Optimal ascertainment strategies to detect linkage to common disease alleles.

Traditionally, extended pedigrees with many affected individuals have been studied for the purpose of detection of linkage. For traits caused by a rare susceptibility allele, this is a productive strategy. However, this sampling strategy may not work well for traits determined by multiple loci in which one or more have common susceptibility alleles. We simulated three single-additive-locus models of inheritance and two-locus models with additive or multiplicative interactions, all with rare or common susceptibility alleles. A trait locus was linked, with no recombination, to a marker locus with four equally frequent alleles. Family structure varied, but the total number of affected individuals was held constant. Two generations of individuals were genotyped. We used three nonparametric affected-sib-pair programs and two nonparametric pedigree-analysis programs to perform linkage analysis. For single-locus, additive, and multiplicative models, we found that, when the susceptibility allele was rare, (frequency .0025), extended pedigrees with first or second cousins had the most power for detection of linkage. However, when the susceptibility allele was common in the single-locus, additive, and multiplicative two-locus models (frequency .25), extended pedigrees were no more powerful than nuclear families. There was also a decrease in power when the pedigrees had a greater number of affected individuals, more so for the single-locus and multiplicative models than for the additive model. We conclude that for single-locus, additive, and multiplicative models of qualitative traits with common alleles, there is no benefit to the collection of extended pedigrees, and there may be a loss of power in the collection of pedigrees with many affected individuals.