have slightly different strengths and weaknesses (see Hedrick 1987; Flint-Garcia et al. 2003). The discussion here will focus on the classical estimator D to develop the conceptual basis of measuring gametic disequilibrium and to understand the genetic processes that cause it.

Gametic disequilibrium The non-random association or combination of alleles at multiple loci in a sample of gametes or haplotypes.

Linkage Co-inheritance of loci caused by physical location on the same chromosome. Recombination fraction The proportion of "repulsion" or recombinant gametes produced by a double heterozygote genotype each generation.

Now that we have developed an estimator of gametic disequilibrium, it can be used to understand how allelic association at two loci changes over time or its dynamic behavior. If a population starts out with some level of gametic disequilibrium, what happens to D over time with recombination? Imagine a population with a given level of gametic disequilibrium at the present time (Dt=n). How much gametic disequilibrium was there a single generation before the present at generation n - 1? Recombination will produce r recombinant gametes each generation so that:

Since gametic disequilibrium decays by a factor of 1 - r each generation,

Dtn = (1 - r)Dtn-1 = a - r)2Dtn-2 = (1 - r)3Dtn_3 . ..

We can predict the amount of gametic disequilibrium over time by using the amount of disequilibrium initially present (Dt0) and multiplying it by (1 - r) raised to the power of the number of generations that have elapsed:

Figure 2.19 shows the decay of gametic disequilibrium over time using equation 2.27. Initially there are only coupling gametes in the population and no repulsion gametes, giving a maximum amount of gametic disequilibrium. As r increases, the approach to gametic equilibrium (D = 0) is more rapid. Equation 2.27 and Fig. 2.19 both assume that there are no other processes acting to counter the mixing effect of recombination. Therefore, the steady-state will always be equal frequencies of all gametes (D = 0), with the recombination rate determining how rapidly gametic equilibrium is attained.

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