Equine Coat Color Genetics - Fundamental Concepts and Terminology

Fundamental Concepts and Terminology

Much of the modern understanding of equine coat color genetics is owed to the work of Dr. Ann T. Bowling of the University of California, Davis and of Dr. Phillip Sponenberg of Virginia Polytechnic Institute. Modern discussions of horse coat color genetics are based on the distinction between "red" and "non-red" coats, a factor determined by a single gene. More detailed discussions of coat color all refer to the differing effects of separate genes on these "base" coat colors.

Coat color alleles affect melanin, the pigment or coloring of the coat. There are two chemically distinct types of melanin: phaeomelanin, which is perceived as red to yellow color, and eumelanin, is perceived as brown to black. All coloration genes in mammals affect either the production or distribution of these two chemicals. Alleles affecting melanocytes (pigment cells) do not alter the pigment chemicals themselves but rather by acting on the placement of pigment cells produce distinct patterns of unpigmented pink skin and corresponding white hair.

Heritable characteristics are transmitted, encoded, and used through a substance called DNA, which is stored in almost every cell in an organism. DNA is organized into storage structures called chromosomes. For the most part, chromosomes come in matched sets, one chromosome from each parent. The location of a gene on a chromosome is called its locus. Alternate forms of a gene are called alleles. The terms Alleles and Modifiers are used interchangeably and describe the same concept. An allele identified with a capital letter is a dominant trait, one identified with a lower-case letter is a recessive trait. Because sex cells (sperm and ova) contain only half the usual number of chromosomes, each parent contributes one allele in each gene set to the ensuing offspring. When an individual's gene set contains two copies of the same allele, it is called homozygous for that gene. When it has two different alleles, it is heterozygous. For a recessive trait to be expressed, it must be homozygous, but a dominant trait will be expressed whether it is heterozygous or homozygous. A horse homozygous for a certain allele will always pass it on to its offspring, while a horse that is heterozygous carries two different alleles and can pass on either one.