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Figure 2.22 Expected levels of gametic disequilibrium (p2) due to the combination of finite effective population size (Ne) and the recombination rate (r). Chance sampling will maintain gametic disequilibrium if very few recombinant gametes are produced (small r), the population is small so that gamete frequencies fluctuate by chance (small Ne), or if both factors in combination cause chance fluctuations in gamete frequency (small Ner).

Figure 2.22 Expected levels of gametic disequilibrium (p2) due to the combination of finite effective population size (Ne) and the recombination rate (r). Chance sampling will maintain gametic disequilibrium if very few recombinant gametes are produced (small r), the population is small so that gamete frequencies fluctuate by chance (small Ne), or if both factors in combination cause chance fluctuations in gamete frequency (small Ner).

Chapter 2 review

• Mendel's experiments with peas lead him to hypothesize particulate inheritance with independent segregation of alleles within loci and independent assortment of multiple loci.

• Expected genotype frequencies predicted by the Hardy-Weinberg equation (for any number of alleles) show that Mendelian inheritance should lead to constant allele frequencies across generations. This prediction has a large set of assumptions about the absence of many population genetic processes. Hardy-Weinberg expected genotype frequencies therefore serve as a null model used as a standard of reference.

• The null model of Hardy-Weinberg expected genotype frequencies can be tested directly or assumed to be approximately true in order to test other hypotheses about Mendelian inheritance.

• The fixation index (F) measures departures from Hardy-Weinberg expected genotype frequencies (excess or deficit of heterozygotes) that can be caused by patterns of mating.

• Mating among relatives or consanguineous mating causes changes in genotype frequencies (specifically a decrease in heterozygosity) but no changes in allele frequencies.

• Consanguineous mating can also be viewed as a process that increases the chances that alleles descended from a common ancestor are found together in a diploid genotype (autozygosity).

• The fixation index, the autozygosity, and the coefficient of inbreeding are all interrelated measures of changes in genotype frequencies with consanguineous mating.

• Consanguineous mating may result in inbreeding depression, which ultimately is caused by over-dominance (heterozygote advantage) or dominance (deleterious recessive alleles).

• The gametic disequilibrium parameter (D) measures the degree of non-random association of alleles at two loci. Gametic disequilibrium is broken down by recombination.

• A wide variety of population genetic processes -natural selection, chance, admixture of populations, mating system, and mutation - can maintain and increase gametic disequilibrium even between loci without physical linkage to reduce recombination.

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