Yes, calico cats need to have two X chromosomes, so they are generally female (XX) not male (XY). But there are “X’ ceptions !
- Male calico cats can have an extra X chromosome (aneuploidy – XXY), similar to Klinefelter syndrome in humans. Such individuals are often infertile because their chromosomes don’t pair properly during generation of sperm.
- Male calico cats can also be a chimera – a fusion of two different individuals early in the pregnancy (as an embryo). If a black and an orange male embryo fuse then you will get a male calico cat that will be fertile, and pass on orange or black, depending on which cells made the testes.
Frequencies (or “I heard that all orange cats were male?”)
In females there are 3 colour choices, so both orange and black are going to be less common than in males. However, this will be more dramatic for orange cats, because the orange allele is not so common. Let’s look at an example. If we had a population where 30% of the alleles are orange, then 30% of male cats will be orange, and 70% black. For females, there will be orange, calico and black and their frequency can be calculated (originally described by Hardy-Weinberg for populations):
Orange = (.3) x (.3) = 9% of females
Calico = (.3) x (.7) + (.7) x (.3) = 42% of females
Black = (.7) x (.7) = 49% of females