= Solution
For the usual no-linkage reference model with unrelated, noninbred parents, define $J$ as the number of homologous <allele> copies shared <identical by descent> by two <full siblings> at the marker. The two siblings receive the same paternal copy with <probability> $1/2$, and independently receive the same maternal copy with <probability> $1/2$. Therefore <Mendelian IBD sharing of full siblings> gives
$$
P(J=0)=\frac14,\quad P(J=1)=\frac12,\quad P(J=2)=\frac14,\qquad J\sim\operatorname{Bin}(2,1/2).
$$
Hence
$$
\boxed{E[J]=1,\qquad \operatorname{Var}(J)=\frac12.}
$$
For affected siblings, absence of <genetic linkage> makes the marker transmissions independent of selection by disease, so these are the relevant null moments. This is a fresh outbred reference calculation, not an application of these sharing probabilities to the inbred siblings $E,F$ in part (b). Without the outbred-parent assumption the three ordinary sharing probabilities require modification.
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