A note on ascertainment probability in the Allen/Hrubec twin model.

In their general model of twin concordance Allen and Hrubec recently introduced the concept of secondary ascertainment rate to consider partial ascertainment of secondary cases. This concept is found to be of limited interest because only pairs where valid information can be obtained from both twins should be included in genetic analyses. It is furthermore shown that the validity of the proband method and the Allen/Hrubec model depends on the equivalence of ascertainment in twins from concordant and discordant pairs. However, if this condition is fulfilled, it has been shown how the ascertainment probability in cotwins of probands is related to the overall ascertainment probability in the total population.