Gregor Mendel’s law of segregation states that if there are two alleles present at a locus in a heterozygotic diploid organism, the probability that one of them will get into a given gamete is 50%. The precision of that probability, perhaps most easily seen in the chance of having a son or a daughter, is impressive. (It probably evolved because the majority of genes mobilized defenses against selfish minorities that sought to distort transmission for their benefit at the expense of majority interests.)