Sample Sizes for the Transmission Disequilibrium Tests: Tdt, S-tdt and 1-tdt

The transmission/disequilibrium test (TDT) is widely used to detect the linkage disequilibrium between a candidate locus (a marker) and a disease locus. The TDT is a family-based design and has the advantage that it is a valid test when population stratification exists. The TDT requires the marker genotypes of affected individuals and their parents. For diseases with late age of onset, it is difficult or impossible to obtain the marker genotypes of the parents. Therefore, when both parents’ marker genotypes are unavailable, Ewens and Spielman extended the TDT to the S-TDT for use in sibships with at least one affected individual and one unaffected individual. When only one of the parents’ genotype is available, Sun et al. proposed a test, the 1-TDT, for use with marker genotypes of affected individuals and only one available parent. Here, we study the sample sizes of TDT, S-TDT, and 1-TDT. We show that the sample size needed for the 1-TDT is roughly the same as the sample size needed for the S-TDT with two sibs and is about twice the sample size needed for the TDT.