Understanding cat genetics is key to ensuring we produce great kittens. Get to grips with the basics at Bombadillo Kittens.
Since we are registered breeders of British Shorthair kittens and cats, this article will focus exclusively on British Shorthair colour genetics. Many of the principles and outcomes are equally applicable to most other breeds as well.
Basic British Shorthair colour genetics
Genetics in all animals are governed by chromosomes. Chromosomes always occur in pairs. This is how it is possible for an animal to be one colour, i.e. black, but also be a carrier of another colour, like chocolate. Where this occurs the cats phenotype is said to be black, because its appearance is black, but its genotype is heterozygous for black and chocolate because it has one chromosome that codes for black and one that codes for chocolate. The black gene is called the dominant gene because it will always manifest itself if a cat has one chromosome coding for black. The chocolate gene is said to be recessive because, unless there are two chocolate genes, the cat will not be chocolate.
- homozygous = both chromosomes are the same
- heterozygous = each chromosome is different (so the cat could be black carrying chocolate, or shorthaired carrying longhair)
The wild type of the gene is the dominant type - this is the gene that would dominate in the wild. So, in this example. Black would be the wild type, as would shorthaired. The wild type is always written as a capital letter as it is the dominant type. So black is a capital b - B, and chocolate is a small b - b.
Red or black?
All British Shorthair cats are either red or black. Every other colour and pattern is a variation on one of these two original colours.
The black gene has mutated over the years. The first mutation of the black gene created chocolate, which is written as ‘b’ and the second mutation of the black gene created cinnamon, which is written as ‘b1’.
- Black is absolutely dominant.
- Chocolate is recessive to black but dominant over cinnamon.
- Cinnamon is absolutely recessive and a cat of a cinnamon phenotype must be of the genotype ‘b1b1’ - if he carried either chocolate or black, he would not be cinnamon in colour. Whereas a black cat can be a carrier of chocolate or cinnamon, but not both.
The dilution gene
Both red and black can be affected by the dilution gene. The dilution gene is written as dd, because the wild type of the gene is the non-dilute version, which is written as DD. The dilution gene is a recessive gene.
A cat that is DD would be red, black, chocolate or cinnamon. The dilution gene is written as ‘d’. A cat that is of genotype ‘DD’ or ‘Dd’ will be either red, black, chocolate or cinnamon. One dilution gene does not affect the appearance of the cat, although some breeders suggest that the colour of the cat may be slightly different in some cases, but there is no conclusive explanation for this yet.
If a cat has two dilution genes and is therefore of the genotype ‘dd’, then a red cat will appear cream, a black cat will appear blue, a lilac cat will appear chocolate and a cinnamon cat will appear fawn. The same rules of colour dominance apply: blue is dominant to lilac, and lilac is dominant to fawn.
- DD = non-dilute, not carrying dilute so the cat would be red, black, chocolate or cinnamon and would never have any offspring that were dilute, even if the other parent was dilute
- Dd = non=dilute, carrying dilute so the cat woule be red, black, chocolate or cinnamon, but in this case, the cat would be able to have dilute offspring (if the other parent either was dilute or carried dilute)
- dd = dilute, so the cat would be cream, blue, lilac or fawn and all of the cat's offspring would carry dilute, or be dilute if the other parent carried/was dilute
British Shorthair patterns
The colourpoint gene
The colourpoint gene was bred into the British Shorthair breed from Siamese and Himalyan cats some time ago. It is a recessive gene, so it must be inherited from both parents for a cat to be a colourpoint, and from only one parent to be a carrier. Because it is the non-wild type it is written as ‘cc’. A non-colourpoint cat that does not carry the colourpoint gene would be represented as ‘CC’, a non-colourpoint cat carrying the colourpoint gene would be represented ‘Cc’ and a colourpoint cat would be represented as ‘cc’.
CC = non colourpoint, not carrying colourpoint - none of this cat's offspring will ever be colourpoint
Cc = non-colourpoint, carrying colourpoint - this cat could produce colourpointed offspring if mated to another cat with the colourpoint gene
cc = colourpoint
Therefore two colourpoint cats will always produce all colourpoint kittens. Both parents must carry the colourpoint gene in order to produce any colourpoint offspring.
The bi-colour gene is a dominant gene in British Shorthair colour genetics. Therefore, if one parent is a bi-colour cat, then around half of the kittens will be bicolours. If both parents are bi-colour cats, then 75% of their offspring will be bicolours or vans. The bicolour gene is called the White Spotting gene, and is written as 'WS'.
The white spotting gene stops the spread of coloured pigment when the kitten is developing in the womb. It usually works from the bottom up, so a bicolour kitten will most often have white legs, paws and tummy, but the white spotting gene can work on any part of the cat.
If a kitten inherits the white spotting gene from one parent the kitten will likely be up to 50% white. If a kitten inherits the gene from both parents, and therefore has 2 white spotting genes, making it homozygous for the gene, then the kitten is likely to be more than 50% white, although the exact genetics of this are not yet fully understand.
- WS WS = high white/van/harlequin
- WS ws = bicolour
- ws ws = self/no white
Below is a blue bicolour kitten with one copy of the White Spotting gene
Agouti, or Tabby British Shorthairs
The agouti gene is a wildtype, dominant gene. Therefore a cat need only inherit one agouti gene from one parent and it will be a tabby. It is one of the more complex aspects of British Shorthair colour genetics. There are different forms of tabby pattern: classic, mackerel and ticked, and it is not entirely clear how each of these different tabby genes interacts with each other. The mackerel tabby pattern is dominant to the classic tabby pattern.
- Therefore two homozygous tabby cats will always have only tabby kittens
- Two non-tabby cats will never have tabby kittens.
- Two cats who are tabbies but also each carry the non-agouti gene will have around 25% non-tabby kittens and 75% tabby kittens.
Below is a silver mackerel tabby British Shorthair kitten at 16 days old.
The Silver, or Inhibitor, gene
Another complex aspect of British Shorthair colour genetics is the Inhibitor gene. This is the gene responsible for making a tabby a silver tabby or smoke and is expressed as 'I' where the inhibitor gene is present or as 'i' where it is not present. A cat need only inherit the silver gene from one parent to appear silver. The silver gene alters the pigment in the hairs, inhibiting the yellow pigment and making the hairs appear as white or grey.
In a tabby cat it turns the background of the pattern to white or silver. In a solid, non tabby, cat it creates a smoke: where the colour in the hairs is restricted the tips of the hair only and the majority of the lengths of the hairs are silver.
British Shorthair colour genetics are quite a complex topic and you will find that over time your understanding of the genetics will continually grow. Nevertheless, there will always be some surprises in store for you no matter how much experience you have!
The Wide Band gene
The Wide band gene is one of the least understood feline genes in British Shorthair colour genetics. The Wide Band gene is responsible for creating the silver shaded and golden shaded British Shorthair varieties. It is not yet known whether the gene is dominant, recessive or is a polygenetic trait (i.e. is caused by multiple genes working together). The Wide Band gene is responsible for creating some incredibly beautiful cats! Read more about the genetics of the Silver Shaded British Shorthair and the Golden Shaded British Shorthair.
British Shorthair colour genetics are a complex topic, but with a bit of practice you will get your head around the topic!