Genetic diversity is a very interesting topic that’s well worth exploring, but it seems to me that Occam’s razor requires us to consider the possibility they just had a really good, well timed dosage of performance enhancing drugs for Secretariat back in the day.

> After he died, Secretariat’s heart was reportedly measured at almost 10 kilograms – twice as much as normal. This appears to have been a physiological fluke, however, as it isn’t a trait reliably passed down to offspring.

In humans, enlargement of internal organs (including heart) is know to be a side effect of using growth hormone as a PED. Wiki says [0] that "Extracted growth hormone was used since the late 1950s", so in 1973 it was already a well known technology.

[0] https://en.wikipedia.org/wiki/Growth_hormone_therapy#Extract...