The B blood group system of chickens. III. The effects of two heterozygous genotypes on the survival and egg production of multiple crosses.

N a previous paper of this series (BRILES, ALLEN and MILLEN 1957) data were 'presented showing that the B blood group system of chickens constituted a case of genetic polymorphism. Of 73 closed populations tested, 71 had two or more alleles. Moreover a detailed serological study disclosed the existence of 21 alleles in 12 lines derived from nine different original stocks. GILMOUR (1959) studied four very highly inbred lines of widely differing origins. (inbreeding coefficients 98.0-99.4) and found two or more B alleles in each, with a total of nine different alleles. Several other blood group systems are also polymorphic, although there is less evidence available for them (BRILES 1956; GILMOUR 1959, 1960). It was concluded by BRILES et al. ( 195 7) that the polymorphism of B was of the balanced rather than the transient type in that segregation was continuing in almost all the populations, even those highly inbred. GILMOUR'S (1959) evidence supported this conclusion for the B system, and extended it to other systems still segregating at very high levels of inbreeding. The most obvious mechanism for such continued segregation would be a selective advantage of heterozygotes over homozygotes (although other mechanisms are possible, e.g., as suggested by HALDANE 1962). Direct evidence of heterozygote superiority during inbreeding was presented by BRILES (1 956) and GILMOUR ( 1960), and more fully for the B system in the second paper of this series (BRILES and ALLEN 1961). This latter paper presented data based on 7732 fertile eggs of seven inbred lines, and provided evidence of association in some lines between B blood type and juvenile viability, adult viability and egg production. The overall effect was one of heterozygote superiority in viability and reproduction. In the present paper experiments are described in which the comparisons of viability and egg production were made, not between heterozygote and homozygote within inbred lines, but between two different heterozygotes in crossbred individuals. The findings may thus have somewhat more relevance to the question of polymorphism in outbreeding populations.