Genetic Variation and Covariation in Egg Production and Other Economic Traits in Chickens

IN THE absence of certain disturbing factors selection is expected to increase the mean of a population each generation by an amount equal to the product of the selection differential and heritability. Lerner and Dempster (1951), however, presented evidence for smaller gains in shank length and egg production than would be expected from the selection practiced. Goodale (1949) presented results of 23 years of selection for egg production and reported decreased gains in the later stages of selection. Gyles et al. (1953) presented data from a selection study showing the actual selection achieved for body weight to be significantly less than that expected. Dickerson (1951) and Kottman (1952) found similar situations in swine. Failure to realize the gains expected would lead one first to question the magnitude of heritability estimates, since selection for a trait tends to be effective until the desired alleles are fixed. Other factors may interfere with …