Tracing Genetic Material from Mares and Stallions

Gender in horses is determined by the sex chromosomes inherited from a foal’s parents. Stallions, like other male animals, have an X chromosome and a Y chromosome, and can pass one or the other to a descendant. Mares, having two X chromosomes, can pass only an X chromosome to their offspring.

Chromosomes carried in the sex cells carry DNA that holds the blueprint for a developing embryo. DNA can be nuclear (found in the nuclei of cells) or mitochondrial (found in the mitochondria, structures that convert energy into a form that can be used by cells). Traits are inherited from both parents, but mitochondrial DNA, the type of genetic material that is helpful in determining the relatedness of individuals and populations, is inherited only from the female parent. This is partly because the ovum contains far more molecules of mitochondrial DNA than a sperm cell, and these molecules from the sperm are destroyed by the ovum after fertilization. Therefore, tracing maternal lineage is possible through mitochondrial DNA, but paternal lineage is not traceable in this way.

The ancestry of mares contains many haplotypes, or lines tracing to a common female ancestor. In Arabian horses alone, there are at least 27 haplotypes, and for horses of other breeds, a much larger number of haplotypes have been identified.

Advances in gene sequencing technology have given researchers the ability to examine genetic diversity of male parentage based on Y chromosomes. A recent study led by scientists from the Vetmeduni-Vienna Institute of Animal Breeding and Genetics University showed that Y chromosomes are very similar in modern horse breeds, with only six haplotypes (HT). HT1 is an ancestral haplotype found in horses of varied breeds and in many locations. The other five haplotypes have developed more recently due to mutations or gene conversion. HT2 is also widespread but is not found in parts of Spain and northern Europe, while HT3 is common in many warmbloods as well as English Thoroughbreds. The remaining three haplotypes occur in small equine populations including Norwegian Fjords, Icelandic ponies, and Shetland ponies. All these haplotypes are separate from the genetic profile of Przewalski’s Horses.