The formation of red and black patches in a female with only one O gene is through a process known as X-chromosome inactivation.
Tortoiseshell cats are therefore heterozygous (not true-breeding) for red colour. The O gene is called a sex-linked gene because it is carried on a sex chromosome. If she inherits only one O gene, she will be tortoiseshell. She must inherit two O genes to be a ginger cat. A normal male cat has XY genetic makeup he only needs to inherit one O gene for him to be a ginger cat. The O gene is carried on the X chromosome. The O gene changes black pigment into a reddish pigment. The ginger colour of cats (known as "yellow", "orange" or "red" to cat breeders) is caused by the "O" gene. Jean Bungartz referred to tortoiseshell-and-white cats as "Spanish cats" in his 1896 book "Die Hauskatze, ihre Rassen und Varietäten" (Housecats, Their Races and Varieties) in " Illustriertes Katzenbuch" (An Illustrated Book of Cats) though he said it was hard to explain why they got the name as they were not restricted to the Pyrenean peninsula. This article only considers normal tortoiseshell cats. These are discussed in detail in " Mosaicism, Tortie Tomcats, Genetically Impossible Kittens and Gender Anomalies". The very few male tortoiseshell cats are caused by genetic aberration or development abnormalities in the foetus.
Because of the way tortoiseshell is inherited, almost all tortoiseshell and calico cats are female.
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