I think I know what you're getting at. However, terms like "large amount" and "knowledge," and to a lesser extent even "required," are somewhat vague (e.g. it could be arguably more difficult to truly understand "depth" more than quantity of material). But anyway, apart from genetics, I'd say hematology/oncology could qualify.
Most of the genetic medicine stuff happens with kids. But on the adult side, you can do some interested things based on the genetics of cystic fibrosis, but at the end of the day it's still all cystic fibrosis. So IM to Pulm and subsubspecialize in CF
If you want to stay in the loop with the cool Heme Onc stuff these days knowing advanced genetics principles is very helpful esp for reading all those cutting edge research papers.