Quantitative genetics of intraspecies hybrids

Quantitative genetics generally is based on the properties of the randomly fertilized (RF) population or inbred derivatives of it. Simple hybrids and hybrid swarms do not conform to this model; and only some properties of hybrid means appear to have been available. In this paper, several genetical properties are derived, including genotype and allele frequencies, genotypic variance, broad-sense heritability, and outbreeding coefficient. The earlier mean is confirmed, and hybrid vigour is examined critically. These results make it possible to evaluate quantitatively both natural selection and forward selection (in plant breeding) from hybrids. An important finding is that hybrids with maximum hybrid vigour do not maximize genetic advance from forward selection, i.e. evolution is unlikely to enhance hybrid vigour. Another finding is that the concepts of additive genetic variance and narrow-sense heritability are inappropriate for hybrids, owing to the genetic disequilibrium inherent from their origin, and to the ephemeral nature of their population structure.