Skin cells multiply during growth and spread out across the skin as the embryo grows the skin cells multiply. One (the "early/late deactivation theory") suggests that the time at which X chromosome deactivation occurs during foetal growth determines whether the cat has well defined patches or is brindled with intermixed black and orange hairs. There are two main theories regarding brindled torties and patched torties. Male cats only inherit one X chromosome so this is active in all skin cells as there is nothing equivalent on the Y chromosome which could "switch off" the O gene. This can only happen in cats with two X chromosomes. The skin is a mosaic of cells where some have the O gene active (making ginger pigment) and some do not (making black pigment). This occurs early on in the embryo and as skin cells multiply, they form patches. This only shows up visibly in skin cells as these produce pigment. Some cells randomly activate the O gene while others activate the gene in the equivalent place on the other X chromosome. 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. Cats with an all-ticked pattern almost shimmer in the sunlight because of the color variation.Because of the way tortoiseshell is inherited, almost all tortoiseshell and calico cats are female. If you look closely, you'll see different bands of color down the length of the cat's hairs.
Mackerel (striped): This is by far the most common pattern, so much so that some people think it should have received the title "classic" designation.Many American shorthair cats demonstrate this pattern. Classic: This pattern usually has whorls ending in a "target" on the side of the cat.
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