Genetic Variation in Populations
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Questions and Answers

What effect did the introduction of sheep from other populations have on the bighorn sheep at the National Bison Range?

  • Increased genetic variation (correct)
  • Had no effect on fitness
  • Decreased genetic variation
  • Increased susceptibility to diseases

    • What does the frequency of an allele in a population represent?

  • The average number of genotypes in the population
  • The total number of alleles present
  • A proportion or percentage of a specific allele (correct)
  • The genetic makeup of an individual organism

    • Why are individuals homozygous for CCR5-∆32 less likely to contract HIV?

  • They possess a stronger genetic variation
  • They lack any form of HIV receptor
  • The mutation prevents HIV from entering cells (correct)
  • They have a higher immune response

    • What is a common consequence of genetic drift in a population?

    <p>Loss of genetic variation</p> Signup and view all the answers

    How is the genotypic frequency calculated in a population?

    <p>Count the number of individuals with the genotype and divide by the total individuals in the sample</p> Signup and view all the answers

    What is implied by the statement that more genetic variation exists in populations than is visible in the phenotype?

    <p>Genotypes can exist without corresponding phenotypic expressions</p> Signup and view all the answers

    What might increase the frequency of the CCR5-∆32 allele in human populations?

    <p>An increase in global AIDS epidemic</p> Signup and view all the answers

    What is a key starting point for understanding the behavior of Mendelian genes in real populations?

    <p>Modeling idealized populations</p> Signup and view all the answers

    What is the first step in testing the Hardy-Weinberg Principle?

    <p>Calculate the allele frequencies</p> Signup and view all the answers

    What does the Hardy-Weinberg equilibrium require for allele frequencies to remain constant?

    <p>The population must be infinitely large</p> Signup and view all the answers

    When does natural selection cause changes in allele frequencies?

    <p>When certain phenotypes have greater survival rates</p> Signup and view all the answers

    Which of the following genotype frequency calculations is NOT part of Hardy-Weinberg equilibrium?

    <p>x2</p> Signup and view all the answers

    What indicates a violation of the Hardy-Weinberg equilibrium principle?

    <p>Differential survival based on phenotypes</p> Signup and view all the answers

    If natural selection favors individuals with an adaptive trait, what is likely to happen?

    <p>Those traits will appear with greater frequency in the next generation</p> Signup and view all the answers

    To calculate the chi-square statistic in Hardy-Weinberg tests, which step should precede it?

    <p>Determine the expected number of each genotype</p> Signup and view all the answers

    Which of these is NOT an assumption that sustains allele frequency according to the Hardy-Weinberg principle?

    <p>Populations are of finite size</p> Signup and view all the answers

    What must the sum of all genotypic frequencies equal?

    <p>1</p> Signup and view all the answers

    Which of the following is NOT a method for calculating allelic frequencies?

    <p>Counting the genomic sequences directly</p> Signup and view all the answers

    How do you calculate the allelic frequency from numbers of genotypes for a locus with multiple alleles?

    <p>Add twice the number of homozygotes and the number of heterozygotes and divide by the total allele count</p> Signup and view all the answers

    In a population of 398 individuals, how would you calculate the frequency of allele LM from the MN blood-type genotypes?

    <p>Add the frequency of LM homozygotes to half the frequency of heterozygotes containing LM</p> Signup and view all the answers

    Which statement is true regarding allelic frequencies in loci with multiple alleles?

    <p>They can include contributions from both homozygotes and heterozygotes</p> Signup and view all the answers

    What is the general formula to determine the genotypic frequencies in a population?

    <p>Total counts of each genotype divided by total alleles</p> Signup and view all the answers

    When calculating the frequency of an allele from genotypic frequencies, what is taken into account?

    <p>The entire contribution of heterozygotes along with homozygotes</p> Signup and view all the answers

    What might be a common misconception regarding allelic frequency calculations?

    <p>The genotypic frequencies must be irrelevant in the calculation</p> Signup and view all the answers

    What is the survival rate of B1B2 individuals in the population described?

    <p>75%</p> Signup and view all the answers

    How does selection affect genotype frequencies according to the information provided?

    <p>Genotype frequencies cannot be calculated by multiplying allele frequencies.</p> Signup and view all the answers

    What was the result of violation of the no-selection assumption in the population?

    <p>The population underwent evolution.</p> Signup and view all the answers

    In the scenario provided, what proportion of B1B1 individuals survived?

    <p>60%</p> Signup and view all the answers

    If each surviving adult donates 10 gametes, what is the total number of gametes contributed by the population?

    <p>800</p> Signup and view all the answers

    According to the information given, what can be inferred about the allele frequencies after selection?

    <p>Both alleles maintained a frequency of 0.5.</p> Signup and view all the answers

    What was observed about Hardy-Weinberg conclusion 1 in the control populations?

    <p>It appeared to hold true.</p> Signup and view all the answers

    Which statement about the survival rates of the genotypes is accurate?

    <p>Survival rates differ among the genotypes.</p> Signup and view all the answers

    What does the Hardy-Weinberg principle state about allele frequencies in a large population with random mating?

    <p>They will remain constant across generations.</p> Signup and view all the answers

    Which of the following is NOT an assumption of the Hardy-Weinberg equilibrium?

    <p>There is natural selection.</p> Signup and view all the answers

    What are the genotypic frequencies at Hardy-Weinberg equilibrium represented by?

    <p>p2, 2pq, and q2</p> Signup and view all the answers

    Under what conditions can a population NOT evolve according to the Hardy-Weinberg principles?

