A note on including phenotypic information from monozygotic twins in variance components QTL linkage analysis

Williams & Blangero (1999) derived closed form expressions for the power of a univariate variance components test of linkage for a variety of pedigree structures. We have extended their results by investigating the effect of including monozygotic twins in the design on the power to detect linkage. Specifically, we determined the power associated with a pedigree of size three, where individuals one and two were monozygotic twins and individual three was a full sibling to the twins. The power of this sampling unit was uniformly greater than the power obtained from a sib-pair under the same genetic model. The reason for this was that addition of a second monozygotic twin provided another estimate of the sibling correlation for the particular IBD class. In addition, when the total heritability of the trait was <50%, the number of individuals that needed to be phenotyped was less than that with sib-pairs alone. However, a pedigree consisting of a monozygotic pair and sibling was never as informative as a sib-trio, presumably because the sib-trio provided information about allele sharing between three individuals, whereas the monozygotic twins and sibling unit only provided one such relationship. We conclude that including a monozygotic twin in the analysis is an economical strategy, since only one twin needs to be genotyped.