[Genetic study in twins].

During the period june 1987-January 1988, 35 consecutive pairs of twins born at two maternity hospitals in the northern area of Santiago were studied to search evidence of genetic or other factors which could eventually favour twinning. Sixteen of these pairs were considered to be monozygotic (MZ) after studying five blood group systems (ABO, Rh, MNSs, Duffy and Kidd) applying the method of Smith and Penrose. Maternal age was higher in dizygotic twins (DZ) than in the MZ group (average 30.1 yrs. v/s 26.8 yrs. respectively (p < 0.05). Family history of twinning was positive in 73% of the MZ and in 82% of the DZ twins. The Duffy and Kidd systems were not in Hardy-Weinberg equilibrium due to a deficit of heterozygotes in MZ as well as in DZ twins. There was a significant increase in MS and Ms haplotypes in both groups of twins respect to the Chilean general population (p = 0.01). These results suggest the presence of a genetic determinant that favors the production of twinning of DZ as well as MZ type. A larger number of cases will be required to test this hypothesis.