Teasing apart the contributions to heritability of environmental factors shared among relatives will soon be possible because the availability of genome-wide markers now provides empirical estimates of identity-by-descent (IBD) allele sharing between pairs of relatives. For example, full sibs share on average half their genetic complement, but this proportion can vary—in one large study it ranged from 0.37 to 0.62 (ref. 38). By relating phenotypic differences to the observed IBD sharing fraction among sib pairs, marker data were used to generate a heritability estimate of 0.8 for height38. This is remarkably consistent with estimates using traditional methods but free of their assumptions, suggesting that for height at least, heritability is not over-estimated. Applying such estimation to distantly related or ‘unrelated’ individuals is now feasible using dense genomic scans39; given the number of people with dense genotyping data, heritability estimates could be generated for a wide variety of traits free of potential confounding by unmeasured shared environment.
