Tortoiseshell is not a breed of cat. Rather, the term tortoiseshell refers to a coat color that can be found in many breeds. They do, however, have certain specific characteristics. Tortoiseshells are almost exclusively female, are not the same as calicos and have distinctive personality traits.
A cat is defined as a tortoiseshell when she has a black and orange or yellow coat. The difference between a tortoiseshell and a calico cat is that calico cats also have white on their bodies, while tortoiseshells do not.
Tortoiseshell cats are almost always female. This is because the genes that code for this coat color are carried on the female, or X, chromosome. Tortoiseshells inherit one X chromosome carrying the gene for the color black from one parent and another X chromosome, carrying the gene for the color yellow or orange, from the other. Two X chromosomes mean that the kitten inheriting them will be female. In the rare case that a tortoiseshell is male, he has three chromosomes: two X's and a Y.
Although tortoiseshells can be one of many breeds, they tend to have specific personality traits. Tortoiseshell cats tend to be feisty and strong-willed. They are also louder and more dominant than other cats and fiercely independent. They do have positive traits as well, of course. They also demand head rubs and extra TLC.
Tortoiseshell cats can be one of many breeds. These include the American shorthair, the British shorthair, the Oriental shorthair, the Persian, the Siamese, the Burmese, the Turkish Van, the colorpoint shorthair, the European shorthair, the Birman, the American bobtail, the Japanese bobtail, the Norwegian forest cat, the Maine coon cat, the Cornish rex, the Devon rex, the American curl, the Manx and the somali cat.
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