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Unformatted text preview: Chapter 25 Population Genetics Dr. Ed Otto George Mason University Key objectives Be able to calculate genotype and allele frequencies for a population Understand the HardyWeinberg Law, its assumptions and applications Understand the impact of inbreeding on genotype frequencies Population Genetics Population genetics studies the genetic makeup of groups of individuals and how a group’s genetic composition changes with time These groups are comprised of interbreeding individuals that have a common set of genes, the gene pool For large populations, it is not practical to determine the genes of all individual members, so a random sample of individuals is evaluated The genotypic and allelic frequencies of the sample are used to represent the gene pool of the overall population Calculating Genotypic and Allelic Frequencies Calculating Genotypic Frequencies Frequency = a proportion or percentage Genotypic frequency = the number of individuals possessing a genotype divided by the total number of individuals in the sample (N) f ( AA ) = f ( Aa ) = f ( aa ) = number of AA individuals N number of Aa individuals N number of aa individuals N f ( AA ) + f ( Aa ) + f ( aa ) = 1 Calculating Allelic Frequencies The gene pool can also be described in terms of allelic frequencies  There are always fewer alleles than genotypes, so the gene pool can be described in fewer terms when the allelic frequencies are used Allelic frequencies can be calculated two ways: 1. from the numbers of genotypes 1. from the frequencies of genotypes 1. Calculating Allelic Frequencies from the Numbers of Genotypes To calculate allelic frequency from the numbers of genotypes, count the number of copies of a particular allele and divide by the total number of all alleles in the sample For a locus with two alleles ( A and a ), the frequencies are usually represented by p and q Frequency of an allele = number of copies of the allele number of copies of all alleles at the locus p = f ( A ) = 2n AA + n Aa 2N q = f ( a ) = 2n aa + n Aa 2N n AA = # AA individuals n Aa = # Aa individuals n aa = # aa individuals N = total number of individuals (divide by 2N because each diploid individual has 2 alleles at a locus) p + q = 1 p = f ( A ) = f ( AA ) + ½ f ( Aa ) To calculate allelic frequencies from the genotypic frequencies, add the frequency of the homozygote for the allele to half the frequency of the heterozygote (because half of the heterozygote's alleles are of each type) q = f ( a ) = f ( aa ) + ½ f ( Aa ) 2. Calculating Allelic Frequencies from Genotypic Frequencies p =...
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 Spring '10
 OTTO
 Genetics, Population Genetics, allelic frequencies, genotypic frequencies

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