Power calculation for QTL linkage analysis can be performed via simple alge
braic formulas for small pedigrees, but requires intensive computation for
large pedigrees, in order to evaluate the expectation of the test statistic
over all possible inheritance vectors at the test position. In this report
, we show that the noncentrality parameter for an arbitrary pedigree can be
approximated by the sum of the variances of the correlations between all p
airs of relatives, each variance being weighted by a factor that is determi
ned by the mean correlation of the pair. We show that this approximation is
sufficiently accurate for practical purposes in small to moderately large
pedigrees, and that large sibships ate more efficient than other family str
uctures under a range of genetic models.