Darwin didn't know about Mendelian Genetics yet, never mind DNA (Mendel was a Christian friar and a meticulous scientist), and recognized that the lack of rules of heredity was a gap in his theory.
With the help of the few books I found on heredity in our small town library (pre-internet), I composed an essay on basic Mendelian inheritance, in which I discussed why some children in a family develop a disease and others do not.