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Linkage disequilibrium $\left(D\right)$ for two bi-allelic loci is defined as:

$$D=X_{11}X_{22} - X_{12}X_{21}$$

, where $X_{11}$, $X_{12}$, $X_{21}$, X$_{22}$ are the frequencies of the haplotypes $A_1B_1$, $A_1B_2$, $A_2B_1$, $A_2B_2$ respectively (the subscript number correspond the two different alleles and $A$ and $B$ correspond to the two different loci)

My questions are

  • How is $D$ defined for more loci and a greater number of alleles?

  • How is $D$ defined when the number of alleles differ from locus to locus?

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