However, in an influential paper by Davis and colleagues (number of cases: 1,061; number of controls: 4,236; number of SNPs: 373,846) (5), there was no evidence for heritability from SNPs with minor allele frequency (MAF) < 0.05 and over 60% of total heritability mapped to the most common variants (MAF > 0.3). In addition, in a meta-analysis of data from OCD Collaborative Genetics Association Study (OCGAS) and Davis et al., ~60% of heritability was accounted for by SNPs with MAF > 0.4 in both the OCGAS sample alone and in the combined sample (16). If this observation were true, it could have profound implications for which evolutionary forces shaped this unusual mapping of risk alleles to their population frequency distribution. For example, balancing selection, where multiple alleles are maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone may play a role in OCD.
