Variability of transferrins in Bovinae is controlled by two loci: Tf (the locus of structural transferrin gene) and T (the locus of a gene responsible for protein modification). Originally, the ancestors of Bovinae, like the other ruminants, had the R type of transferrin (T-/T- genotype, inactive T gene). Later on, the T gene activation occurred and the B type appeared (T+/T-, T+/T+ genotypes). The subsequent evolution of Bovinae was accompanied by almost complete fixation of T+ allele. In one of the Bovinae representatives (Bos taurus L.) the frequency of T- allele remained at the level of approx 0.1. A hypothesis is proposed which explains the change of the transferrin type in Bovinae by imitation of multiple allelic forms of this protein present in Tf heterozygotes, by supplementary modificatory multiple transferrin forms arising on activation of the minor gene (T+). This process is assumed to involve the reduction of Tf locus variability. Since this hypothesis proceeds from the assumption of TF heterozygote advantage, the question is considered, whether this assumption is compatible with high polymorphism of Tf locus which significantly exceeds that for other loci.