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.
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.Gene Inactivation
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's Difficult to Breed Calicos
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.
Do you have a calico? Looking for a great name? Check out this article Naming Your Calico Cat: Name Ideas for Calico Cats