Sibship disequilibrium test (SDT)

The dataset consists of sibships having at least one affected and unaffected sibling.

It tests whether particular alleles are commoner among affecteds than unaffecteds in more than half the sibships studied (ignoring sibships where the frequency is equal).

The frequency of the A allele is higher among affected than unaffected subjects in both sibships.

This method uses all members of sibships (unlike SIBASSOC, which uses only one affected subject) and is a test for association as well as linkage (unlike S-TDT).

A combined test on multiple alleles can be performed.

Data can be combined with genotypes from families having parents available for genotyping.

It is expected to be less powerful as a test for linkage than S-TDT, but is preferable as a test for association.