The genes of any mammal are found in the nucleus of each cell on structures called chromosomes. Each species has a specific number of chromosomes. Chromosomes contain all the genes that code for the control of embryonic development, metabolic function and reproduction.

Included in the chromosomes are those which determine sex of the animal. The mare contributes a chromosome called an X and the stallion either an X or a Y. In horses, each parent contributes 32 chromosomes in the sperm or egg to form a normal equine embryo with 64 chromosomes. Thus, the normal karyotype for a mare is 64, XX and a stallion 64, XY.

Rarely, chromosomal material may be lost or rearranged during early embryonic development. Often this results in embryonic death and pregnancy termination. In some cases, though, if the chromosomal changes are less serious, a foal may be born, but may show abnormalities in development or reproduction.

Karyotyping tests the number of chromosomes and the presence of the x or y sex chromosome. It requires the attending vet to take a blood sample and submit it by overnight carrier to a genetics laboratory where this test is performed.