The ABC's and X's and Y's of Feline Genetics

Chapter 1 -- Mr. Mendel's Peas

Most cat breeders are aware that certain breedings produce cats that have certain physical traits. However, many of us are uncomfortable trying to explain exactly how this happens. Through this series of articles, you will be provided with an understanding of the principles of genetic inheritance in cats, and specifically, the genetics of color.

The 'laws' of genetics were discovered by a Franciscan monk named Gregor Mendel. He studied the changes in flower color and seed shape in successive generations of garden peas that he cross pollinated. Mendel found that by crossing a white flowered pea to a red flowered pea, the next generation of plants would have pink flowers. If he then took the pink flowered plants and cross pollinated them with each other, he would get flowers of all colors in the resulting generations of plants. After many experiments like this, Mendel discovered that the results of these crosses could be expressed as statistical averages. For example, the cross of pink flowered plants to pink flowered plants yielded the following ratios: approximately 1/4 of resulting generation would have white flowers, 1/2 would have pink flowers and the remaining 1/4 would have red flowers.

		p	p	pp
PP	P	Pp	Pp
	P	Pp	Pp	(all pink flowered)

		P	p	Pp
Pp	P	PP	Pp
	p	Pp	pp	(red, pink and white)

Mendel theorized that something in the pollen and pistils of the plants controlled the expression of color in the flowers. He realized that a white flowered plant could only pass on white flowers to its offspring, and that a red flowered plant could likewise pass only red flowers. The pink plant that resulted from the white x red cross, was because only 1/2 of the offspring's traits came from either parent. As we now know, each cell in an individual contains protein strands called chromosomes, and that those chromosomes are composed of smaller units of inheritance called genes. All the cells of an individual, except for the reproductive cells (egg and sperm), contain the same number of chromosomes. The reproductive cells contain only 1/2 this number, so that when they combine during fertilization, the resulting offspring has the correct number of chromosomes for a normal healthy individual.

The ratios that Mendel observed regarding flower color in peas result because of simple recessive inheritance, and co-dominance. The gene for flower color in peas exists in 2 forms or alleles, one for red color and one for no color (white). In order to be white, a pea must posses 2 copies of the gene for no color; called a homozygous ('same egg') recessive. In order to be red flowered, the plant must have 2 copies of the red color gene; called homozygous dominant. Pink flowered plants posses one copy of each allele of the gene; one red and one white, and are called heterozygous. The exact genetic representation of the alleles (forms of genes) possessed by an individual is called their genotype. The physical expression of an individual's genes is called a phenotype. It is very important to realize that a phenotype may be the result of more than one genotype. In simple recessive inheritance, the heterozygous genotype and the dominant homozygous genotype often have the same phenotypic expression. In the case of the flower color in peas, red is incompletely dominant over white resulting in the pink color in the heterozygous genotype, rather than red, as might be expected.

So how does all this relate to breeding cats? In order to understand what color kittens may result from a breeding, it is necessary to understand what genes control color (and coat length, ear shape, coat texture, etc.) and how they behave in relation to each other.

In the most basic sense, genetically a cat is either black or it is not. The gene for black coat color (exclusive of tabby or solid pattern) is represented by the symbols B/b. The dominant allele B represents black, and the recessive, b, represents chocolate. This gene is influenced by the dilution gene (D/d). The dominant allele, D is full coat color, and the recessive d, is dilute coat color. These genes always act together in determining coat color.

Suppose that we have a black cat, that we know is homozygous for black coat color. This cat's coat color would be represented by the genotype BBDD (homozygous for Black over chocolate, and homozygous for full coat color over dilute). We decide to mate this cat to a blue cat, also homozygous. The genotype for this blue cat is represented as BBdd, homozygous for black coat, over chocolate, and homozygous for dilute over full coat color. It is the d gene that makes the cat blue. If the cat was not homozygous for d, it would be black.

One of the simplest methods to demonstrate how alleles are inherited is with a Pennet square. For our theoretical breeding, since both parents can only contribute the B allele of the black gene, a Pennet square used to show the inheritance of the dilute gene alleles would look like this:

		d	d	(Blue)
(Black)	D	Dd	Dd
	D	Dd	Dd	all black

All the kittens resulting from the above breeding would be phenotypically black, but they would carry the allele for blue (dilute) coat color; genotypically represented as BBDd.

If we then crossed 2 of the resulting kittens (this is not to advocate inbreeding), the results would look like this:

		D	d	(Black)
(Black)	D	DD	Dd
	d	Dd	dd	black and blue, with 1/2 carrying blue

The result is the same ratio that Mendel found in the peas, 1/4 of the kittens would have the possibility to be homozygous for full coat color (black, homozygous dominant), DD; 1/2 of the kittens have the possibility be heterozygous for full coat color (black carrying blue), Dd and 1/4 of the kittens could be homozygous for dilute coat color (blue, homozygous recessive), dd. Please remember that these probabilities affect each kitten and not the whole litter. Thus each kitten has a 1/4 chance to be blue, not 1/4 of the kittens in a litter.

Many of the genes that control coat color, length and pattern (as well as other traits), are genes that exhibit simple recessive inheritance or simple dominant inheritance. In the case of a simple dominant gene, only one copy of the gene is required to express the trait, so that a genotypically heterozygous cat phenotypically has the charateristic coded for by the gene. Below is a chart of the genes and their alleles, both in symbolic representation and as the dominant and recessive traits physically expressed. When we selectively breed cats for a certain trait, color or otherwise, we are artificially increasing the occurrence of some alleles in the breeding population (genepool). Increasing the occurrence of an allele means that we also increase the probability of the desired physical trait. This is exactly how breeds like the American Curl were developed.

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©Copyright 1996-1997, Gayle Hunter
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Created: 9/15/96 Updated: 9/14/97