The potential for improved performance is obvious. That s where this discussion reverts to a detective story, with Marianna Haun playing the role of lead investigator, or at least, coordinator of information. The Australians first noted the possibility of sex linkage of the inheritance of large hearts in Thoroughbreds. If that is correct, the large heart characteristic is transmitted on the X chromosome. Stallions have but one X chromosome, which passes to their daughters.
Mares have two X chromosomes, of which one is passed to their sons and one to their daughters.
A trait which is based in the genes of an X chromosome cannot be passed to a stallion s sons, but to his maternal grandsons. Males inherit these traits from their dams. The support for the idea could come from looking at heart scores in families that contain superior runners, and confirmation could come from mapping the specific genes involved. The former information on families is the subject of a major portion of the book, told in the sequences of deductive reasoning that make for a good detective yarn. Haun begins with her initial awareness of the heart size of the great Secretariat, which was described after his death in 1989.