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(coupling term) (repulsion term)

where gxy stands for a gamete frequency. D is the difference between the product of the coupling gamete frequencies and the product of the repulsion gamete frequencies. This makes intuitive sense: with independent assortment the frequencies of the coupling and repulsion gamete types are identical and cancel out to give D = 0, or gametic equilibrium. Another way to think of the gametic disequilibrium parameter is as a measure of the difference between observed and expected gamete frequencies: g11 = p1q1 + D, g22 = p2q2 + D, g12 = p1q2 - D, and g21 = p2q1 - D (note that observed and expected gamete frequencies cannot be negative). In this sense, D measures the deviation of gamete frequencies from what is expected under independent assortment. Since D can be both positive as well as negative, both coupling and repulsion gametes can be in excess or deficit relative to the expectations of independent assortment.

Gametic disequilibrium can be measured using several estimators, including the squared correlation coefficient (p2, where p is pronounced "rho"), where p2 = D2/(p1p2q1q2), which has a range of -1 to +1. Different estimators of gametic disequilibrium

Table 2.12

Expected frequencies

of gametes for two diallelic loci in a randomly mating population with a

recombination rate between the loci of r. The first eight genotypes have non-recombinant and recombinant

gametes that are identical. The last two genotypes produce novel recombinant gametes, requiring inclusion

of the recombination rate to predict gamete frequencies. Summing down each column of the table gives the

total frequency of each gamete in

the next generation.

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