Population Genetics - Biochemistry

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