Conditions for Young's equation:
(1) autosomal dominant condition
(2) both parents clinically unaffected
(3) calculating the risk for the sibling of an affected child
(4) integrates the condition of having normal siblings in addition to the affected child
risk of recurrence =
= (0.5 * (penetrance) * (1 - (penetrance)) * ((1 - ((penetrance) / 2)) ^ (number of unaffected siblings))) / (((1 - (penetrance)) * ((1 - ((penetrance) / 2))^(number of unaffected siblings))) + ((penetrance) * (1 - (fitness of affected heterozygotes))))
where:
• fitness is that of the affected (= penetrant) heterozygote (Friedman's equation uses the fitness of all heterozygotes)
• penetrance and fitness are decimal fractions from 0 to 1.
If only the affected child is present (no healthy siblings), this equation becomes
risk of recurrence =
= (0.5 * (penetrance) * (1 - (penetrance))) / ((1 - ((penetrance) * (fitness of affected heterozygotes))))
