Curtis D and Sham PC. A note on the application of the transmission disequilibrium test when a parent is missing. Am J Hum Genet 1995 56: 811-812.

The transmission disequlibrium test (TDT) is useful for detecting a susceptibility locus in linkage and linkage disequilibrium with a marker locus (Spielman et al, 1993). Unlike association studies which use unrelated controls it is not prone to produce false positive results in the presence of hidden population stratifications. We wish to point out a situation which may have escaped the attention of some readers where there may be a risk of misapplying the test in a way which can produce false positive results....

....We conclude that when a parental genotype is missing then the remaining parent-child pair should be discarded not only when the child has the same genotype as the parent but also if the child is homozygous. For markers with only two alleles this means that all cases in which one parental genotype is missing must be discarded. For markers with more than two alleles, no bias will arise by including information from single parent-child pairs where the child is heterozygous but has a different genotype from the parent, for example where the parent is AB and the child is AC or BC, and such pairs may be included in the TDT even if the marker genotype for the other parent is missing.

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