Genetics - Population Genetics

• Natural selection: a major force controlling extent and patterns of genetic variation
• Charles Darwin; On the origin of species by means of natural selection (1859)
• Natural Selection

  • Overproduction
  • Struggle for existence
  • Survival of the fittest
  • Accumulation and retention of favorable genes and their inheritance
• Genetic variation directly affects the morphology, physiology, and anatomy of organisms
• Variation is due to

  • Addition, deletion, translocation, and inversions
  • Mutation, transposition, genetic recombination, crossing over
• Chromosomal variation

  • Studied by Theodosius Dobzhansky, Russian geneticist (moved to US in 1920s)
  • Drosophila pseudoobscura
  • New Mexico
  • 1930-1980
• Types of D. pseudoobscura

  • Studied 4 inversion types - variation
  • AR Arrowhead (most common) - 30 years
  • PP Pikes peak - increase in # in 1965
  • CH chiricahua
  • ST standard (always present)
• Inversions occurred at different genes, due to spraying of DDT
• Usually disease causing alleles of the genes are found in low frequencies in human populations
• Genetic diseases in general are kept in low levels
• Some disease causing alleles are found in high frequencies

  • Ex. B-globin locus - sickle cell disease
  • Presence of sickle cell allele can be easily detected by electrophoresis
  • Separation of protein is done in this process instead of DNA
• Gel electrophoresis

  • Imposing an electric field across a gelatinous supporting medium (starch gel or polyacrylamide)
  • Protein placed on the gel, proteins migrate
  • Relative mobility of protein is observed
  • Mobility depends on the size and molecular wt. Of the protein
• Three B-globin genotypes

  • Homozygous normal genotype A1A1
  • Heterozygous carrier genotype A1A2
  • Homozygous sickle cell disease genotypes A2A2
  • B-globin genotypes are: A1A1, A1A2, A2A2
• Population - a group of interbreeding individuals of the same species that exist together in time and space
• For B-globin gene, N11, N12, N22 indicate the number of individuals counted for A1A1, A1A2, and A2A2
• N= total # of individuals in the sample
• Estimated frequencies for 3 genotypes will be

  • P= N11/N for A1A1
  • H= N12/N for A1A2
  • Q = N22/N for A2A2
  • P+H+Q=1
• Polymorphic - If either A1 or A2 occurs <99%
• Monomorphic - If allele A1 or A2 occurs >99%
• Hardy-Weinberg Principle

  • 1908
  • English mathematician Godfrey Hardy
  • German Physician Wilhelm Weinberg
  • Simple relationship between allelic frequencies and genotypic frequencies
• In some populations, random union of gametes doesn’t occur
• Allelic frequencies of 3 different genotypes for a population according to Hardy-Weinberg Proportions varies
• The Hardy-Weinberg model assumes that:

  • There is no selection
  • No new alleles arise from mutation
  • There is no migration into or out of the population
  • The population is infinitely large
  • Random mating occurs
• If individuals of all genotypes are subject to natural selection and don’t have equal rates of survival and reproductive success, allele frequencies may change from one generation to the next
• Natural selection is the principal force that shifts allele frequencies within large populations
• Mutation is the only process that creates new alleles in a gene pool
• Because most mutations are recessive, indirect methods using probability and statistics are often employed to determine the mutation rate
• If the mutation rate is known, the extent to which mutation can cause alleles frequencies to change from one generation to the next can be estimated