Encyclopedia of Life Sciences Linkage Equilibrium and Disequilibrium

Haplotypes Linkage equilibrium and disequilibrium Additive, multiplicative, epistatic tness regimes Causes of linkage disequilibrium Extent of disequilibrium in nature Multilocus population genetics is the study of the distribution and dynamics of genetic variation at several loci in biological populations. Haplotypes Even the simplest known biological organisms have thousands of genes and millions of DNA base pairs. This makes it necessary to study the distribution and dynamics of genetic variation at many loci (or genetic markers) simultaneously. A particular combination of genes (or genetic markers) for a DNA segment under consideration is referred to as haplotype. In what follows diierent haplotypes will be represented by sequences of bold letters corresponding to diierent alleles (for example A, aB, ABC) whereas the frequencies of these haplotypes in the population will be represented by variable x with a corresponding subscript (for example x A ; x aB ; x ABC). If a diploid population is at Hardy-Weinberg equilibrium, then genotype frequencies can be found as products of the corresponding haplotype frequencies. In this case, the population state can be described in terms of the haplotype frequencies. If there is no statistical association of alleles in haplotypes, then the frequency of a haplotype is equal to the product of the corresponding allele frequencies. In this case, one says that the population is at linkage equilibrium. For example, in such a population x aB = x a x B ; x ABC = x A x B x C and so on. If there is statistical association of alleles in haplotypes, one says that the population is at linkage disequilibrium. The term \linkage disequilibrium" is somewhat misleading because even unlinked loci can be in \linkage disequilibrium" and because other factors besides linkage can aaect the degree of statistical association of diierent alleles. Alternative terms used are \gametic (dis)equilibrium" and \gametic phase (dis)equilibrium".]