Population genetics

• Hardy-Weinberg equilibrium:

  Genotype frequencies: p^2 + 2pq + q^2

  Dominant homozygous and heterozygous frequencies: p + q = 1
• After one generation of mating, the frequency of recessive homozygotes drop but the frequency of the recessive allele stays the same
• Deviations from Hardy-Weinberg:

  Natural selection - HW assumes all genotypes have the same fitness, but not true (lethal alleles)

  • MRSA due to antibiotic resistance
  • geographical variation in lactase persistence and alcohol tolerance
  • many top long-distance runners from Africa
• Selection coefficient = the reduction in the ability to pass on an allele compared to that of an individual who reproduces with complete success
• Founder effects - a loss of variation caused by a large population arising from a small number of individuals coming from a larger population

  e.g. connexin deafness allele in the Himalayas

  European population of drosophila introduced to America came from a single vial
• Bottlenecks - the loss of genetic diversity caused by a large reduction in population size

  e.g. after a natural disaster, random alleles are lost
• Genetic drift - random change in allele frequencies due to random chance, greatest in small populations
• Sampling error and genetic drift - standard deviation

  root of (pq / 2N)

  the larger the sampling size, the smaller the sampling variance

  i.e. more random change in small populations
• Tristan de Cunha (Atlantic Ocean) - human population arose from a small number of individuals (founder effect) and has seen great fluctuations (bottlenecks)

  This population has a high incidence of retinoblastoma, a type of eye cancer
• Genetic diversity in humans negatively correlates with the distance from Addis Ababa (due to this invasive species undergoing founder effect and a loss of genetic diversity)
• Migration - gene flow between populations can bring new alleles to a population

  e.g. origin of different populations in London

  Transatlantic slave trade
• Assumption of HW that mating is random is not true - individuals of the same genotype more likely to mate, historically illegal in US to marry outside your racial group
• Positive assortative mating - mating of individuals of a similar genotype or phenotype (like with like)

  e.g. in humans for traits such as height, weight, skin colour (historically), educational level
• Negative assortative mating - mating of individuals of a different genotype of phenotype (like with unlike)

  e.g. wolves more likely to mate with individuals of a different coat colour

  Paramecium can mate with 5 other different genotypes

  Cultural taboo in humans to prevent like with like (inbreeding)

  Neanderthal interbreeding with Homo sapiens introduced genetic diseases into our population
• BRUCE effect in mice - if a pregnant female who was impregnated by a male mouse from the same inbred line is placed in a cage with a male from a different line, she will abort the pregnancy to mate with the different line (due to scent receptors)
• Tay-sachs: stops the nerves working properly and leads to brain degeneration in the young

  prevented by Dor Yeshorim - a genetic screening process to avoid heterozygotes marrying each other
• Mutation - allele frequencies change if new alleles emerge in a population

  e.g. diversity of primates, whale DNA and fossil record

  • varying rates in genomes (fibrin clotting mechanism has a fast rate, haemoglobin has a slow rate)