Examination of h2 vs. gene density builds upon a literature demonstrating that essential genes expressed in many tissues can occur in dense clusters of high expression, including instances of transcriptional co-localization.65–67 Essential genes identified in mouse mutagenesis screens show high linkage conservation;68 and intergenic regions in humans have higher SNP densities than in introns, along with higher rates of neutral polymorphisms.69,70 Our observations appear concordant with these reports, whether selection directly inhibits heritability in gene-dense regions or due to the relative paucity of genotype variation in such regions.
