Well, we could have used weights to determine the second parent as well, but this way we ensure every parenting individual has a chance to participate in the breeding process.

The breeding selection algorithm picks one parent by looking at the weights - in effect, every individual gets a chance to be a parent, but the number of parent slots is finite.

Although the number of bones is roughly the same in all breeds, there is considerable variation in the size and shape of individual bones, as the result of selective breeding.