A dominant allele shows even when there is only one. A recessive allele only shows when the kitten inherits two – one from the sire and one from the dam. A cat with a single recessive allele is a carrier: it looks like the dominant variant but can pass the recessive trait on.

Black, chocolate and cinnamon

The B gene sets the shade of eumelanin, the dark pigment. Black (B) is dominant over chocolate (b), which is dominant over cinnamon (bˡ). A chocolate cat can therefore carry cinnamon, but never black.

Dilution: blue, lilac, fawn and cream

The D gene controls pigment density. Full colour (D) is dominant, dilute (d) recessive. A cat with two d alleles has a diluted colour:

Red and tortie: a sex-linked colour

The O gene for red (orange) colour sits on the X chromosome. Males have one X chromosome, females two. As a result:

• A male is either red (cream) or has no red at all – he cannot be tortie.
• A female with two O alleles is red, with one she is tortie, without O she has no red.
• A male inherits his X chromosome only from his mother, so his red-series colour comes from her, not from his father.
• A tortie male is rare, usually has XXY chromosomes and is typically infertile.

Tabby pattern and the agouti gene

Every cat genetically carries some type of tabby pattern. Whether it shows depends on the agouti gene: dominant A reveals the pattern, two recessive a alleles (non-agouti) produce a solid colour. Red and cream are the exception – the pattern shows even in non-agouti cats, which is why solid red cats usually have at least some visible markings.

The type of pattern is controlled by other genes: mackerel is dominant over blotched, ticked shows over both, and spotted forms when other genes break the stripes up into spots.

Silver, white and white spotting

• Silver (the I gene, inhibitor) is dominant. A silver cat must have at least one silver parent. On tabbies it gives silver tabby, on solid colours smoke.
• Dominant white (W) masks every other colour – a white cat can carry any colour underneath. White cats with blue eyes have a higher rate of congenital deafness.
• White spotting (the S gene) is incompletely dominant: one copy usually gives less white, two copies more – from bicolour to van.

Siamese and Burmese points

Points are recessive alleles of the C gene. Siamese (cˢ) and Burmese (cᵇ) only show in two copies. One Siamese and one Burmese allele together give the Tonkinese colouring. A pointed kitten can be born to two non-pointed parents if both carry points.

Why kitten colours cannot be predicted exactly

A cat’s colour shows its dominant genes but not the recessive ones it carries hidden. A black cat can carry chocolate, dilution and points without any of it being visible. Exact probabilities can only be calculated when you know the parents’ genotype – from genetic tests or from the colours of their ancestors and earlier litters.

Probabilities also describe the average across many litters. A single litter can easily be all blue even when one in four is expected.

How to estimate kitten colours in Breedio

The phenotype calculator in the Genetics module works from both parents’ EMS codes. Because hidden recessive genes cannot be read from an EMS code, it shows the range of colours and patterns that can be born rather than exact percentages.