Linkage Disequilibrium

Two linked loci are said to be in linkage disequilibrium if specific combinations of alleles at the loci are seen together on chromosomes more often than expected by chance. For example, if die frequency of allele 1 at locus A (Al) is 0.7 in a population and the frequency of allele 1 at locus B (Bl) is 0.6, then the expected proportion of chromosomes bearing the combination Al/Bl is given by the product of the frequencies of Al and Bl: 0.7 X 0.6 = 0.42. If we examine a series of chromosomes in a population and find that Al and Bl are found on the same chromosome significandy more often or less often than expected (e.g., suppose the frequency of chromosomes containing both alleles is only 0.3)> then the two loci are in linkage disequilibrium. Through time, recombination between two loci tends to distribute alleles so that they are in linkage equilibrium. Because recombination is rare for very closely linked loci, such loci are more likely to exhibit linkage disequilibrium (Fig II-4-3). Thus, the measurement of linkage disequilibrium in populations can be useful in gene mapping.

3 C7

6 is smalt d-0

6 is smalt

t (generations)

Figure 11-4-3. Duration of Linkage Disequilibrium as a Function of Time for Loci Showing Different Recombination Frequencies (6)

t (generations)

Figure 11-4-3. Duration of Linkage Disequilibrium as a Function of Time for Loci Showing Different Recombination Frequencies (6)

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