Population Genetics: Hardy-Weinberg Theorem

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Questions and Answers

What is a gene pool?

  • The sum of all organisms within a specific habitat
  • A group of organisms of the same species that cannot interbreed
  • The total sum of all alleles at all gene loci in a population (correct)
  • The total number of individuals in a population

Natural selection acts directly on genotypes.

False (B)

Who proposed the theory that individuals inherit traits acquired during their lifetime?

Lamarck

The change in populations over time is known as __________.

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

Match the following terms with their definitions:

<p>Population = A group of organisms of the same species in a specific area Fitness = The number of surviving offspring produced by an individual Natural Selection = The process through which advantageous traits become more common Dominant Allele = An allele that masks the presence of another allele</p> Signup and view all the answers

Why don't dominant alleles completely eliminate recessive alleles in a population?

<p>Recessive alleles can remain hidden in heterozygous individuals (C)</p> Signup and view all the answers

Evolution can be observed in a single generation.

<p>False (B)</p> Signup and view all the answers

What does the term 'fitness' refer to in the context of population genetics?

<p>The number of surviving offspring an individual produces</p> Signup and view all the answers

Which of the following conditions is NOT required for Hardy-Weinberg equilibrium?

<p>There is a high mutation rate (C)</p> Signup and view all the answers

The Hardy-Weinberg principle states that allele frequencies change over time if specific conditions are met.

<p>False (B)</p> Signup and view all the answers

What is the effect of genetic drift in small populations?

<p>Changes in allele frequencies due to chance events.</p> Signup and view all the answers

The _____ effect occurs when a small group of individuals becomes isolated from a larger population.

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

Which of the following is NOT an agent of evolutionary change?

<p>Random Evolution (D)</p> Signup and view all the answers

Match the definitions with their corresponding concepts:

<p>Founder effect = Isolated small population differing in allele frequency Bottleneck effect = Reduction in population size affecting genetic diversity Assortative mating = Similar phenotypes mate with each other Mutation = Change in genetic code that can alter phenotypes</p> Signup and view all the answers

In non-random mating, individuals prefer mates based on their genetic traits.

<p>True (A)</p> Signup and view all the answers

What happens to allele frequencies when one or more Hardy-Weinberg assumptions are violated?

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

What is the effect of gene flow on populations over time?

<p>It can decrease or increase fitness depending on the alleles introduced. (D)</p> Signup and view all the answers

Disruptive selection eliminates both extremes from a phenotypic array.

<p>False (B)</p> Signup and view all the answers

What must exist among individuals for natural selection to occur?

<p>Variation among individuals/phenotypes.</p> Signup and view all the answers

In the Hardy-Weinberg principle, the sum of the frequencies of the two alleles must equal _____ .

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

Match the type of selection with its description:

<p>Disruptive Selection = Eliminates intermediate types Directional Selection = Eliminates one extreme from an array of phenotypes Stabilizing Selection = Eliminates both extremes from an array of phenotypes Artificial Selection = Human exerted selection</p> Signup and view all the answers

What is an example of heterozygote advantage?

<p>Heterozygotes are less susceptible to malaria. (A)</p> Signup and view all the answers

Natural selection is the process that leads to evolution as the outcome.

<p>True (A)</p> Signup and view all the answers

Gene flow can occur via migration of _____ individuals or gametes.

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

In the Hardy-Weinberg equilibrium equation, what does the term $p^2$ represent?

<p>The frequency of genotype AA (A)</p> Signup and view all the answers

The sum of allele frequencies in a population always equals 1.

<p>True (A)</p> Signup and view all the answers

If allele frequency q is 0.25, what is the frequency of allele p?

<p>0.75</p> Signup and view all the answers

In the Hardy-Weinberg model, the expected frequency of genotype Aa is given by ______.

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

What is the expected frequency of genotype aa if p = 0.5 and q = 0.5?

<p>0.25 (C)</p> Signup and view all the answers

If observed genotypic frequencies are significantly different from expected frequencies, evolution is not occurring.

<p>False (B)</p> Signup and view all the answers

What role does the Chi-square test play in evaluating Hardy-Weinberg equilibrium?

<p>It tests the significance of the difference between observed and expected genotypic frequencies.</p> Signup and view all the answers

What does a chi-squared value greater than the table threshold indicate in terms of evolutionary change?

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

All individuals in a population have the same genes which leads to identical traits.

<p>False (B)</p> Signup and view all the answers

What is the fundamental concept behind Hardy-Weinberg equilibrium?

<p>It describes a population that is not evolving, where allele frequencies remain constant over time.</p> Signup and view all the answers

In a population, alleles that produce successful phenotypes tend to ________ in frequency over time.

