How to Mix and Match Chromosome Numbers

The seventh chromosome is just over 159 million base pairs. It contains 989 working genes, which means there's 988 places between them where you could have a crossover event. The probability of having one child and then having another identical child is so unlikely, it's the equivalent of out of the entire observable universe,. picking out a single proton and going, oh, that’s cool, mixing it back into the observable universe... And in fact, doing that four times. That seems to be the way it works.