Discworld genetics are not purely particulate. It is not rare that more than bits of little thingies called chromosomes are passed on to descendants. The exemplary case study is Susan Sto Helit, Death's granddaughter. Death has the ability to walk through walls and doors, be invisible, and do things outside of the mainstream time. Death's adopted daughter, Ysabell, didn't seem to have these abilities. Death's rather short-term apprentice, Mort, had these abilities while he was on the job. Mort and Ysabell (who were both otherwise ordinary humans) went back to the normal world, married, and had a daughter Susan. Susan automatically has all of these abilities except for when she deliberately tries to forget them; it required no learning or practice whatsoever.
The wizards of Unseen University, in particular Professor Catbury, have also dabbled in genetics, using magic to crossbreed a pea and a fruit fly. The result was a green thing that flew. It led a short, sad life before being eaten by a vegetarian spider.
The God of Evolution briefly experimented with genetics (The Last Continent) by trying to replace the soggy bit of a cow with the inflammable bit of an oak tree (to make it easier for his followers to sacrifice/burn cows), but ended up with a bush that squirted milk.