Sib transmission/disequilibrium test (S-TDT)

The dataset consists of sibships having at least one affected and unaffected sibling, not all having the same genotype.

It tests whether an allele is commoner among affected rather than unaffected subjects by seeing whether the observed frequency in affected subjects exceeds the expected frequency.

The frequency of allele A in affected subjects is 4/6, but the overall frequency of A is only 5/14.

The method only considers one allele at a time. Data can be combined with TDT data from families having available parents.

When sibships are used it is more powerful as a test for linkage than SIBASSOC, but when there are more than two sibs S-TDT is a test only for linkage, not for association.