Re: Why are calico cats all female?

Dear Laura -

The calico appearance, or phenotype, results from X inactivation. The genes controlling feline color coat reside on the X chromosome. As you are probably well aware, females have two X chromosomes, (XX), while males have only one (XY). During X-inactivation parts of each X are turned off to leave the equivalent of one functioning X. The genes for black, white and orange coat color reside on the X. Depending on which genes are turned off a specific region will be back, white or orange.

Since males have only one X, no such inactivation occurs, producing a uniform coat color. The only time a male cat can be a calico is when it is a "Klinefelter" -- XXY genotype. I'm not certain about the process in cats, but humans with Klinefelter's Syndrome undergo X inactivation ^*. These cats, as with human XXY males, are sterile and cannot reproduce.

References:
1. The calico cat, the munchkin and achondroplasia
2. Zuffardi O. Maraschio P. Lo Curto F. Muller U. Giarola A. Perotti L. The role of Yp in sex determination: new evidence from X/Y translocations. American Journal of Medical Genetics. 12:175-84, 1982.