Tortoiseshell cat

The term tortoiseshell cat refers to a coloration pattern caused by a combination of specific genetic traits. It is not a specific breed of cat. The tortoiseshell markings appear in many different breeds.

The coats of tortoiseshell cats are a mixture of colours. It is often reserved for cats with coats that look like black and orange patches, while calico cat is typically two-thirds white, with orange and black patches on their backs. Their coats may also combine red or ginger, and occasionally silver or ginger tabby patterns. The size of the patches can vary from a fine speckled pattern to large areas of colour. Cats with a basic white colour and red and black patches are known as tortoiseshell-and-white in Britain, or, in the United States, calico.

Coat coloration in cats is complex. The genes involved include the Orange gene, O, which in its dominant form, O, produces orange fur, and in its recessive form, o, produces black fur.

For a cat to be calico, it must simultaneously express two genes, O and o, which are located at the same location on the X chromosome. Males normally cannot do this: they can have only one allele, as they have only one X chromosome. Over 90% of tortoiseshell cats are females. Occasionally a male is born. These have Klinefelter's syndrome and are almost always sterile.

The position of the patches depends on which X-chromosome is active in each cell and which is inactivated to become a Barr body.