> For instance there is a condition in mice that lives on the male chromosome. It causes all of it's offspring to be male. So all of them also carry this condit

> For instance there is a condition in mice that lives on the male chromosome. It causes all of it's offspring to be male. So all of them also carry this condition. Thus the condition spreads itself twice as much as the competing genes. And so it quickly fixes itself in the population and then there are no more females. Only if the offspring of regular males can't compete well enough to keep producing females! I don't recall the name offhand, but there is a species complex of fish that live stably in a similar situation- effectively, there is one species that consists of normal males and females, and another species that consists solely of males which parasitizes the other's females for reproduction, and thus steals genes from the "normal" species.