“Nope, you end up with the cat that made it alive across the chasm. When you breed that cat, you’ll multiply the number of cats with the extra strong leg gene that enables them to jump the chasm.”
Alright, your 10,000 cats are on an island. An earthquake makes the 10’ chasm, leaving the food on one side and the fresh water on the other. Each cat tries to leap across, only your one cat makes it. Luckily, she is pregnant, from one of the deceased 6’ jumpers, and she gives birth to a litter of 8’ jumpers. The end.
Not quite. We'll assume that my cat is extra strong because of a de novo single point mutation in one of the muscle protein genes. The mutation is most likely dominant, since the mutation would have occurred in a parental germ cell, either the egg or the sperm. Since the other parental germ cell would have had the normal protein gene, my super cat has one normal and one mutant "super" gene. Thus, when she delivers her kittens, half of them have the same mutation (because she gives half of her genes to each kitten) and can leap 10 feet. The other half, with the normal copy of the gene, end up at the bottom of the chasm with the 9,999 cats who were already there. As long as the cats have to jump across the chasm to get their food and water, only the ones who can leap 10 feet survive to reproduce. Eventually, the "normal" gene all but disappears, and the majority of kittens born are all super jumpers.
Meanwhile, all those cats down in the chasm are busy trying to learn to build ladders. But that's another story...