Coloring A horse's color is determined by its genetic makeup, or genotype. The visual appearance of the horse is its phenotype. The genotype determines the phenotype.

Chromosomes are threadlike structures located within the cells of an animal. Each horse has 32 pairs of chromosomes within each cell. The genes, which control the animal's traits, are located on the chromosomes. Genes exist in matching pairs, with one of each type of gene on each pair of chromosomes. Each member of a pair of genes is called an allele. Each allele exists at a certain spot on the chromosomes, called a locus (the plural is loci).