By Anne Peters
A couple of months ago Byron Rogers of Performance Genetics forwarded a link to a recently published study on population genetics. A team of researchers in China and Australia generated statistics that suggest a mother’s influence may actually be more important than a father’s influence when it comes to racing ability. Is this study a fluke of statistical analysis or is there solid science behind it?
In the paper “Potential role of maternal lineage in the Thoroughbred breeding strategy,” published in Reproduction, Fertility and Development in May 2015, by Xiang Lin, et al., the racing ability of 675 Australian Thoroughbreds was categorized by average earnings per start as an indicator of quality of performance and ranked from highest to lowest. The top 30% of earners were put into the “Elite” group of performers and the remaining 70% into the “Poor” group.
The runners were divided into four groups based on their parentage: Elite dam vs. Elite sire (EE), Elite dam vs. Poor sire (EP), Poor dam vs. Elite sire (PE), and Poor dam vs. Poor sire (PP).
The results indicated the best runners were from the EE group (Elite dam and sire), while the worst were from the PE group (Poor dam, Elite sire). Surprisingly, the PP group (Poor dam and sire) was not the worst, as one would expect.
Here’s the interesting part: Foals with Elite dams (EE and EP) had statistically similar superior results while foals with Elite sires (EE and PE) did not demonstrate the same similarities, instead, showing an influence related more closely to the performance of their dam. The foals of Poor dams (PE and PP) were poor across the board, even those by Elite sires.
The study concludes “that maternal heritability of athletic performance may be a stronger contributor than paternal heritability to race ability.”
The researchers hypothesize that this is due to mitochondrial DNA (mtDNA), which is inherited only through the female parent and has a strong influence in the production of ATP, the prime source of energy for any activity, in particular, physical exercise. Sires pass on a lot of important characteristics to their progeny, but they don’t pass on mtDNA as mares do. If superior mtDNA is the thing that makes the critical difference, then we’ve been putting emphasis in the wrong place, relying too heavily on stallions.
Since Joe Estes’ research in the 1940s and 1950s, studies have proved time and again that the best racemares make the best broodmares, but up until now it hasn’t been demonstrated that the mare’s input may actually be more important than that of the sire’s. Good racemares are almost always bred to the best stallions available, so testing them against members of the “poor” community hasn’t been something that most breeders would attempt, which makes these results especially intriguing.
Theories have tried to prove the greater importance of good mares with mixed success. Geneticists have already discounted the so-called “X Factor” theory, which proposed that weaving their way down the breed were certain x-chromosomes that were deemed responsible for producing larger-than-average hearts, which, in turn, were presumed to translate into greatness. While several genes are related to heart size, researchers mapping the equine genome didn’t find any of them on the x-chromosome. This doesn’t dismiss the idea that certain females could pass on certain superior traits, only that the X Factor, as it relates to heart size, wasn’t on the x-chromosome.
The so-called “Rasmussen Factor” is a theory focused on inbreeding to superior females. Inbreeding to any superior individual sounds like a good idea, but often the parents are simply not good enough to pull it off or the key ancestors are so distant that their influence is too diluted to be effective. There are many clever examples of the Rasmussen Factor, but it hasn’t proved itself under statistical scrutiny.
Bruce Lowe’s numbered female families are based on the direct tail-female path of the mtDNA, and his Family 1 remains the most prominent in producing top runners, for some reason, even today. Is it due to mtDNA? Some of his ideas seemed far-fetched, but maybe not so much now.
This new study on maternal influence could help explain why so few stallions succeed in the highly competitive stallion market, or, rather, why so many apparently well-qualified stallion prospects prove mediocre or fail altogether.
It could also help us understand why the stallions we have valued as good or great can sire stakes winners at a rate as low as 5% of their foals, or a few rare ones, such as Danzig, as high as 19%. Compare this to a remarkable broodmare such as Juddmonte Farms’ Hasili, who produced 70% stakes winners to foals.