Answer
$P(E)=0.25$
If the husband and wife have 4 children, we would expect that one will not have Huntington’s disease.
Work Step by Step
Event = "the offspring will not have Huntington’s disease" = {ss}. So, $N(E)=1$.
The sample space: S = {SS, Ss, sS, ss}. So, $N(S)=4$.
Using the Classical Method:
$P(E)=\frac{N(E)}{N(S)}=\frac{1}{4}=0.25$