Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 112

Research has shown that it is possible for alleles at two loci to be in random mating frequencies and yet not in equilibrium with respect to each other. Further, equilibrium between two loci is not attained after one generation of random mating as Hardy‐Weinberg law concluded, but is attained slowly over many generations. Also, the presence of genetic linkage will further slow the rate of attainment of equilibrium (Figure 3.2). If there is no linkage (c = 0.5), the differential between actual frequency and the equilibrium frequency is reduced by 50% in each generation. At this rate, it would take about seven generations to reach approximate equilibrium. However, at c = 0.01 and c = 0.001, it would take about 69 and 693 generations, respectively, to reach equilibrium. A composite gene frequency can be calculated for genes at the two loci. For example, if the frequency at locus Aa = 0.2 and that for locus bb = 0.7, the composite frequency of a genotype Aabb = 0.2 × 0.7 = 0.14.

Figure 3.2 The approach to linkage equilibrium under random mating of two loci considered together. The value of “c” gives the linkage frequency between two loci. The effect of linkage is to slow down the rate of approach, the closer the linkage, the slower the rate. For c = 5, there is no linkage. The equilibrium value is approached slowly and theoretically unattainable.