Frozen_Heart
Hero
Genes can still be passed down even with less fertile offspring. One of the reasons why it's speculated that sapiens and neanderthals may have had only partial fertility (only fertile female offspring, and not fertile male offspring), is due to the tiny percentage of their DNA which has survived, and the complete lack of any part of the neanderthal Y chromosome.Modern Europeans have a small fraction fraction of Neanderthal DNA. So the production of fertile offspring must be possible! As I understand it, the viability of offspring depends on not inheriting incompatible DNA on a shared gene. If we assume the two species of human have 99% of the same genes (I’m not sure an exact figure is known) then if the 1% that is different gets left out in the genetic shuffle the offspring is viable.
Infertile male hybrid offspring is extremely common in mammals. Though with reptiles (including birds) it's the opposite, with the female offspring being the infertile ones.