Read this in a review of racehorse breeding book
"...all Thoroughbreds trace back to a single broodmare who appears in the pedigree of each of the three foundation stallions. As well, most breeding theories focus on sires, however, traits such as heart size and DNA in mitochondria (the energy sources in cells) are actually carried by the females, explaining why large-heart horses like Secretariat were most successful as broodmare sires."
Concession made in another article: "The dam is much overlooked, because a stallion can have a hundred foals a year whereas a mare can have one."