Why are tortoiseshell cats female? why are tortoiseshell cats female?

All tortoiseshell and tortoiseshell and white (Calico) cats are female. But not all gingers are male, though most are. The reason is that in cats the colour gene is carried on the X chromosome. As females have two Xs they can express two colours in the coat, and if black is inherited from one parent and ginger (red) from the other, the result is a calico cat. The calico's brother, having only one X chromosome, would be either ginger or black, depending on the colour of the mother. I believe there was a case of an apparently male calico cat but it was found to have a mutation giving an XXY chromosome set-up, and was sterile. For a female to be ginger she would have to inherit red from both parents. This could happen if the father was ginger and the mother tortoiseshell for example. The white is inherited on a separate gene and is independent of the colour gene. It can appear on either gender and with any colour of coat. For some reason not understood it is usually found that in tortoiseshell and white cats the patches of black and ginger are larger and more distinct than in the plain tortoiseshell without white. A tabby pattern of stripes can also be inherited separately, resulting in a cat known as a "torby" (Tabby-tortoiseshell).

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