Validation and aplication of an interval factor in estimating age at onset of Huntington's disease.

The time interval between the first appearance of signs in the transmitting parent and the birth of the subsequently affected child has been shown by Brackenridge and Telscher (1975) to influence the age at onset of Huntington's disease. The cirticism by Burke (1976) that the interval factor offers no predictive advantage over parental onset age is refuted. The advantage of small sibship sizes in familial correlation studies is noted and an equation to estimate onset age is derived to control for ascertainment bias. The interval factor is shown to surpass parental onset age as a determinant of offspring onset age. When applied to Queensland material, reasonable agreement is obtained between predicted and reported onset ages. Evidence for the desirability for parents at risk who intend to have families to plan them early is discussed.