ABC of clinical genetics. Estimation of risk.

1/4 1/4 The 50% risk of developing a condition for the offspring of an affected person may be modified by age in disorders whose onset is in adult life, such as Huntington's chorea. In examples 1 and 2 the risk to person B of developing Huntington's chorea is still 50% at age 30 years, but by the age of 65 the residual risk to a healthy person has fallen to 10%. The risk to person C therefore falls from 25% in example 1 to 5% in example 2. In example 3 the risk for C cannot be reduced below 25% because parent B, although clinically unaffected, died aged 30 while still at 50% risk.