Two kinds of ranking

There are two kinds of ranking known as variable ranking (Fig. 4.1) and constant ranking (Fig. 4.2), and these are related to single-gene and many-gene resistances.

Variable ranking is often called a differential interaction in which more than one host differential is required to identify one parasitic ability, and more than one parasite differential is required to identify one resistance. Variable ranking normally involves qualitative differences such as single-gene resistances. A special differential interaction called the Person/Habgood differential interaction (Fig. 4.4) is the definitive characteristic of a gene-forgene relationship, and the vertical subsystem (Robinson 1976). However, other differential interactions are possible (see 6.1).

Constant ranking normally involves many-gene, quantitative differences. Once a constant ranking is established, a single host individual will measure any level of parasitic ability, and a single parasite individual will measure any level of resistance.

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