Calico cats – Why are they always female?

It is no secret that I am absolutely obsessed with cats – they are by far my favourite animal. Despite being sassy and most of the time a pain in the ass, they are absolutely adorable and there is nothing I love more than having a purring cat curled up on my lap.

I have had multiple cats in my life, and recently we adopted two turkish angora cats (we think that’s their breed).


The cats are sisters, yet they have very different coat markings. As you can see, the one on the right is covered in white, black and ginger patches, making her look even more adorable (which is probably why she gets away with almost anything…). The word to describe this coat colouring is ‘calico‘. Tortoiseshell cats have two colours, demonstrated by the cute left one.

Now here is an interesting fact: most calico cats are female. But why is this? Well, the answer is down to genetics.

Either trying to be a bird, or trying to catch one…

There are specific genes that determine a cat’s coat colours. These genes are found on the sex chromosomes – the X and Y.

Most females have two X chromosomes, and males have an X and  Y.  The genes coding for tortoise shell colouring are on the X chromosome.

One gene produces one characteristic. In this example, the coat colour gene controls coat colour. However, as we know, cats can be white, black, ginger, brown (even blue), so how can so many colours come from only one gene?

The answer is to do with alleles. Alleles are genes that ‘code for’ the same characteristic, in this case colour, but result in different qualities. So, a gene can either make the cat produce a black pigment in the fur, or ginger etc.

So each different allele produces a different colour. Male cats, having a genotype of XY, can only ever inherit one allele on the X chromosome from its mother. Females however have two X chromosomes, so will inherit two alleles.

Lets do a bit of genetics. An allele that produces an orange pigment will be represented by uppercase O. If it is black, it can be a lowercase o. The orange allele produces the pigment melanin (the same protein that gives you a sun tan), and the black allele codes for eumelanin, which has a black-brown colour. When we talk about genes on a sex chromosome, we put a big X or Y in front of the letter to show where it is found.

So what colour will the cat be?

MALES can either be:

  • XO Y (orange)
  • Xo Y (black)

FEMALES can be:

  • XO XO (Orange)                         “homozygous” – the same allele on each 
  • Xo Xo (Black)
  • XO Xo ( Orange AND black)    “heterozygous” – different alleles on the chromosomes

So as you can see, in order to produce a cat with at least two colours, it must have two of the alleles and because of this it has to be a female.

However, in the first picture the tortoiseshell cat on the left is orange and black (it looks brown), but the calico cat on the right is patchy with white spots. What causes this difference?

The devil cat

Well there are two factors that contribute to her colour.

Firstly, lets talk about the big coloured patches. In all females, there are two X chromosomes, which we already know. Yet if we take a closer look at the chromosomes in a female cell, we will see something fishy going on.

barr bodies 2.jpg

The right photo is of female nuclei (hence the XX). The arrows are pointing to darker condensed structures – these are called Barr bodies. 

But what are they? In females, the X chromosomes contain the same genes so code for the same proteins. These proteins are not needed in high concentrations, and if too much is produced then it can damage the cell. In fact, one X chromosome is sufficient for cell survival, and two is too much. To solve this issue, once an embryo is a few cells large,  called a blastula, one of the two X chromosomes is silenced by condensing and supercoiling it into a smaller structure known as the Barr body. This process is known as Lyonization, and only the uncoiled one is expressed.

(to learn more about how this works, visit


Surprisingly, this process is completely random. Either one of the chromosomes is inactivated, and it doesn’t matter which one. In the case of our cat in question, in her black patches, the X chromosome with the orange allele XO was silenced. In the orange patches, the Xo was the victim.

After the blastula stage, the cells continue to divide until a foetus is produced. Thus, all the tissue that came from cells with an active orange allele will eventually produce orange fur, and those that originated from one of the cells containing an active black allele will be black. The figure below illustrates this quite well.


This explains the colour, but not the white. Where did that come from? You may notice that there is not an allele coding for white fur. This is because the gene controlling white patches is not the same that codes for orange/ black fur – it is at a different location (locus) , S, to the orange/black gene. There are fewer than 10 genes that control coat colour, and one of these controls this white colouring: such colouring is called piebald spotting or bicolouring (white paws and legs, white on the breast and in the face). The gene for this is represented by an S.

There are three possible genotypes for this gene:

Note: when there are two different alleles, one might dominate over the other so it is called dominant. In this case, if it is present it is expressed regardless of whether the other allele is there or not. The ‘other allele’ is the recessive one. For this to be expressed, there must be two of them present.

  • SS (two dominant alleles). This causes high grades of white spotting (sometimes resulting in a solid-looking white cat or a white cat with just a few coloured hairs)
  • Ss (one dominant, one recessive allele), leading to medium grades of white spotting and is the genotype present in bicoloured cats.
  • ss (two recessive alleles), which results in a solid colour or low grades of white spotting (sometimes as little as a few white hairs)


So as you can see, the white patches can range from being large or relatively small and dispersed, as the picture shows.

Larger patches may be caused by:

  • Slightly earlier Lyonization during ontogeny (the development of an organism). The earlier the inactivation, the larger the patch of skin derived from each Lyonized progenitor cell
  • The same X chromosome being inactivated in adjacent progenitor cells

My Calico cat has the genotype Ss as mentioned above. The dominant allele of this gene prevents the cells containing the coloured pigment, melanocytes, from migrating into the developing hair follicles, so those follicles will not express any colour and will be white. The gene itself is a mutant – meaning it is a mutated version of another gene.

So if you see a bicoloured or calico cat and want to impress your friends, tell them that it’s a female – you will almost always be right (you have a 1 in 3,000 chance of being right, but if you are an unlucky, its probably due to a condition called Klienfelter syndrome,

I’ll leave you with some more pictures of the devil and the other just-as-adorable cat


don’t be fooled by how cute she looks…



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