Description of The Test
We first describe the TDT in the case where families consist of trios (two parents and one affected child). Our description follows the notations used in Spielman, McGinnis & Ewens (1993).
The TDT measures the overtransmission of an allele from heterozygous parents to affected offsprings. The n affected offsprings have 2n parents. These can be represented by the transmitted and the nontransmitted alleles and at some genetic locus. Summarizing the data in a 2 by 2 table gives:


Total  
Transmitted allele  
a  b  a + b  
c  d  c + d  
Total  a + c  b + d  2n 
The derivation of the TDT shows that one should only use the heterozygous parents (total number b+c). The TDT tests whether the proportions b/(b+c) and c/(b+c) are compatible with probabilities (0.5, 0.5). This hypothesis can be tested using a binomial (asymptotically chisquare) test with one degree of freedom:
Read more about this topic: Transmission Disequilibrium Test, The Case of Trios: One Affected Child Per Family
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