The mutation that gives male cats a ginger-colored coat and females ginger, tortoiseshell, or calico coats produced a particularly telling map. The orange mutant gene is found only on the X, or female, chromosome. As with humans, female cats have paired sex chromosomes, XX, and male cats have XY sex chromosomes. The female cat, therefore, can have the orange mutant gene on one X chromosome and the gene for a black coat on the other. The piebald gene is on a different chromosome. If expressed, this gene codes for white, or no color, and is dominant over the alleles that code for a certain color (i. e. orange or black), making the white spots on calico cats. If that is the case, those several genes will be expressed in a blotchy coat of the tortoiseshell or calico kind. But the male, with his single X chromosome, has only one of that particular coat-color gene: he can be not-ginger or he can be ginger (although some modifier genes can add a bit of white here and there), but unless he has a chromosomal abnormality he cannot be a calico cat.