Linkage disequilibrium refers to tendency for "two alleles" to be present on the same chromosome (positive linkage disequilibrium), or not to segregate together (negative linkage disequilibrium). As a result, specific alleles at two different loci are found together more or less than expected by chance. Linkage disequilibrium is the non-independence, at a population level, of the alleles carried at different positions in the genome. In this case, the expected frequency of a two-locus haplotype can be calculated as the probability of the occurance of two independent (or joint) events simply by multiplying their gene frequencies. The same situation may exist for more than two alleles. Its magnitude is expressed as the delta value and corresponds to the difference between expected and observed haplotype frequency. If there is no linkage disequilibrium, delta will be zero (or not significantly different from zero), and if there is positive linkage disequilibrium, it will be positive value
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