Lecture 4 Heritability

The reason for our focus, indeed obsession, on the heritability is that it determines the degree of resemblance between parents and offspring, which in turn determines the response to selection. In particular, the slope of a midparent-offspring regression is just h = VA/VP . The fact that the regression involves midparents implies sexual reproduction. In many plant breeding settings, the parent-offspring regression involves offspring that are asexual clones of the single parent. In this case, the parent-offspring regression has slope given by the broad-sense heritability,H = VG/VP . When we refer to heritability (without making use of eitherh orH), we are by default referring to the narrow-sense heritability h. Use of the broad-sense heritability H is generally restricted to discussions of clones (such as identical twins or asexual propagates of an individual). While H also gives the total fraction of variation in a trait due to differences in genotypic values, for sexually reproducing species only variation in breeding values is (easily) converted into selection response. Hence, h rather than H is a better measure for sexual species of the fraction of (easily) usable genetic variance.