Analysing case and sibling control results for a multiallelic marker

Siblng case-control data for a multiallelic marker can be presented in a table showing the number of times a case receives one allele while their sibling contorl receives another:

                    Cases

                 1    2    3    4

             1        4    4    2

Siblings     2  13        15    5

             3   9   12         3
 
             4   6    3    6    

Thus the 4 in the second column of the first row indicates that in 4 case-control pairs the case had a 1 allele while the control had a 2 allele, while the 13 in the first column of the second row indicates that in 13 pairs the case had a 2 allele while the control had a 1 allele.

If there were no association we would expect diagonally opposite elements to be equal. There are three ways for testing for association:

• Examining each allele in turn
• Testing all allele-pairs
• Allele-wise analysis