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Ch. 17_Population Genetics

Ch. 17_Population Genetics - Ch 17 Population Genetics...

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Ch. 17. Population Genetics Molecular Evolution . This topic will be discussed at the end of the 2 Population Genetics lectures if time permits. Therefore, I am skipping this section for now. It was very briefly mentioned in chapter 12 (Recombinant DNA) also. Population Genetics (section 17.2; p. 618) Population Genetics is the study of heredity of group of individuals for traits by one, (or a few) genes. This field was begun in 1929 by 3 scientists- R. Fisher, S. Wright and JBS Haldane. The variation in allele (gene) distributions within groups of individuals is studied. One examines geographic variations or variations over time in these allele (gene) frequencies in a population rather than in a single mating or within one family. A Mendelian population consists of a group of inter-breeding individuals with a shared set of genes, the "Gene Pool". Questions commonly studied by Population Geneticists are- a. How much natural variation exists in a population? b. What controls the observed variations? c. What evolutionary forces are shaping the population? d. What is causing genetic divergence? e. How do biological characteristics of a population affect the gene pool? i. Monogamy, polygamy, polyandry? ii Child mortality rate? iii Age distribution in the population? iv Rates of migration from a different gene pool, etc. The field of population genetics frequently uses mathematical models and equations to describe and explain these forces. Allele frequencies and genotype frequencies. One first determines the frequencies of alleles and genotypes in a population ( Figs. 17.5, 17.6, 17.7 ). Genotype Frequency = # of individuals of a given genotype/Total # of individuals. Among 1,000 French people, when the DNA was genotyped for the wild type (A) or mutant CCR5 gene (a), the 3 genotypes were found in the following proportions-
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795 AA, 190 Aa and 15 aa (Total = 1,000) The genotypic frequencies are: AA = 795/1,000= 0.795 ; Aa = 190/1,000= 0.19 ; aa = 15/1,000 = 0.015 . The allelic frequencies are: A” allele = (2x AA + Aa)/2xTotal = (2x795 + 190)/2x1,000 = 0.89 . a” allele = (Aa + 2x aa)/2xTotal = (190 + 2x 15)/2x1,000 = 0.11 . If only 2 alleles are present, then frequency of (a) = 1- frequency of (A). One need calculate only one of the 2 frequencies. Effect of mating preferences. The frequencies of gametes are determined by the frequencies of the alleles in the parent’s generation ( Figs. 17.6, 17.7 ). What the frequencies of progeny genotypes will be is determined by how gametes come together in fertilization? This is determined by the mating preferences of the individuals in the parent’s generation. In random mating , the matings occur without any preference for one genotype or another. Assortative (non-random) mating
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Ch. 17_Population Genetics - Ch 17 Population Genetics...

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