Definition
A gene is a segment of DNA that carries the instructions for a specific heritable trait or biological function. Each gene occupies a fixed position, called a locus, on a chromosome. Horses have 32 pairs of chromosomes, and the traits passed from dam and stallion to foal are determined by which versions of each gene — called alleles — the foal inherits.
Genes and Horse Coat Color
Coat color genetics provides the clearest applied example of gene action in horses. The Extension locus (E gene) determines whether a horse produces black or red pigment as its base color. The Agouti locus (A gene) controls the distribution of black pigment on a horse with black base. The Cream gene produces dilute colors including buckskin and palomino in a single copy and cremello in two copies. The Dun gene creates the dun coat pattern with its characteristic primitive markings. The how pigment loci stack is built from the interaction of a relatively small number of genes, each with two or more alleles.
Genes and Breed Traits
Beyond color, genes determine physical traits, gait capacity, and disease predispositions. The DMRT3 gene mutation underlies the ambling gait seen in ambling breeds that carry this variant breeds. The HYPP mutation in some Quarter Horse bloodlines causes a muscle disorder. SCID (severe combined immunodeficiency) in Arabians results from a single-gene autosomal recessive mutation, meaning both parents must carry one copy for a foal to be affected. Understanding the relevant genes allows breeders to use genetic testing to make informed decisions and avoid producing affected offspring.
Genetic Testing
Commercial genetic panels now allow horse owners to test for coat color alleles, performance-linked variants, and disease-associated mutations from a single hair or blood sample. Results are most useful in the context of a known the wider allele landscape and a specific breeding goal, since a gene’s effect depends on what other genes are present in the same individual.
Not all coat patterns in horses are produced by Mendelian alleles at defined loci. Brindle — a striped coat pattern — is one documented exception: it can arise from somatic mosaicism, a postzygotic mutation that generates two genetically distinct cell populations within one animal. Because the mutation occurs after fertilization, it does not follow standard inheritance ratios and cannot be predicted from pedigree analysis of the dam and sire’s alleles at the Extension or Agouti loci.