Identical twins
Wilhelm Weinberg made 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 a formula for estimating the frequency of MZ and DZ twins from the ratio of same and opposite sex twins to the total of maternities. Weinberg also estimated that the heritability of twinning itself was close to zero. That means the capacity to have twins is not hereditary.