Extended TDT

TDT information for a multiallelic marker can be presented in a table showing the number of times a heterozygous parent transmits one allele while not transmitting the other:

                    Transmitted

                 1    2    3    4

             1        4    4    2

Not          2  13        15    5
transmitted  
             3   9   12         3
 
             4   6    3    6    

Thus the 4 in the second column of the first row indicates that 4 parents with genotype 12 transmitted allele 2 to an affected offspring, while the 13 in the first column of the second row indicates that the 13 parents with genotype 12 transmitted allele 1 instead.

If there were no transmission distortion we would expect diagonally opposite elements to be equal. There are three ways for testing for a positive TDT:

• Examining each allele in turn
• Genotype-wise analysis
• Allele-wise analysis