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Population Genetics

Population Genetics. Allele Frequencies Genotype Frequencies The Hardy-Weinberg Equation. Allele Frequency. Allele frequency = number of copies of an allele in a popuation ÷ total number of alleles in a population Example: A population of 100 pea plants have the following genotypes

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Population Genetics

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  1. Population Genetics Allele Frequencies Genotype Frequencies The Hardy-Weinberg Equation

  2. Allele Frequency • Allele frequency = number of copies of an allele in a popuation÷total number of alleles in a population • Example: • A population of 100 pea plants have the following genotypes • 64 tall plants with the genotype TT • 32 tall plants with the genotype Tt • 4 dwarf plants with the genotype tt t = 32 + 2(4) ÷ 2 (64) + 2(32) + 2(4) = 40 ÷ 200 = .2 or 20%

  3. Genotype Frequency • Genotype frequency = number of individuals with a particular genotype in a population ÷ total number of individuals in a population • Example: tt = 4 ÷ 64 + 32 + 4 = 4 ÷ 100 = .04, or 4%

  4. The Hardy Weinberg Equation • Is a way to study whether allele and genotype frequencies will change over the course of many generations. • 1908 = Godfrey Harold Hardy and Wilhelm Weinberg independently discovered a mathematical expression that predicted the stability of allele and genotype frequencies from one generation to the next. • Hardy-Weinberg equilibrium = the allele and genotype frequencies do not change over the course of many generations (under a given set of conditions).

  5. Factors that cause change in allele and genotype frequencies: • 1. Mutation • 2. Random Genetic Drift • 3. Migration • 4. Natural Selection • 5. Nonrandom Mating • Assignment: describe each of the above factors on a sheet of paper (pg. 670).

  6. Hardy-Weinberg Equation Example = If a gene exists as 2 different alleles (A and a), then the variable p represents the allele frequency of A and the variable q represents the allele frequency of a. The following equation shows that the allele frequency of A plus the allele frequency of a equals the 100% of the alleles for a particular gene: p + q = 1

  7. Hardy-Weinberg Equation p²+ 2pq + q² = 1 • p² = the genotype frequency of AA • q² = the genotype frequency of aa • 2pq = the genotype frequency of Aa

  8. Example If: • p = .8 • q = .2 Then: AA = p² = (.8)² = .64 Aa = 2pq = 2(.8)(.2) = .32 aa = q² = (.2)² = .04 (In other words, the allele frequency of A = 80% and the allele frequency of a is 20%.) The genotype frequency of AA is 64 % The genotype frequency of Aa is 32 % The genotype frequency of aa is 4%

  9. A .8 a .2 A .8 a .2

  10. The African Cheetah • This species has a relatively low level of genetic variation because the population was reduced to a small size approximately 10,000 to 12,000 years ago.

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