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Calico cats, with their orange, black and white coloration, are almost always female. So are black and orange tortoiseshells. Why?

The answer has to do with genetics. Every cat has 38 pairs of chromosomes; half of the pairs are from the mother, the other half is from the father. Within every chromosome there are thousands of different genes.

Every female cat receives one X chromosome from her mother and one X chromosome from her father, while a male receives one X chromosome from his mother and one Y chromosome from his father. Within the X chromosome is a gene for coat color.

In calicos and tortoiseshells, one X has the black gene; the other X has the orange gene. White coat color is associated with a completely separate gene.

At conception, the kitten is a one-celled organism, which divides until there are millions of cells that make up the final kitten. Each time a cell divides, it passes on its genetic material.

For the sex chromosomes, there is a battle for power. This is especially true for the X chromosome. If two X chromosomes are present, which determines female sex, one X chromosome will become inactivated at some point in fetal development. When this happens, all the cells descended from the activated X chromosome will have the same characteristics, including coat color.

In calicos, if the X-chromosome that is left functioning carries the orange gene, then all the cells descended from it will result in an orange color. The same is true if the functioning X chromosome has the black gene. If both X-chromosomes carry the same color gene, then the calico pattern will not appear.

Since X chromosomes inactivate at various times in each individual cat, color patches vary.

The story is different for male cats. Males have only one X chromosome, and it is never inactivated. Whatever color gene is present on this X chromosome will determine the color of the cat. Males can be calico or tortoiseshell only if they are born with 2 X-chromosomes and a Y (XXY), allowing one X to be inactivated. This genetic defect (XXY) is very rare.

It is difficult to breed specifically for calico or tortoiseshell cats. Breeding an orange cat to a black cat may increase the chances, but it all depends on whether the kitten is female and at what stage during development the X-chromosome becomes inactive. There is no way to predict or force an X chromosome to inactivate at a certain point in development.

Source: Dr.Jon