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Genetic information that determines the traits of a living organism is stored in the nucleus of a cell. Using a microscope, it is possible to observe oblong structures in the nucleus during certain time periods in the life cycle of a cell. These structures, called chromosomes, contain deoxyribonucleic acid (DNA), which is the direct carrier of hereditary information. The molecular building blocks of DNA are the four nucleotides known as adenine, thymine, cytosine, and guanine. A DNA molecule has two strands aligned with each other (like the tracks of a train), forming the familiar double helix. Segments of DNA are called genes. They are essentially the ingredients for the recipe to make a living thing and determine the kind of “products” the cell will manufacture and the characteristics they determine—for ­example, eye or hair color.

The DNA sequence of an organism and the presence or absence of proteins that code for certain traits is called genotype; their external manifestation (the organism’s observable traits and characteristics) is phenotype. Most multicellular organisms obtain half of their chromosomes and genes from each parent. Horses have 32 pairs of chromosomes for 64 in total, and each gene normally exists in the same place on the same chromosome—this location is called the locus. The locus is the physical site on the chromosome where one form of a gene can be found. The different forms of genes that determine possible phenotypes are known as alleles. The alleles for a gene normally will be found in the same place on a chromosome from one parent as on the same chromosome from the other parent. As one locus is paired with another locus, the horse has two allelic genes matched from the series of alleles possible for a specific characteristic.