Greg Cochran pointed out something that I’d been considering about the MacArthur et al. paper: if the average human (OK, non-African human) has ~100 loss-of-function variants, then the standard deviation should be ~10. That’s because the distribution is presumably poisson, and variance = mean, and the square root of the of the variance (~100) is the standard deviation (~10). In plainer English there should be a substantial variation in the number of loss-of-function variants within a population, and across siblings. Though by definition these loss-of-function variants don’t kill you, in general there is the assumption that this class of mutants does exhibit some fitness drag (e.g., the fitness of a heterozygote for a variant which is lethal as a homozygote genotype may be ~0.90). A quick back of the envelope calculation implies to me that there is a 1 out of several hundreds of thousands probability that two siblings may exhibit a range of 60 loss-function-variants. But a 40 unit gap is more like a 1 out of one thousand chance.
This variance in mutational load has been the hobby-horse of intellectuals for a while now. Armand Leroi suggested that it correlated with beauty. Geoffrey Miller with intelligence. In the near future presumably we’ll get to see if there’s anything real in this. And obviously we don’t need to leave it to scientists. We’ll all know the summary statistics about own genomes, and probably be able to intuit rough patterns…if they exist.