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Introduction
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).
Genes can be dominant or recessive. The dominant form of a gene is identified with a capital letter, and the recessive form with a lower-case letter. For an example, let's use a simple gene, T, which controls tobiano spotting. T is the dominant form of the gene, causing a horse to show tobiano spotting, and t is the recessive (nonspotted) form. Since chromosomes exist in pairs, one of the chromosomes in the pair will be T or t, and the other one will also be T or t. Of course, they do not necessarily match. Each horse has one of three combinations of T genes at the T loci: TT, Tt, or tt. Horses that are TT or tt are homozygous, while horses that are Tt are heterozygous.
T
is a dominant gene, so the horse only needs to possess one copy of T to display the tobiano pattern. Therefore, a horse with TT or Tt will be tobiano, and a horse with tt will not be tobiano (its color and markings are determined by genes at other loci). The presence of T masks the expression of t, so T is dominant over t. The non-tobiano horse is said to be "recessive at the T locus," and the tobiano is dominant at that locus.A tobiano horse is usually described as T-. The dash (-) means that the gene present at that locus has no effect on the horse's visual appearance. TT and Tt horses exhibit the same phenotype.
