Why are calico cats only female?

A female cat with the distinctive calico colour pattern.
Image credit: Chance Agrella
Date:25 April 2017 Author: Nikky Knijf Tags:, , ,

With their mostly white bodies and unique coloured patterns of orange and black, calico cats are a firm favourite of many cat lovers. And very recognisable. But did you know calico cats are always female? If you did, do you perhaps know the reason why?

Well, it all comes down to DNA

Like humans, cats’ DNA is a paring of chromosomes inherited from the parents. The 19 chromosome pairs in cats comprise of 18 autosomes and one sex chromosome pair. The sex chromosome in female mammals will comprise of two X chromosomes, while males have one X and one Y chromosome.

In males both sex chromosomes can remain active for their entire lives. But in females one X chromosome needs to be inactivated before development can take place.

In the very early stages of embryonic development one of the two is permanently inactivated. This is called X-inactivation or Lyonization. When exactly this happens, writes the Miami College of Arts and Sciences, can determine the size and patterns in calico cats. But regardless of when it happens, the X chromosome that remains active is completely unpredictable. So some cells will be left with the X chromosome inherited from the father, others are left with the X chromosome inherited from the mother.

In calico cats this pattern of X chromosomes is seen in their fur. This is because colour genes are located on the X chromosomes. Tortoiseshell colouring occurs in much the same way.

Calico cats can be male

Now we must add it is possible for a male cat to be calico or tortoiseshell, but this is very rare. Only about one in every 3 000 calico cats could be male. This happens when the cat is born with two X chromosomes and one Y chromosome. This is known as Klinefelter syndrome, a serious condition that affects male cognitive development. This syndrome can also be found in humans.

The United States Library of Medicine says humans born with this chromosome pattern will have less testosterone than the average male, resulting in delayed or incomplete puberty. They also often have breast enlargement, less body hair and an incapability to reproduce. Klinefelter syndrome could also have learning disabilities and delayed speech and language development. People living with the syndrome also tend to be quieter, more sensitive and unassertive as a result, but these characteristics would also vary dependent on the individual, they write.

Want to know more?

In the video above Derek Muller from the YouTube channel Veritasum explains the entire process in much finer detail, check it out.


Image credit: Chance Agrella
Video credit: Veritasum

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