Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 37 / 6 / iv / Solution

For binary traits, $Y_1{Y}_2$ is the indicator that both individuals are affected. Hence the pair probability is its expectation, or equivalently the covariance plus the product of individual means. Using the preceding results,These statements retain the same conditional-independence and common-population assumptions as the multiplicative diallelic penetrance covariance calculation.