The specificity of phenotypic expression and inheritance of immunoglobulin allotypes IgG1a and IgG2b in pigs are discussed. It was shown that a negative state by both these allotypes is repeatedly found among newborn pigs but is extremely rare in pigs older than one month. A model, which simultaneously describes the genetic determination of allogroups formed by allotypes IgG1a and IgG2b and the dynamics of ontogenetic expression of individual genotype by system IgG with a result of a visually registered phenotype, was developed. Allele frequencies by system IgG were assessed in populations of domestic pigs of productive breeds, laboratory miniature pigs, and wild Eurasian pigs.