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Genetic Drift

Genetic Drift. Random change in allele frequency Just by chance or chance events (migrations, natural disasters, etc) Most effect on smaller populations (≤100) Founder effect- allele frequencies change as a result of migration of a small subgroup of a pop.

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Genetic Drift

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  1. Genetic Drift • Random change in allele frequency • Just by chance or chance events (migrations, natural disasters, etc) • Most effect on smaller populations (≤100) • Founder effect- allele frequencies change as a result of migration of a small subgroup of a pop. • Chance events can reduce pop. size or create new, smaller pop.s from an original larger pop.

  2. Genetic equilibrium • Hardy-Weinberg principle- allele frequencies in a population will remain constant unless 1 or  factors causes those alleles to change • Genetic Equilibrium- allele frequencies in the pop. remain the same, nothing is changing • Conditions under which evolution will not occur • 5 Conditions • Random Mating • No Immigration or Emigration (Migration) • No Mutations • No Natural Selection • Large Populations

  3. Hardy-Weinberg Equation • p2 + 2pq + q2 = 1 • p + q = 1 • p=frequency of dominant allele, q= frequency of recessive allele • p2 = frequency of individuals who are homozygous dominant, q2 = frequency of individuals who are homozygous recessive, 2pq= frequency of individuals who are heterozygous • If the frequency of people in the United States that have a certain recessive trait is 2% (or 0.02), what is the frequency of the allele for that trait? • q2 = frequency of individuals who are homozygous recessive = 0.02 • q= frequency of recessive allele= √0.02 = 0.14 or 14%

  4. Sample Problem • A certain recessive disease has a frequency of 1 in 2500 people. What is the frequency of each allele and the frequency of carriers? (Assuming the population is at Hardy-Weinberg equilibrium) • What is the genotype of a recessive disease? • So q2= 1/2500= 0.0004, and q= √0.0004 = 0.02 • If q is present 0.02 or 2% of the time, and p is the only other allele, how often is p present? • So q= 0.02, p= 0.98, those are the frequencies of the alleles! • What is the genotype of a carrier of a recessive disease? • p2 + 2pq + q2 = 1 • So 2pq= 2 x 0.02 x 0.98= 0.04 • Frequency of homozygous dominant= 0.982= 0.96 • Frequency of homozygous recessive= 0.022= 0.0004 • 0.96 + 0.04 + 0.0004= 1

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