 One plausible explanation of the limited success of GWA studies depends on the distinction between genotypic variance and variance in actual genotypes being clearly made. Even if the classical quantitative genetic heritability is high and similarity between twins or a set of close relatives is associated with the similarity of yet-to-be-identified genotypes or genetic factors, the factors may not be the same from one set of relatives to the next, or from one location (environment) to the next. In other words, the underlying factors and the pathways of development that they influence may be heterogeneous. It could be that pairs of alleles, say, AAbbcbDDee, subject to a sequence of environmental factors, say, FghiJ, are associated, all other things being equal, with the same outcomes as alleles aabbCCDDEE subject to a sequence of environmental factors FgHiJ (Taylor 2012). The possibility of heterogeneous factors underlying similarity in traits obviously recedes if the biological materials and locations are close to the original set of relatives and environmental factors. The corollary is that, when users of quantitative genetic models overlook that possibility, they are, in effect, assuming tight control of biological materials and conditions.