Genetic polymorphism was analyzed for five blood proteins: albumin – Al, esterase – Es, α₁B-glycoprotein – Xk, transferrin – Tf and hemoglobin – Hb in 200 Thoroughbred (TB) and 124 Argentine Creole (AC) horses. Of the five systems examined, Tf and Hb were not in Hardy-Weinberg equilibrium in either breed and Es was not in equilibrium in the Creole breed. Genetic variability, estimated as average heterozygosity, was higher in AC (H = 0.585 ± 0.131) than in TB (H = 0.353 ± 0.065). The genetic differentiation between these two populations (F_ST) was 0.109. Thus, of the total genetic differences between breeds, the proportion of genetic variation attributable to breed differences was about 10%; the remaining 90% was due to individual variation within breeds. The high degree of genetic variability seen in Argentine Creole horses could be a consequence of natural selection. Selection of TB through the centuries has most likely modified the gene pool of the ancestral population, with a consequent reduction in variability at certain loci. Probably, different mechanisms exist for maintaining polymorphism at these loci in TB and in AC horses. Heterozygosity may have played a fundamental role in adaptation.

Keywords: Horses, Argentine Creole, Thoroughbred, Heterozygosity, Genetic variability, Serum protein polymorphysm.