We take the $1-alpha$ quantiles of the individual LOD
scores. In the case of X-chr-specific permutations, we use the combined length
of the autosomes, $L_A$, and the length of the X chromosome,
$L_X$, and calculate the area of the A:A, A:X, and X:X regions as
$L_A^2/2$, $L_A L_X$, and $L_X^2/2$, and then use the
nominal significance levels of $1 - (1-alpha)^p$,
where $p$ is the proportional area for that region.