Under the multiplicative polygenic model and the logistic model for lifetime risk of breast cancer, risk is assumed to be due to the multiplicative actions of many 'polygenes' that are assumed to have a normal distribution across the population. The relative frequency is indicated by the solid blue line scaled to the left-hand vertical axis. The lifetime risk (cumulative risk to age 75 years) for women in the general population (assumed to be, on average, 11%) increases logistically, as indicated by the solid black line scaled to the right-hand vertical axis. For women with a strong family history, their polygene distribution is shifted to the right by a little more than one standard deviation such that they have, on average, a three-fold increased risk. Their relative frequency is indicated by the dotted red line scaled to the left-hand vertical axis.