Wednesday, May 8, 2013


Tortoiseshell and calico cats
Did you know that Tortoiseshell cats are almost always female? Have you ever wondered why that is? As I am sure you are aware, female cats, just like human females, have two X chromosomes. Males on the other hand only have one X and have one Y. The Y chromosome does not carry genes that code for color.
 A picture of a Tortoiseshell cat
taken from
So since females have two X chromosomes does that mean that they are twice as dark colored as the males since they would have double the amount of color proteins? The answer to this question is no. This is because one of the X chromosomes inactivates itself. This is a random process that occurs early in development. If a cat has one X that carries a gene for black pigment and a different X chromosome that carries a gene for orange pigment then she expresses both colors in random blotches depending on which X chromosome remains active. Once a cell inactivates one of the X chromosomes all the cells that are descendants from that cell will all have that same chromosome inactivated. This is why there are blotches of color rather than the cat looking more like a checker board with lots of tiny dots of color. The earlier in development the cell inactivates one of its X chromosomes then the bigger the blotches will be. Also since inactivation of a X chromosome is a random event, even if two cats shared the exact same genetics they would not have the exact same color pattern.
Picture of a Calico cat taken from

Calico cats work the same way as Tortoiseshell cats, except calico cats express a different gene that is independent of the X chromosomes that codes for a white color.   



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