Robinson (1987) also showed that there were other categories of differential interaction that were not due to a gene-for-gene relationship, and which should not be confused with vertical resistance. Nor should the term 'vertical' be applied to these alternative differential interactions. They all have genetic differences between the differentials that are greater than those of the gene-for-gene relationship. Ten such differential interactions have been recognised but, no doubt, there are others. In general, they are not important in plant breeding, but their occurrence should be noted, as they can occasionally give a false appearance of vertical resistance. Anyone requiring additional information should consult the original description.
Perhaps the only important consideration here concerns interspecific hybridisation. Hybridisation in the parasite leads to a loss of parasitic ability. Hybridisation in the host leads to a gain in resistance. But this gain is lost when there is a corresponding degree of hybridisation in the parasite. These changes can be misleading during studies of inter-specific hybrids.
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