Conditional-Sibling Trait Distribution under Polygenic Architecture.
A: The conditional-sibling trait distribution according to Equation 4 for index siblings at the 1st, 25th, 50th, 75th, and 99th percentile of the standardised trait distribution, when heritability is high (h2 = 0.95, in orange) and moderate (h2 = 0.5, in blue). When heritability is 0.95 conditional-sibling expectation is almost half of the index sibling z-score, when heritability is 0.5 the conditionalsibling expectation is equal to 1/4 of the index sibling z-score. B: The conditional distribution transformed into rank space. An individual whose sibling is at the 99% percentile is expected to have a trait value in the 80% percentile when heritability is high and in the 67% percentile when heritability is moderate.