Population genetics and natural selection

• We need to estimate frequency to predict how many individuals will predict a genetic disease and to estimate the proportion of individuals who are carriers of a genetic disease.
• To predict genotypes in a proportion using Hardy-Weinburg equation.
• Hardy-Weinburg equation:

  Allele frequency: p + q = 1

  p - dominant allele

  q - recessive allele
• Genotype frequency = P2+ 2pq + q^2 = 1
• Carriers = heterzygous = 2pq
• Allele frequencies can change via: non random mating, bottle neck effect, natural selection, random genetic drift, founder effect and gene flow/migration.
• Genetic drift is a change in relative frequency, which an allele occurs in a population due to random events.
• A bottle effect is where the original population have equal frequencies of 1 and 2, with few 3. The population reduction or bottle neck effect will lead to 3 being completely lost and 2 are now rare.
• Founder effect is a small group leaving a large group and becoming seperate populations. Emigrants have only a small sample of the gene pool. the small group establishes a new population.
• Stabilising selection means medium sized individuals are favoured and phenotypes are favoured by natural selection. The peaks get higher and narrower.
• Stabilising selection reduces variation, but does not change the mean.
• Directional selection means larger individuals are favoured. The peak shifts in one direction.
• Directional selection changes the mean value towards one extreme.
• Disruptive selection means large and small individuals are favoured. Two peaks are formed.
• Disruptive selection favours the two extremes producing two peaks.
• Frequency dependent selection in Perissodus microlepis: natural selection maintains equal proportion of left and right mouthed individuals of the population.
• The fitness of a phenotype or genotype, if positive, the phenotype and genotype increases as it becomes more common.
• Spatial Distribution of genetic variation, the gradual geographic change in genetic/phenotypic composition is defined as CLINE.