Additional contributions by Weinberg to statistical genetics included the first estimate of the rate of twinning . Realizing that identical twins would have to be the same sex , while non-identical twins could be either same or opposite sex , Weinberg derived the formula for estimating the frequency of MZ and DZ twins from the ratio of same and opposite sex twins to the total of maternities . Additional contributions by Weinberg to statistical genetics included the first estimate of the rate of twinning - Realizing that identical twins would have to be same-sex , while dizygotic twins could be either same or opposite sex , Weinberg derived the formula for estimating the frequency of monozygotic and dizygotic twins from the ratio of same sex and opposite twins to the total of maternities .