Answer
$P(E)=0.75$
Work Step by Step
Event = "the offspring will have Huntington’s disease" = {SS, Ss, sS}. So, $N(E)=3$.
The sample space: S = {SS, Ss, sS, ss}. So, $N(S)=4$.
Using the Classical Method:
$P(E)=\frac{N(E)}{N(S)}=\frac{3}{4}=0.75$