Genetic differences in the frequency of the hinge variants of porcine IgA is breed dependent

The distribution of the IgA(a) and IgA(b) alleles of porcine IgA in over 16 0 randomly-selected animals revealed an abundance of heterozygotes but only two b/b homozygotes. Since the IgA(b) allotype is a splice site mutant lac king two-thirds of the hinge, this study tests the hypothesis that pigs wit h this genotype may be at a selective disadvantage while heterozygous indiv iduals may be at some advantage. This hypothesis was tested by collecting data on 374 animals of known breed and often parentage. We show here that when breed was not considered, youn g animals of known parentage had genotypic frequencies identical to that ex pected for Mendelian alleles but that a/b heterozygotes were overrepresente d in adults. However, when analyzed with regard to breed, a very strong ass ociation between breed and the frequency of the IgA(a) and IgA(b) alleles w as discovered. Meishan and NIH minipigs were homozygous for IgA while heter ozygotes predominated in Berkshire, Chester White, Durocs, Hampshire and La ndrace. Animals homozygous for IgA(b) were best represented in the White Cr oss line. We show here that this very strong breed dependency of IgA alloty py in swine can produce a sample bias that can explain why only two b/b hom ozygotes (1.3%) were found in the 160 randomly-selected samples since the o riginal samples came from primarily Landrace and Yorkshire animals. The exp ected frequency of b/b homozygotes in these breeds would be <3%. Thus, the data presented here reject the hypothesis that swine homozygous for a trait that results in loss of two-thirds of the IgA hinge, are selected against and that heterozygotes are positively selected. Rather, the study shows tha t IgA(a) and IgA(b) appear to be simple, breed-dependent allotypic markers. (C) 2000 Elsevier Science B.V. All rights reserved.