Phenotypic, Genetic and Environmental Correlations

and in many instances the estimate of a phenotypic correlation is reported smaller in magnitude than that of the corresponding genetic correlation, e.g. with certain poultry records, in Lerner & Cruden [1948], sheep records in Morley [1951] and with certain dairy records in VanVleck [1960] and Searle [1961]. Such results may seem a little unexpected at first sight since phenotype includes genotype and one might anticipate the correlation between phenotypes to be larger than that between genotypes. When estimates have not followed this pattern the explanation is sometimes given that a phenotypic correlation less than a genetic correlation is the result of a negative environmental correlation in the records of the two traits. This paper investigates the relationship between these three correlations on the basis of a linear model, and demonstrates the situations in which this explanation is correct. Other comparisons are also made.