Dairy cattle eventually graduate to McDonalds, so there is some interest in the genetics of beef production in dairy breeds. There is course more interest in the genetics of beef production in beef breeds of cattle.
Usually you don’t find a single allele that makes a lot of difference, but in some beef breeds, there are myostatin mutations that result in a ridiculous-looking, ‘double-muscled’ beast. Homozygosity for myostatin mutations causes difficulties in birth, so it takes really strong selection for beef production to make a myostatin null common. I don’t think you ever see this in dairy breeds.
But, as it turns out, there is a deletion that is pretty common in some dairy breeds that significantly increases milk production while killing homozygotes before birth.
Breeds under weaker selection for single traits, your typical cow of the past, probably have neither.
The point is that the genetic architecture of a quantitative trait does not have to be the same in different populations of the same species. For example, I have the impression that height is not as highly polygenic in Pygmies as it is most other human populations. There’s a particular region on chromosome 3 that seems to influence height- you don’t see such a concentration in Europeans.