The distribution of the IgAa and IgAb alleles of porcine IgA in over 160 randomly-selected animals revealed an abundance of heterozygotes but only two b/b homozygotes. Since the IgAb allotype is a splice site mutant lacking two-thirds of the hinge, this study tests the hypothesis that pigs with this genotype may be at a selective disadvantage while heterozygous individuals 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, young animals of known parentage had genotypic frequencies identical to that expected for Mendelian alleles but that a/b heterozygotes were overrepresented in adults. However, when analyzed with regard to breed, a very strong association between breed and the frequency of the IgAa and IgAb alleles was discovered. Meishan and NIH minipigs were homozygous for IgA while heterozygotes predominated in Berkshire, Chester White, Durocs, Hampshire and Landrace. Animals homozygous for IgAb were best represented in the White Cross line. We show here that this very strong breed dependency of IgA allotypy in swine can produce a sample bias that can explain why only two b/b homozygotes (1.3%) were found in the 160 randomly-selected samples since the original samples came from primarily Landrace and Yorkshire animals. The expected 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 that IgAa and IgAb appear to be simple, breed-dependent allotypic markers.