|MadSci Network: Genetics|
Many phenotypic traits result from the interaction of alleles at more than one gene locus and the environment; that is, the trait is multifactorial. When more than one locus is involved in a trait, the loci can be either connected or independent. If the alleles at each locus assort randomly into gametes, one would expect that the frequency of a given allele at one locus would have no effect on the frequency of an allele at the second locus involved in the trait; that is, they are independent from one another. Such a situation is linkage equilibrium. Linkage equilibrium occurs in a population with a trait involving N loci when the frequency of any genotype is equal to the product of the individual frequencies at each of the N loci. For example, if a trait is determined by 2 loci, let's say A and B, each with two alleles (A/a and B/b) and at each locus an allele appears in the population with a frequency. At one locus, we can call these freq(A), freq(a) and at the other, freq(B) and freq(b). There there are four possible gamete types: AB, aB, Ab, and ab and with a population in linkage equilibrium, we would expect: freq(AB) = freq(A) x freq(B) freq(aB) = freq(a) x freq(B) freq(Ab) = freq(A) x freq(b) freq(ab) = freq(a) x freq(b). In other words, D, the coefficient of linkage disequilibrium = Freq(AB) x Freq(ab) - Freq(Ab) X Freq(aB) As you can see, if A assorts independently of B, then Freq(AB) x Freq(ab) = Freq(Ab) X Freq(aB) AND D = 0. If A is linked to B, then Freq(AB) will not equal Freq(A) X Freq(B), etc to a point in the extreme, where A always goes with B, and a always goes with b, such that A and B are tightly linked and thus D = 1. In this case, the population is in complete linkage disequilibrium. Hope this helps,
Try the links in the MadSci Library for more information on Genetics.