I don't get it. If these mice have all male offspring, why won't they be out-bred by the mice that have females too? Why would a non-advantageous mouse gene be passed down and take over? Wouldn't natural selection kill off the genetically modified mice? If there end up being a bunch of males with very few females, most of the males won't pass down their genes, but all of the females will. Thus making sure the female-offspring-having normal mice survive and the males-only mice die off. What am I missing?