As an alternative to the analysis of recurrence risk for OCD within pedigrees, heritability can also be estimated from individuals drawn from a population who have no obvious familial relationships, as long as they have been characterized for genetic variation across their genomes. Usually, this genetic characterization employs genotypes of single nucleotide polymorphisms (SNPs) for which alleles are common in the population. In this approach, which we will call SNP-based, the central idea is that the multiplicity of SNP genotypes allows estimation of familial relationships, albeit distant, among subjects as well as the covariance of their phenotypes, and these are the key elements for estimating heritability. When the heritability of OCD is computed in this manner, estimates range from 25–43% (5, 14–16).
