For many years horse owners and breeders have witnessed a phenomenon known as the maternal grandsire effect. The most notable example of this is Secretariat, one of the greatest Thoroughbred racehorses of our time. While Secretariat was an impressive athlete, his offspring did not race at the same competitive level. However, his daughters were exceptional broodmares; notably, his daughter Terlingua. Not only was she a record-breaking stakes winner herself, but she was also the dam of two-time leading North American sire, Storm Cat.

In recent years, equine scientists have begun to shed light on the genetic mechanisms that lead to the maternal grandsire effect in horses. Important studies in the 1960s revealed that the stallion’s role in breeding does not stop at conception, but continues to ensure pregnancy maintenance in the mare. Scientists measured equine chorionic gonadotropin (eCG), the placenta-derived hormone of pregnancy maintenance, in horse and donkey pregnancies. While eCG is high in horse x horse pregnancies, it is low in donkey x donkey pregnancies. Interestingly, crossing a mare and a jack (male donkey) results in low eCG, but when a jenny (female donkey) is crossed to a stallion, eCG is high. In fact, eCG levels in those jennies were similar to those seen in horse x horse pregnancies. This crucial finding pushed those in the field to investigate the role of the stallion’s genetics in pregnancy success.

Equine geneticists combined these previous findings with the novel discovery of offspring genes in which expression is exclusively derived from maternally (dam) or paternally (sire) inherited genes, but not both. These genes, known as imprinted genes, are present in the placenta. Nearly 50 years after the eCG experiments, scientists uncovered that almost 100% of imprinted genes in the equine placenta are paternally expressed, i.e. a result of the sire’s DNA sequence being passed down. Therefore, the stallion plays a central role not only in conception, but also in the maintenance of pregnancy through placental gene expression and thus health of the foal.

In the last 10 years, leading scientists have also determined the “speed gene” which is believed to contribute to racing success in Thoroughbred horses. The gene-encoding myostatin is crucial for skeletal muscle development and mutations play a role in equine athleticism. Interestingly, when looking at other racing breeds, including Arabians, the gene-encoding neurotrimin was determined to be important. Neurotrimin is involved in neurologic development and was strongly correlated with race starts, and thus believed to play a role in race learning ability.

With the rapid pace of science and technology, the breeding industry will likely add parental genetics to determine the best crosses for creating the ideal equine athlete that carries these “speed genes” from the sire or dam. The maternal grandsire effect has been proven to be beyond an observation, instead becoming the prelude to a scientific breakthrough that has longstanding implications for horse breeding and beyond.