Kasept-

First, you have it backwards. If it is carried on the X chromosome, then a sire that possesses it would transmit it to all his daughters but to none of his sons, since the only sex chromosome a son would get from his sire would be the Y chromosome.

A mare that carries the gene, given that she has two X chromosomes, can have either one or two copies. If she has one copy, then 50% of her offspring will inherit her gene. If she has two copies, then all of her offspring will get one of her copies, and all of her sons would express this gene, since they have only one X chromosome.

Of course, all this supposes that the theory is true in the first place. I challenge you to give me a single citation from a reputable scientific journal that supports this theory. This is just dosage in another emperor's suit. The author appears to be the one who has done the research, but has published none of it except in a book she is selling. And the promo refers in an oblique way to some support from Dr. Fregin at Virginia Tech, but provides no details, and he has not published on this either.

While I am sure that there is good research to show that heart size may be related to racing ability, showing heritability is another thing entirely.

I'd be greatly interested to see a reference with any data on the sex-linked heritability of heart size in horses.