Cat Genetics

Most breeders waiting for a litter from a particular pairing will be able to tell you in advance what colors the kittens are likely to be and whether they will be long- or short-haired. These breeders ae clairvoyant, they have simply learned something about feline genetics. Characteristics like fur colors and coat length, build, conformation, and a great many other features are inherited, and the breeder is familiar with the characteristics of the kittens' progenitors a fairly straightforward probability calculation can be made.

A cat has 19 pairs of chromosomes in every single cell in its body. This is where the genetic information is stored. The only exception to this are the egg cells in the female and the sperm cells in the male. These cells contain 19 single chromosomes—each is the random half of a pair of chromosomes. This makes sense, because when the egg and the sperm fuse together, the 19 single chromosomes from the female and the 19 single chromosomes from the male join together to form the pairs of chromosomes that together contain all the hereditary characteristics needed to create a cat. This means that a kitten gets half of the male's genetic characteristics and half of the female's. These genes do not mix. Mixing would mean that a cross between the red cat and a white cat would produce cream kittens, and a longhair a shorthair cross should result in a litter of semi-longhair. This is not what happens, however, because some genes are dominant and others are recessive.

Dominant means that this characteristic dominates and will always show up in the appearance of an individual cat carrying that gene. Recessive means that the characteristic only emerges if it is not suppressed by a dominant gene. If a recessively inherited characteristic is to emerge, the kitten will have to get the same recessive gene from both its father and its mother.

For example, the black coat color, indicated by a capital D, is dominant over the blue coat color, indicated by a small d. If we were to mate a purebred black tom (DD) with a purebred blue queen (dd), the kitten would get a single dominant gene for black (D) from its father and a recessive gene for blue (d) form its mother. Because the dominant gene always overrides a recessive one, the kitten will have a black coat, but it will carry not only the dominant gene for black (which everyone can see), but also the recessive gene for blue which he inherited from the queen. This kitten's genetic code is consequently Dd. Now we can theoretically mate this kitten with his litter sister, who carries the same combination of genes (Dd) for coat color. The kittens from this mating will also get a random half of the chromosome pairs from each parent. This means two dominant genes, two recessive genes or a dominant and a recessive gene can come together.

Because a different combination can occur in each fusion of an egg and a sperm cell, the following colors can be expected from this cross:

As a rule, you would expect to get more black than blue kittens in a litter like this one, but it is impossible for there to be no blue kittens at all. However, we can only work out the probabilities on the basis of the genetic codes for color, markings and coat length.

In term of percentage, the chance of black kittens carrying two genes for black resulting from this combination is 25%, the chance of black kittens that carry both the gene for black and the gene for blue is 50% and the chance of blue kittens, which of course carry two genes for blue, is again 25%.

Dominant and Recessive Characteristics

Not all of the cat's characteristics are passed on in as clearly a dominant or recessive form as those outlined above. Some dominant charcteristics, for example, will only show up in the cat's appearance if there's another gene present to activate that specific gene, while other characterisitcs are gender linked.