They threw up the occaisional star like Bikile and Kieno in the 60's when they had the advantage of being full-time athletes when the rest of the world were amateur part-timers and forced to retire by age 25 (22 in the case of Herb Elliott), but they were far from dominant, despite distance running being the only sport that any young sportsman could make money from, and the greatest stars from that period were non-African. Jim Ryun was better than Keino. Steve Ovett was better than Henry Rono even at Rono's distance of two miles.
Before 1988 or so, no Kenyan sniffed the top 300 marathon all time list, and there were more Japanese than Ethiopians in it. Only with the advent of EPO did Kenyans start to dominate.
So why bring genetics into it?