The idea of inheritable trait was well known before the findings of genetics. This is most obvious in the case of animal and plant husbandry that have existed for centuries. Breeding animals or plants with certain traits was not something that we suddenly found out or realized with Charles Darwin or Rosalind Franklin(Because fuck you James Watson!). If you look at some of the letters that Darwin received, there is a letter from a Farmer, who talks about how he already knew what Darwin was talking about, because as a farmer the idea of inheritable trait is already known, that the theory of natural selection fit in perfectly in terms of inheriting certain traits from mother and father. This theory is then further backed by discovery of genetic codes which explained the method of inheriting different traits.

Source: Most of this is from my biology class, For Darwin its from "Darwin's Ghosts" Neither are historical sources however.