Population Genetics (Learning Objectives) 130/lectures/Population Genetics... · PDF...

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  • Population Genetics (Learning Objectives)

    Define the terms population, species, allelic and genotypic frequencies, gene pool, and fixed allele, genetic drift, bottle-neck effect, founder effect.

    Explain the difference between microevolution and macroevolution. Review how genotypic and allelic frequencies are calculated.

    Given the appropriate information about a population you should be able to calculate the genotypic and allelic frequencies of homozygous dominant, recessive, or heterozygous individuals (following the example discussed in class).

    Visit this website to learn the factors that lead to changes in genotypic and allelic frequencies between generations: http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf

    What is the Hardy-Weinberg Equilibrium and what are its conditions. What are the factors that lead to microevolution? What is the source of new alleles within any population?


  • A population is a localized group of interbreeding individuals in a given geographic area

    A species is a group of populations who interbreed and produce fertile offspring


  • Definitions Gene pool = The collection of all alleles in the

    members of the population

    Population genetics = The study of the genetics of a population and how the alleles vary with time

    Gene Flow = Movement of alleles between populations when people migrate and mate

  • Changes allelic frequencies in populations

  • Populations not individuals are the units of evolution

    - If all members of a population are homozygous for the same allele, that allele is said to be fixed

  • Allele Frequencies

    Allele frequency =# of particular allele

    Total # of alleles in the population

    Count both chromosomes of each individual Allele frequencies affect the frequencies of the three


  • Frequency of a trait varies in different populations. Example: PKU an autosomal recessive trait

    Table 14.1

    Phenotype Frequencies

  • Evolution

    Microevolution small changes due to changing allelic frequencies within a population from generation to generation

    Macroevolution large changes in allelic frequencies over 100s and 1000s of generations leading to the formation of new species

  • What is the allelic frequency (of R and r) in this population?

    Calculating the allelic frequencies from the genotypic frequencies

  • Genotypic frequencyRR= 320/500 = 0.64Rr = 160/500= 0.32rr = 20/500 = 0.04

  • What is the allelic frequency in a population of 500 flowers?

    How many total alleles are there? 500 X 2 = 1000

    Frequency of R allele in populationRR + Rr = 320 X 2 + 160= 640+160= 800

    800/1000 = 0.8 =80%Frequency of r allele = 1- 0.8 = 0.2 =20%

    or rr +Rr = 20 X 2+ 160= 200200/1000 = 0.2

  • - Meiosis and random fertilization do not change the allele and genotype frequencies between generations

    - The shuffling of alleles that accompanies sexual reproduction does not alter the genetic makeup of the population

  • The Hardy-Weinberg theorem describes the gene pool of a non-evolving population

    Hardy Weinberg animationhttp://zoology.okstate.edu/zoo_lrc/biol1114/t


    practice questionshttp://science.nhmccd.edu/biol/hwe.html


  • p + q = 1

    p = allele frequency of one alleleq = allele frequency of a second allele

    p2 + 2pq + q2 = 1

    p2 and q2 Frequencies for each homozygote

    2pq Frequency for heterozygotes

    All of the allele frequencies together equals 1

    All of the genotype frequencies together equals 1

    Hardy-Weinberg Equation

  • Populations at Hardy-Weinberg equilibrium must satisfy five conditions.(1) Very large population size. In small populations,

    chance fluctuations in the gene pool, genetic drift, can cause genotype frequencies to change over time.

    (2) No migrations. Gene flow, the transfer of alleles due to the movement of individuals or gametes into or out of our target population can change the proportions of alleles.

    (3) No net mutations. If one allele can mutate into another, the gene pool will be altered.

  • (4) Random mating. If individuals pick mates with certain genotypes, then the mixing of gametes will not be random and the Hardy-Weinberg equilibrium does not occur.

    (5) No natural selection. If there is differential survival or mating success among genotypes, then the frequencies of alleles in the next variation will deviate from the frequencies predicted by the Hardy-Weinberg equation.

    Evolution results when any of these five conditions are not met - when a population experiences deviations from the stability predicted by the Hardy-Weinberg theory.

  • Genetic Driftchanges allelic frequencies in populations

  • The bottleneck effect

    The founder effect

  • Caused by four factors:1. Non-Random mating2. Genetic drift due to sampling/ bottleneck &

    founder effects, geographic & cultural separation

    3. Migration- of fertile individuals4. Mutation- in germline cells transmitted in

    gamete5. Natural selection- accumulates and maintains

    favorable genotypes in a population


  • Figure 14.3

    Source of the Hardy-Weinberg Equation

    Figure 14.3

  • Solving a Problem

    Figure 14.4

  • Solving a Problem

    Figure 14.4

  • Table 14.3

    Calculating the Carrier Frequency of an Autosomal Recessive

  • Figure 14.3

    Calculating the Carrier Frequency of an Autosomal Recessive

    Figure 14.5

  • Figure 14.3

    Calculating the Carrier Frequency of an Autosomal Recessive

    What is the probability that two unrelated Caucasians will have an affected child?

    Probability that both are carriers =1/23 x 1/23 = 1/529

    Probability that their child has CF = 1/4 Therefore, probability = 1/529 x 1/4 =


  • Calculation of % PKU carriers from screening

    About 1 in 10,000 babies in US are born with PKU- The frequency of homozygous recessive individuals = q2 = 1

    in 10,000 or 0.0001.- The frequency of the recessive allele (q) is the square root

    of 0.0001 = 0.01.- The frequency of the dominant allele (p) is p = 1 - q or 1 -

    0.01 = 0.99.The frequency of carriers (heterozygous individuals) is

    2pq = 2 x 0.99 x 0.01 = 0.0198 or about 2%.About 2% of the U.S. population carries the PKU allele.

  • Question

    What is the chance or probability that two unrelated US individuals will have an affected child?

    Slide Number 1DefinitionsDefinitionsChanges allelic frequencies in populations Slide Number 5Allele FrequenciesSlide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Genetic Driftchanges allelic frequencies in populations Slide Number 19MicroevolutionSlide Number 21Solving a ProblemSolving a ProblemSlide Number 24Slide Number 25Slide Number 26Slide Number 27Question