ALTHOUGH the importance of genotype-environmental interactions has long been recognised, it is only in the last decade that it has been possible to describe and predict this component of an individual's phenotype. This success followed the recognition that the phenotypic character under study itself provides a quantitative assessment of an environment (Yates and Cochran, 1938), and the finding that phenotype was often linearly related to such environmental values (Finlay and Wilkinson, 1963; Perkins and Jinks, 1968). The various regression approaches that have been used have been described and compared in a previous paper (Fripp and Caten, 1971). Inmost previous studies (e.g. Finlay and Wilkinson, 1963; Eberhart and Russell, 1966; Perkins and Jinks, 1968; Breese, 1969) the environmental values used were the mean yield in each environment of the particular set of genotypes under investigation, calculated from the actual data being analysed for its genotype-environmental interactions. This use as independent variable of values which are not independent of the phenotypic variable regressed on to them, has been criticised by Freeman and Perkins (1971). The regression technique can still be used to study genotypeenvironmental interactions provided the environmental values used are independent of the data being analysed. Freeman and Perkins (1971) list and discuss a number of ways in which independent environmental values might be obtained and describe a biometrical-genetical model and a regression analysis (approach 3 in Fripp and Caten, 1971) appropriate to such values. The many possible methods of assessing the environments may be grouped into four major categories:
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