Mendelian breeding emphasises single genes, and genetransfers by pedigree breeding. Biometricians' breeding emphasises polygenes, and changes in their gene frequencies by population breeding, recurrent mass selection and transgressive segregation. For example, horizontal resistance is a polygenic character, and its level can be increased by increasing the frequency of its polygenes in a single individual. Gene pool
The totality of genes possessed by a population of sexually reproducing organisms. Gene-for-gene relationship
A gene-for-gene relationship exists when each gene for resistance in the host has a corresponding (or matching) gene for parasitic ability in the parasite. This phenomenon is the definitive characteristic of the term vertical, and the concept of the vertical subsystem. The gene-for-gene relationship was discovered by H.H. Flor in 1940. When the host and parasite genes match, the vertical resistance does not operate, the infection is successful, and parasitism occurs. When the genes do not match, the vertical resistance functions, the infection is unsuccessful, and parasitism does not occur.
The sole function of the gene-for-gene relationship is to control the population explosion of an r-strategists parasite, which usually has an asexual reproduction that leads to a particularly rapid multiplication. This control is commonly achieved by reducing the proportion of allo-infections that are matching infections. But it can also function by reducing the growth, and hence the reproduction, of a non-matching parasite (see quantitative vertical resistance). A gene-for-gene relationship will evolve only in a discontinuous pathosystem, and in seasonal host tissue (i.e., annual plants, or the leaves and fruit of deciduous trees or shrubs).
For mathematical reasons, it is thought that all individuals of both host and parasite in a wild plant pathosystem have half of the total genes available. This provides the maximum heterogeneity, and the maximum effectiveness, for a given number of pairs of genes in a system of biochemical locks and keys (see n/2 model).
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