Transmission Disequilibrium Test (TDT)

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Definition: Transmission Disequilibrium Test (TDT)

To test the presence of a genetic linkage between a genetic marker and a trait, Spielman invented the TDT (transmission disequilibrium test). It will detect genetic linkage only in presence of a family-based association. Later this test was established as a statistical test for finding out association. The null hypothesis can be tested by population based test or family based test. The power of TDT is more when association is higher.

E.g.: Let us consider a basic family with one affected child. In this case, TDT is considered to be trios: two parents and one affected child. This is a case of n affected off springs having 2n parents.  The tdt aims to measure the transmission of altered gene from heterozygous parents to the affected offspring. We can summarize the data into the following table:




Non-transmitted gene






Transmitted gene







A + B




C + D


A+ C



Here, we find that the TDT indicates that only the heterozygous parents should be used which indicates a total of B + C and also, if the proportions b/ (b+c) and c/ (b+c) are well-matched with equal probabilities i.e. (0.5, 0.5).

Hence, this concludes the definition of Transmission Disequilibrium Test (TDT) along with its overview.

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