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

Match the following scenarios with their potential outcomes:

<p>High resource availability = Population growth Limited resources = Increased competition Successful phenotype alleles = Increased fitness Less successful phenotypes = Decreased prevalence</p> Signup and view all the answers

What is the total observed population size in the flower study?

<p>120 (A)</p> Signup and view all the answers

The allele frequency for pink flowers in the study is 0.2.

<p>False (B)</p> Signup and view all the answers

Calculate the expected frequency of white flowers if the total population is 120 and the allele frequency for white is 0.2.

<p>4.8</p> Signup and view all the answers

The Hardy-Weinberg principle uses __________ to calculate expected genotype frequencies.

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

Match the following components with their definitions related to allele frequencies:

<p>p = Frequency of the dominant allele q = Frequency of the recessive allele p² = Frequency of homozygous dominant genotype q² = Frequency of homozygous recessive genotype</p> Signup and view all the answers

What is the expected number of red flowers if the population size is 120 and the allele frequency for red is 0.8?

<p>76.8 (C)</p> Signup and view all the answers

If observed genotypic frequencies are not significantly different from expected frequencies, evolution is occurring.

<p>False (B)</p> Signup and view all the answers

What test can be used to evaluate if a population is in Hardy-Weinberg equilibrium?

<p>Chi-square test</p> Signup and view all the answers

Flashcards

Population

A group of the same species living in the same area, able to interbreed and produce fertile offspring.

Gene pool

The total collection of all alleles (different versions of a gene) in a population.

Evolution

Change in populations over time.

Natural Selection

Favorable traits help individuals survive and reproduce better, leading to more of those traits in the population.

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Fitness

Measure of an individual's ability to survive and pass on their genes in a specific environment.

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Population Genetics

Study of genes in populations; combining evolutionary biology and population genetics to see how change happens through selection.

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Species

Group of organisms that can interbreed and produce fertile offspring.

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Hardy-Weinberg Theorem

A principle describing the conditions under which allele and genotype frequencies in a population will stay constant from one generation to the next.

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Hardy-Weinberg Equilibrium

A condition where allele frequencies in a population remain constant from generation to generation, meaning no evolution is occurring.

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Genetic Drift

Change in allele frequencies due to chance events, especially important in small populations.

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Founder Effect

Genetic drift that occurs when a small group of individuals establishes a new population, carrying a different allele frequency than the original population.

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Bottleneck Effect

Genetic drift caused by a sharp reduction in population size, which can affect the gene pool.

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Non-random mating

Mating where individuals with similar phenotypes mate more frequently than random mating.

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Mutation

Changes in the genetic code that can result in altered phenotypes.

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Disruptive Selection

A type of natural selection that favors extreme phenotypes over intermediate ones.

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Directional Selection

A type of natural selection that favors one extreme phenotype over the other extreme or the average phenotype.

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Stabilizing Selection

Natural selection that favors the average phenotype and eliminates extreme phenotypes.

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Heterozygote Advantage

A situation where heterozygous individuals have a higher fitness (survival and reproduction) compared to both homozygous genotypes.

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Allele Frequency

The proportion of a particular allele within a population.

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Degrees of Freedom

The number of independent values that can vary in a statistical test. It reflects the number of categories in the data minus one.

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Genotype Frequency

The proportion of a specific genotype combination(combination of genes) in a population

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Chi-squared Test

A statistical test used to compare observed frequencies of data to expected frequencies, determining if there's a significant difference between them.

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Evolutionary Change

A shift in allele frequencies within a population over generations, indicating a change in genetic makeup.

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p + q = 1

The sum of the frequencies of alleles (p and q, usually for the two major variants) in a population equals 1. This equation models a simple 2-allele system.

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Population Capacity

The maximum number of individuals that a particular environment can sustain indefinitely, limited by resources and carrying capacity.

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p² + 2pq + q² = 1

The Hardy-Weinberg equation, showing the expected genotype frequencies (AA, Aa, aa) for a 2-allele system, given the allele frequencies (p and q).

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Homozygous Dominant

An individual carrying two copies of the same dominant allele (e.g., AA).

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Homozygous Recessive

An individual carrying two copies of the same recessive allele (e.g., aa).

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Heterozygous

An individual carrying two different alleles (e.g., Aa).

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Observed Genotypic Frequencies

The actual proportions of different genotypes (like homozygous dominant, heterozygous, homozygous recessive) observed in a population sample.

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Expected Genotypic Frequencies

The theoretical proportions of genotypes expected in a population based on Hardy-Weinberg equilibrium and allele frequencies.

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How to test for evolution?

Compare observed genotypic frequencies to expected frequencies. If they differ significantly, evolution is likely occurring.

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Calculate Expected Genotype Frequencies

Use known allele frequencies (p and q) and the Hardy-Weinberg equations (p² + 2pq + q² = 1) to determine the expected proportions of each genotype.

