How did the selective breeding of stallions affect horse Y chromosomes?

The selective breeding of horses has significantly impacted the Y chromosome, resulting in a domestic horse population dominated by a single, paternally inherited lineage with almost no genetic variation.1

This lack of diversity contrasts sharply with mitochondrial DNA, which shows high maternal diversity. The discrepancy is attributed to the historical management of horses:

• Breeding Practices: Because stallions can be difficult to handle, breeders likely favored a small number of cooperative males to impregnate large numbers of mares.1
• Selection Pressure: Breeders directed most selection toward males because a single stallion can produce a significantly larger number of offspring than a female.1

While most modern domestic horses share this uniform paternal lineage, rare exceptions—such as specific populations in southwestern China—have been found to possess unique Y chromosome variants, potentially representing distinct lineages that survived or recent mutations. Overall, the limited variation on the Y chromosome reflects a history where human intervention prioritized specific male traits, effectively narrowing the paternal gene pool compared to the maternal side.1