Power of concordant versus discordant sib pairs at different penetrance levels

Knowing the answers, we used the GAW11 data set to compare the power and ef ficiency of discordant versus concordant affected sib pairs for qualitative traits at different levels of penetrance. Samples of 200 concordant sib pa irs outperformed discordant sib pairs for low penetrance (40%) and 70% pene trance models while at 90% penetrance they performed equally well. Increasi ng the sample size of discordant sib pairs to twice that of concordant pair s was not enough to reach the power of concordant sib pairs at the 40% and 70% penetrance models. For low penetrance using a combination of concordant and discordant sib pairs resulted in higher power than using discordant si b pairs alone. At 90% penetrance, the power of concordant and discordant si b pairs was similar in the region close to the gene while concordant sib pa irs performed better at locations further from the gene. (C) 1999 Wiley-Lis s, Inc.