Chapter 25

• Population genetics


  The study of the genetic composition of a population of organisms

  
• Population


  A group of organisms that actively interbreed and exchange a common set of genes (share a gene pool)
• Why study population genetics?
• Genotype frequency


  How common is a particular genotype? (AA, Aa, or aa)

  
• Allele frequency


  How common is a particular allele? (A or a)

  
• We usually can't study every member in a population, so we take a sample and use it to make predictions
• The relationship between genotype frequency and allele frequency teaches us about the trait we're studying

  
• If you don't understand the difference between genotype and allele frequency, you will get everything wrong in this part of the class

  
• M/N blood type


  • LMLM = M-antigen on red blood cells
  • LNLN = N-antigen on red blood cells
  • LMLN = M-antigen and N-antigen on red blood cells

  
• Co-dominance


  In this case the phenotypes tell us genotypes, and we calculate both genotype frequencies and allele frequencies

  
• Calculate allele frequencies from genotype numbers


  1. p = frequency of LM in the population

  2. q = frequency of LN in the population

  
• In this case, since LM and LN are the only alleles in the population, p + q = 1
• Calculate allele frequencies from genotype frequencies


  1. p = f(M) = f(MM) + 1/2 f(MN)

  2. q = f(N) = f(NN) + 1/2 f(MN)

  
• Hardy-Weinberg Equilibrium


  Under certain conditions (or assumptions), the relationship between allele frequencies and genotype frequencies will follow these rules:

  
• Genotype Frequency


  • AA = p^2
  • Aa = 2pq
  • aa = q^2

  
• Hardy-Weinberg frequencies are based on a set of assumptions
• We can use those assumptions to estimate allele frequencies that we can't otherwise observe, or we can test whether a population is in HW equilibrium, to see if one or more of our assumptions are incorrect

  
• Estimating allele frequencies at a locus


  This assumes the population is in HW equilibrium with respect to the gene being studied, and uses the frequency of homozygous recessives to estimate the allele frequency in the population

  
• Testing whether a population is in HW equilibrium with respect to a particular gene


  This requires knowing the actual allele frequency in a population (not estimated), and uses chi-square test to see if genotype frequencies fit the predictions of HW equilibrium

  
• Albinism


  • Frequency: 1/20,000
  • Assuming Hardy-Weinberg, chance that a random person is a carrier is 1.4%

  
• Testing for equilibrium: χ2 goodness of fit test


  1. Calculate the allele frequencies

  2. Calculate the expected genotype numbers

  3. Test whether our observed genotype numbers match the expected using a χ2 goodness of fit test

  
• If a population is NOT in Hardy-Weinberg equilibrium, one (or more!) of the Hardy-Weinberg assumptions must be wrong
• Hardy-Weinberg assumptions


  • Mating is random
  • No new alleles arise by mutation
  • There is no genetic exchange with outside populations
  • The population is large enough to avoid chance fluctuations
  • All genotypes are equally viable and fertile

  
• Genotypes


  • LMLM
  • LMLN
  • LNLN

  
• LMLM expected frequency


  p^2 = 0.2911
• LMLN expected frequency


  2pq = 0.4968
• LNLN expected frequency


  q^2 = 0.2121
• The population is in Hardy-Weinberg equilibrium
• Population genetics


  The study of the genetic composition of populations and how it changes over time
• Hardy-Weinberg principle


  A mathematical model that describes the genetic composition of a population and how it remains constant from generation to generation in the absence of evolutionary forces
• Exceptions to Hardy-Weinberg


  • Nonrandom mating
  • Mutation
  • Gene flow
  • Genetic drift
  • Unequal fitness
• Genetic equilibrium


  A state where allele and genotype frequencies remain constant from generation to generation
• If a population is NOT in Hardy-Weinberg equilibrium, one (or more!) of the assumptions must be wrong
• Hardy-Weinberg assumptions


  • Mating is random
  • No new alleles arise by mutation
  • No genetic exchange with outside populations
  • Population is large enough to avoid chance fluctuations
  • All genotypes have equal survival and reproductive ability

  
• Sickle cell anemia


  Caused by 2 main alleles: HBBA (A) wild-type and HBBS (S) recessive mutant allele
• Sickle cell genotypes


  • A/A: wild-type, normal blood
  • A/S: mostly unaffected, mild anemia
  • S/S: severe, often fatal anemia
• The West African population study of 12,387 individuals does not fit Hardy-Weinberg equilibrium
• Genotypes in West African population


  • A/A
  • A/S
  • S/S
• Expected A/A frequency


  p^2 = 0.769
• Expected A/S frequency


  2pq = 0.216