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