A gene-for-gene relationship can be demonstrated genotypically or phenotypically.
A genotypic demonstration of a gene-for-gene relationship requires genetic studies in both the host and the parasite. Such studies can be a lengthy business, often requiring many years of painstaking work. Genetic studies in the parasite are often extremely difficult, as with aphids or rusts, for example. In many other parasites, genetic studies are impossible because the reproduction is entirely asexual. A genotypic demonstration is sometimes assumed from genetic studies in only the host. For example, Black, et all (1953) assumed a gene-for-gene relationship in potato blight (Phytophthora infestans) before the A2 mating type (see 7.20.4) was either known or available.
Genotypic demonstrations were necessary when gene-for-gene relationships were largely unknown, and were a subject of academic study. Today, however, a phenotypic demonstration is entirely adequate for all practical purposes.
Loegering and Powers (1962) designed the 'quadratic check' and suggested that it provided a demonstration of a gene-for-gene relationship. However, Day (1974) pointed out that other situations, such as the presence or absence of an antibiotic, can also produce a quadratic check. F.E. Williams (Private Communication, 1984) determined the minimum requirements for a phenotypic demonstration of a gene-for-gene relationship. These require at least two pairs of matching genes. The minimum matrix consists of two host differentials and three parasite differentials, or vice versa. This matrix must fit into a Person/Habgood differential interaction, and it must include the two by two matrix produced when the two genes are tested, each with the other two (Fig. 4.5).
Figure 4.5 Phenotypic demonstration of a gene-for-gene relationship.
In this diagram, which is derived from the Person/Habgood differential interaction (see fig. 4.4), the Habgood nomenclature is used throughout (i.e., Habgood '3' consists of genes 1 & 2). A red spot is a matching interaction, and a white spot is a non-matching interaction. Note the central matrix obtained from the interaction of genes 1 & 2; this is known as the 'transagonal' matrix.
The minimum requirements for a phenotypic demonstration of a gene-for-gene relationship are a transagonal matrix plus the pair of interactions that occurs either above or below, or to the right or left of it. In other words, the minimum differential interaction must involve (3 x 2) or (2 x 3) genes that include a transagonal differential interaction.
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