    <p>The population meets all Hardy-Weinberg assumptions.</p> Signup and view all the answers

    What does the 2pq term in the Hardy-Weinberg equation represent?

    <p>Frequency of heterozygous individuals.</p> Signup and view all the answers

    What is indicated if a population is not in Hardy-Weinberg equilibrium?

    <p>The population is evolving.</p> Signup and view all the answers

    Which frequency never exceeds 0.5 in a population at Hardy-Weinberg equilibrium?

    <p>Frequency of heterozygous individuals.</p> Signup and view all the answers

    What is the purpose of the Hardy-Weinberg principle in genetics?

    <p>To serve as a null model for allele and genotype frequency examination.</p> Signup and view all the answers

    Study Notes

    Genetic Variation in Populations

    • Genetic variation is evident in all organisms.
    • Populations possess more genetic variation than is observable in their phenotypes.
    • A population's genetic structure can be described by enumerating genotypes and alleles present, as well as their frequencies.

    Allele and Genotype Frequencies

    • Allele Frequency: The proportion or percentage of a specific allele within a population.
    • Genotype Frequency: The proportion of individuals in a population possessing a specific genotype.
    • The sum of all genotype frequencies always equals 1.
    • Calculating Genotype Frequencies: Divide the number of individuals with a specific genotype by the total population size.
    • Calculating Allelic Frequencies:
      • Count the number of copies of a specific allele, adding twice the number of homozygotes and the number of heterozygotes carrying the allele, and divide this sum by twice the total population size.
      • Alternatively, add the frequency of the homozygote to half the frequency of each heterozygous genotype containing the allele.

    Hardy-Weinberg Equilibrium Principle

    • This principle is a mathematical model that evaluates the effect of reproduction on allele and genotype frequencies in a population.
    • Assumptions:
      • Large Population Size: Prevents random fluctuations in allele frequencies.
      • Random Mating: Individuals choose mates without preference for specific genotypes.
      • No Mutation: No new alleles are introduced or existing ones altered.
      • No Migration: Individuals do not move into or out of the population.
      • No Natural Selection: All genotypes have equal survival and reproductive rates.
    • Conclusions:
      • Allele Frequencies Remain Constant: Under Hardy-Weinberg conditions, allele frequencies do not change from generation to generation.
      • Genotypic Frequencies Stabilize: Genotype frequencies reach a predictable equilibrium after one generation.
        • AA: p² (where p is the frequency of allele A)
        • Aa: 2pq (where q is the frequency of allele a)
        • aa: q²

    Implications of the Hardy-Weinberg Law

    • It serves as a null model for population genetics.
    • Deviation from Hardy-Weinberg equilibrium indicates that one or more of its assumptions are being violated, suggesting potential evolutionary processes at play.

    Testing the Hardy-Weinberg Principle

    • Steps:
      • 1. Calculate Allele Frequencies: Determine the frequency of each allele in the population.
      • 2. Calculate Expected Genotype Frequencies: Using the allele frequencies, calculate the expected genotype frequencies under Hardy-Weinberg equilibrium.
      • 3. Calculate Expected Number of Individuals: Multiply the expected genotype frequency by the total population size to estimate the expected number of individuals for each genotype.
      • 4. Calculate Chi-square Statistic: Compare the observed genotype frequencies to the expected ones using a chi-square test to determine if there is a significant difference.
      • 5. Interpret Significance: A significant chi-square value suggests a deviation from Hardy-Weinberg equilibrium.

    Selection and the Hardy-Weinberg Equilibrium

    • Selection violates the Hardy-Weinberg assumption of no selective pressure and can cause allele frequencies to change across generations.
    • Natural Selection: Individuals with advantageous traits tend to survive and reproduce more successfully, increasing the frequency of these traits over time.
    • Example: ADH locus in Fruit Flies
      • AdhF allele (faster ethanol breakdown) demonstrates higher survival rates in populations exposed to ethanol, leading to increased frequency of this allele.

    Impact of Selection on Genotype Frequencies

    • Selection can alter genotype frequencies in a manner that prevents direct calculation from allele frequencies.
    • Example: Allele Frequencies Remain Constant with Selection
      • Despite selection favoring survival of individuals with the B1 allele, the overall allele frequencies may remain unchanged due to different survival rates of genotypes.
      • This implies that the Hardy-Weinberg law's conclusion 1 (allele frequencies remain constant) is not met.
    • Example: Genotype Frequencies Shift Despite Constant Allele Frequencies
      • Due to differential genotype survival, the observed genotype frequencies deviate from the expected frequencies predicted by the Hardy-Weinberg equilibrium.
      • This indicates that the Hardy-Weinberg law's conclusion 2 (genotype frequencies stabilize) is also not met.

    The CCR5-∆32 Allele

    • CCR5-∆32 Mutation: A mutation in the CCR5 gene, which reduces susceptibility to HIV infection.
    • Individuals Homozygous for CCR5-∆32: These individuals are significantly less likely to contract HIV.
    • Potential Impact of HIV Epidemic: The global AIDS epidemic is predicted to increase the frequency of the CCR5-∆32 allele in human populations.

    The Hardy-Weinberg Principle as a Null Model

    • The Hardy-Weinberg principle serves as a baseline against which real population data can be compared.
    • It allows scientists to identify potential evolutionary forces that might be altering allele and genotype frequencies in natural populations.

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    Description

    Explore the concepts of genetic variation, allele frequencies, and genotype frequencies in populations. This quiz covers methods to calculate these frequencies and the significance of genetic structure in populations. Test your understanding of how genetic diversity is measured and its implications in biology.

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