The first secret is don't bother looking for a genetic explanation or wading through pages of studies.
Seriously, we don't know much about chickens despite centuries of of chicken keeping. The whole genetics completely determines behaviour is just plain wrong. It's not that genes don't play their role but there is a balance in there somewhere between environment and genes.
But there are many people who have a flock with several breeds, all kept together in one pen and with one kind of feed and one style of management, and their Silkies go broody while their Leghorns do not. Or maybe it's the Cochins going broody while the ISA Browns don't. But across many different such flocks, certain breeds are much more likely to be the ones that go broody, and certain other breeds almost never do.
So I am firmly convinced that genetics have SOME effect, even though they do not seem to account for all of it.
On the other hand, environment is obviously important too (I am including feeding, housing, care, climate, and many other things under "environment.") I've encountered stories of a hen that goes broody frequently while living in one person's flock, and not at all in a different person's flock, which seem to clearly show that changing environment can change whether a particular hen goes broody, since the hen certainly did not change her own genes when she moved to a new home! (I've encountered such stories in either direction, starting with a frequently-broody hen or with a not-broody hen, and switching to the other condition after being rehomed.) @Shadrach your own stories are part of why I am sure that environment is part of the answer. I notice that some of your hens didn't even have to move, just have different care in the same location, which shows that something about the care/feeding/housing you provide is able to make a difference in how many hens go broody.