The classic transmission/disequilibrium test (TDT) proposed by Spielman et al.(1993) for analysis of familybased case-control studies is a matched x2 test referred to as “McNemar’s test.” However, the same data also could be analyzed by a likelihood model, as suggested by Terwilliger (1995). In the present letter, we study the maximum likelihood (ML) statistic derived from this model and show that, when a common asymptotic threshold is used, the ML statistic is expected to be slightly more powerful than the classic McNemar x2. We also investigate the influence of linkage disequilibrium and allelic frequencies on the power of the ML-TDT, with comparison with the results obtained by means of the classic TDT in a recent study (Risch and Merikangas 1996).

Using the notation in Spielman et al.’s (1993) table 2, we consider a marker locus with codominant alleles M1 and M2, where b (c) is the number of alleles M1 (M2) transmitted from a heterozygous M1M2 parent to an affected child. The classic TDT test is, 2 2 x (b c)/n