The gene can be propagated by individuals who did not express it - it could be sex-linked, it could be recessive, it could require two or three conditions to be

The gene can be propagated by individuals who did not express it - it could be sex-linked, it could be recessive, it could require two or three conditions to be met, etc. (Also, propagation doesn't require 'more children than his peers', it just requires children that go on to reproduce).