For quantitative traits, in which phenotypic scores are (or can be transformed to be) normally distributed, the efficacy of a genomic profile is naturally expressed as the proportion of the genetic variance explained by the profile. The variance in phenotypes, VP, can be partitioned into variance of genetic values, VG, so that the proportion of the variance that is genetic is the heritability VG/VP. Genomic profiling provides a direct estimate, , of true genetic values, G, for individuals in a population and the efficacy of a genomic profile can be expressed as the proportion of the genetic variance explained by the profile /VG. We define = /VG, since in selection theory [11], used in livestock and plant breeding, the correlation between predicted and true genetic risk () is used as the measure of accuracy of prediction, , and if the predictor is unbiased (the regression of G on is 1), . The ratio /VP is estimated as the R 2 from the regression of P on and is interpreted recognising its upper limit to be VG/VP or heritability. These measures