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Deviation

The difference between the observed genotypic frequencies and the expected genotypic frequencies.

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Study Notes

Population Genetics: Hardy-Weinberg Theorem

  • Population: A group of organisms of the same species that occur in the same area and can interbreed to produce fertile offspring.
  • Gene pool: The sum of all alleles at all gene loci of all individuals in a population.
  • Species: A group of organisms that can interbreed with each other and produce fertile offspring.
  • Evolution: The change in populations over time. Populations evolve; individuals do not. (Requires at least 2 generations to observe evolution.)

How Evolution Happens (Darwin/Wallace)

  • Natural Selection:
    • Gene variation causes variability in traits.
    • Some individuals have inherited traits allowing more surviving offspring than others without those traits.
    • Populations gradually include more individuals with advantageous characteristics.
    • Natural selection acts directly on phenotypes and indirectly on genotypes.
    • Fitness: the number of surviving offspring an individual produces for the next generation (relative measure). Selection favors phenotypes with the greatest fitness.

Darwin vs Lamarck

  • Lamarck: Individuals accumulate changes during their lifetime that are advantageous and pass these traits to their offspring. Traits acquired during an individual's life are passed on.
  • Darwin: Individuals are born with either advantageous or disadvantageous traits. Traits that are advantageous are passed on.

Population Genetics

  • Fuses evolutionary biology with genetics of populations, investigating evolutionary change due to natural selection and other factors.
  • Hardy-Weinberg Principle (1908):
    • If certain conditions are met, frequencies of alleles in a population remain constant from generation to generation.
    • If this principle is met (allele frequencies do not change across generations), genotypes are in Hardy-Weinberg Equilibrium (HWE). HWE means evolution is not occurring.

Conditions Required for HWE

  • Infinitely large population to avoid genetic drift (change in allele frequencies due to chance).
  • Random mating with respect to traits/alleles.
  • No new mutations at the gene locus.
  • No immigration or emigration (no gene flow).
  • No selection: all genotypes have equal reproductive success.

Genetic Drift

  • Frequencies of alleles may change by chance alone.
  • Particularly important in small populations.
  • Founder effect: Few individuals found a new population - small allelic pool.
  • Bottleneck effect: drastic reduction in population size causing a smaller gene pool.

Non-random Mating & Mutation

  • Assortative mating: Individuals mate with similar phenotypes (e.g., blue with blue).
  • Mutation: Changes to the genetic code. May cause a change in phenotype. Mutation rates are typically low.

Gene Flow/Migration

  • Movement of alleles among populations.
  • Alleles move through individual or gamete migration (e.g., pollen).
  • Reduction in variation across populations over time.
  • Can increase or decrease population fitness depending on alleles introduced.

Selection

  • Disruptive selection: Eliminates intermediate types.
  • Directional selection: Eliminates one extreme phenotype.
  • Stabilizing selection: Eliminates both extremes for a phenotypic array.

Natural Selection (Examples)

  • Adaptive melanism: Variation in coloration related to environment. (e.g., pocket mice on lava rocks).
  • Heterozygote advantage: Heterozygotes are favored over homozygotes (e.g., sickle cell anemia with malaria resistance).

Artificial Selection

  • Human-exerted selection (e.g., dog breeds, agriculture).

Hardy-Weinberg Rule: Calculating Allele Frequencies

  • Analyzing frequencies of alleles in successive generations to predict allele frequencies.
  • If conditions for HWE are met, you can predict the frequency of alleles in a population.
  • Two alleles at one locus: A and a. Allele frequencies designated as p (dominant) and q (recessive). Sum of p and q must always equal 1 (p+q=1).

Hardy-Weinberg Rule: Calculating Allele Frequencies (continued)

  • Possible genotypes are: AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive)
  • In algebraic terms: p² + 2pq + q² = 1
  • Frequency of genotype AA = p²
  • Frequency of genotype aa = q²
  • Frequency of genotype Aa = 2pq

Further Hardy-Weinberg Examples

  • Illustrates calculating allele and genotypic frequencies using actual data from a population.

More Practice (Stepwise work)

  • Step-by-step calculations to illustrate how to solve Hardy-Weinberg problems, using MC1R gene as an example

Hardy-Weinberg Practice.

  • Illustrates chi-square test to analyze whether a population is in HWE.

Further notes on Variation and Population Capacity.

  • All populations have capacity to increase, but not indefinitely.
  • Competition for resources.
  • Alleles that produce successful phenotypes will increase.
  • Change leads to increased fitness and adaptation.

Outputs

  • Lists exercises related to Hardy-Weinberg equilibrium and simulation of evolutionary change.

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Lab 16: Population Genetics and Evolution (MK Fall 2024) PDF

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