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Updated: Dec 20 2019

Population Genetics

  • Overview
    • Forces responsible for genetic variation
      • mutation
        • de novo mutation rates constant among populations
          • intrinsic error rate in DNA polymerase
      • founder effect
        • if one member of a small community carries a triat, as the population expands there will be a higher frequency of that trait in the new community than there is in the general population
        • Ex.) Pennsylvania Amish and Ellis-van Creveld syndrome
      • genetic drift
        • a dramatic change in allele frequency based on chance
          • small populations are more vulnerable to genetic drift
      • natural selection
        • ↑ in allelic frequency that ↑ species fitness
        • ↓ in allelic frequency that ↓ species fitness
        • some genes ↑ species fitness as heterozygote but ↓ species fitness as a homozygote
          • ex.) sickle cell trait lowers malarial infections, while sickle cell anemia is detrimental
      • bottleneck
        • Even when fitness is equal for all phenotypes, a population bottleneck can result in disrupted allelic frequencies or loss of a genotype all together by chance
      • gene flow
        • transfer of alleles from one population to another
    • Hardy-Weinberg equilibrium
      • states that genotype and allele frequencies remain constant through generations
      • disease prevalence equation
        • p2+ 2pq + q2 = 1
          • where p = frequency of allele A
          • where q = frequency of allele B
          • p2 = frequency of homozygous individuals for allele A
          • q2 = frequency of homozygous individuals for allele B
          • 2pq = frequency of heterozygotes
      • requirements for validity
        • large population
        • random mating
      • the genotypic frequencies of the population will remain stable from generation to generation
      • assumptions
        • no mutation
        • no selection for any of the genotypes at the locus
        • no migration
      • other notes
        • prevalence of an X-linked recessive disease in males = q
          • prevalence of an X-linked recessive disease in females = q2
        • possible to assume in most cases that p = 1 as the wild-type allele is approximately 1
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