The figure of 165 homozygous protein-truncating variants per average genome seems awfully high. Indeed, the text indicates that the “real” number is probably rather lower, due to things like the inadvertent consideration of pseudogenes. But it would still be pretty high, say over 100. If there are 20,000 translated genes, that’s 1 in 200! The simplest cocktail-napkin calculation would then suggest that ~7% of translated autosomal genes are expressed by only one allele. Again, a non-intuitively high figure–especially considering the dosage effects of one-allele expression of some genes, famously BRCA1 and BRCA2.

Consider one of the more reliable associations between genes and behaviour: a gene known as KIAA0319 which has been found to relate to reading ability in several different samples. In one study, an overall association was reported with a p value of 0.0001, indicating that the likelihood of the association being a fluke is 1 in 10,000.