Allele Distribution

• Genetic drift


  The change in the composition of a gene pool as a result of chance or random events
• Genetic drift

  • It will occur faster and be more significant in smaller populations, where chance events have a bigger impact on the gene pool
  • Larger populations will be less affected by random events and maintain more stable allele frequencies with low genetic drift
• Population bottleneck


  An event that reduces population size by an order of magnitude (~ >50%)
• Population bottleneck


  • May result from natural occurrences (e.g. fires, floods, etc.) or be human induced (e.g. overhunting)
  • The surviving population has less genetic variability than before and will be subject to a higher level of genetic drift
  • As the surviving members begin to repopulate, the newly developing gene pool will be divergent to the original
• Founder effect


  Occurs when a small group breaks away from a larger population to colonise a new territory
• Founder effect


  • The population subset does not have the same degree of diversity as a larger population, it is subject to more genetic drift
  • As this new colony increases in size, its gene pool will no longer be representative of the original gene pool
• Allele frequency


  The prevalence of a particular allele in a population, as a proportion of all the alleles for that gene
• Changes in allele frequency can reflect either random processes (genetic drift) or differential processes (natural selection)
• Population bottlenecks and the founder effect will exacerbate genetic differences between geographically isolated populations
• How to compare allele frequencies


  1. Go to the Allele Frequency Database (ALFRED)

  2. Type a gene name into the search parameter

  3. Choose a specific gene loci

  4. Select a polymorphism

  5. Choose a frequency display format
• SCL45A2

  • Involved in human melanin production (allele frequency differs in Northern Europe)
• ABO


  • Blood groups show regional variation (e.g. O blood more common in Africa)
• LCT

  • Lactase is required for lactose tolerance (more common in Northern Europe / Africa)
• SCL45 is one of a number of genes contributing to melanin production and the pigmentation of human skin
• A particular allele responsible for lighter pigmentation is more frequent in Northern Europe