NOTE 17L: An exact calculation, made by integrating the differential equation as above, gives 7770 generations. This is substantially shorter, mainly because the strong advantage in the homozygote greatly shortens the time taken to go from 0.1 to 0.99. However, the rough estimate of 13,800 generations is much closer to the exact figure of 7770 than the time taken for a strictly recessive allele of ~10^{6} generations.