Codominance and Cats

If genes are dominant or recessive, why can some cats be multiple colours?

Anybody who knows me will be able to attest to how much I love cats. They’re honestly one of the best creatures on the planet. I have cats myself, and occasionally they’ll crop up over on my twitter, if you’re interested. But this blog post is not an ode to my beloved kitties. No, this blog is about cats in general and just what genetic concepts can be demonstrated simply by looking at a simple housecat.


Many people are aware of genes and how some versions of genes (alleles) are dominant (such as brown eye colour). However, not many people will consider this: If genes are completely either dominant or recessive, why do tortoiseshell cats exist? Surely, they should be one colour, not a patchwork of different ones? The answer, dear readers, is codominance.


Codominance is a simple concept, really. If two dominant alleles are combined, a mixture will be present in the offspring. For example, one of my cats is black and white. I don’t know what his parents were like, but somewhere down his family line a white cat will have been crossed with a black cat. Hence the pattern. Ever considered why two black and white cats can have litters that contain all black, all white or mixed offspring? The alleles they pass on result in the cat’s fur colour, and if both a black and a white allele are passed on, codominance occurs.


Incomplete dominance is another concept in genetics which is similar to codominance. However, instead of both of the alleles battling it out in the final product, they get mixed together to create something else. For example, a red flower is crossed with a white flower to get a pink flower. The two colours are both seen in the final product, but not individually. Instead, they’ve combined their qualities to merge into a new one. This is incomplete dominance – neither are dominant enough to override the other, but they’re both dominant enough to be expressed.


And that, dear readers, is a very short summary of two concepts which can occur in genetics. So next time you see a tortoiseshell cat, be sure to appreciate just how beautiful it is. If you don’t, it would be an absolute CATastrophe.


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