Large Heart Genetics
The last time I looked this one was still just a well thought out and respected theory at this point. As far as I know they have no test for it yet but they do have various ways of making educated guesses as to whether a horse has it or not.
They call it the X Factor and it is thought that the gene is only contributed from the mother and down through her female lines. This would explain why in Thorughbreds the dam line is so highly evaluated when breeding decisions are made. This "X Factor is thought to be a mutation and thought to trace back to a mare named Pocohontas - (and seem feel they can trace it back further than her yet.
Thorughbreds like Secretariat (whos heart they actually weighed) and older ones like PrinceQuillo, War Admiral, Mahmoud and Blue Larkspur, to name a few were all thought to possess the Large Heart Genetics.
It is a fact that some of the best race horses had a larger heart than normal. The theory being that the larger the heart, the bigger the motor as more oxygenated blood can pump through the system delivering more speed.
In Our database we mark this as LH (large heart) - so the options we have are nn. n/LH or LH LH - we do not have these results in the online database at this time but will continue to track them and at various times spend more time on researching possible positives. At the time of writing we had results reocrded for approx 150 horses from internet sources.