Hardy Weinberg Equilibrium

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Hardy Weinberg Equilibrium. p 2 + 2pq + q 2 = 1. Two scientists independently derived the basic principle of population genetics called the Hardy – Weinberg Principle . This principle states that:. - PowerPoint PPT Presentation

Transcript of Hardy Weinberg Equilibrium

  • Hardy Weinberg Equilibriump2 + 2pq + q2 = 1

  • Two scientists independently derived the basic principle of population genetics called the Hardy Weinberg Principle. This principle states that:

    If all factors remain constant, the gene pool in a population will have exactly the same composition generation after generation. This condition is called genetic equilibrium.

    If the genetic equilibrium of a population is upset, the population is said to be evolving.

  • Evolution"the sum total of the genetically inherited changes in the individuals who are the members of a population's gene pool."

    Evolution is simply a change in frequencies of alleles in the gene pool of a population.

  • PopulationA group of the same species living in the same place at the same time

  • Gene pool all of the genes / alleles that occur in a population.

    Ex) human gene pool for blood type are IA, IB, and i.

  • Allele frequency % or proportion of that allele in the population

  • ConditionsEvolution will NOT occur and Hardy-Weinberg equilibrium will be met if the following conditions are met:

  • Conditions1.No Mutation

  • Conditions2.The population is infinitely large- laws of probability must apply

  • Conditions3. All members of the population breed

  • Conditions4. All mating is totally random

  • Conditions5. Everyone produces the same number of offspring

  • Conditions6. There is no migration in or out of the population

  • EquationEquation used to find genotype frequencies:p+2pq+q=1


    p + q = 1

  • p is the frequency of the dominant allele

    q is the frequency of the recessive allele

    p2 is the frequency of the homozygous dominant genotypes

    q2 is the frequency of the homozygous recessive genotypes

  • Equation

    2pq is the frequency of the heterozygotes

  • ExampleAlbinism is only expressed in the phenotype of homozygous recessive individuals (aa).

    The average human frequency of albinism in North America is only about 1 in 20,000.

  • QuestionCalculate the frequencies of the alleles and all three genotypes in this population.

  • SolutionSynthetic Theory of Evolution: Sample Hardy-Weinberg Problem

  • Examples:

    1. In a population, 21% of the individuals are homozygous dominant, 49% are heterozygous and 30% are homozygous recessive. What percentage of the next generation are predicted to be homozygous recessive?

  • 2. 16% of a population is observed to have a continuous hairline (recessive). What percentage of the population possesses the dominant allele? If there are 500 members in the population, how many would be heterozygous?

  • 3. A recessive genetic disorder occurs in 9% of the population. What percentage of the population will be carriers for the disorder? What percentage will be homozygous dominant?

  • Quiz Theoretical Ideashttp://w3.dwm.ks.edu.tw/bio/activelearner/18/ch18summary.html

  • Disturbances to EquilibriumThere are some situations that may make H-W equilibrium of alleles more likely to change:

  • 1) Mutations Whether a mutation is good or bad, often depends on the environment. A harmful mutation can turn out to have a selective advantage if the environment changes over time.

  • 2) Non-random Mating Individuals are often attracted to one another because they value specific traits. Ex. In humans, wolves, elk

  • -will reduce genetic diversity, thus decrease frequency of some alleles3) Inbreeding

  • 4) Genetic Drift - a reduction in the gene pool variation caused purely by chance. Usually in small populations. If a specific allele doesnt reproduce (by chance) it may be lost entirely.

  • Genetic Drift Example

  • 5 ) Gene Flow Migration is the movement of genes into (immigration) / out of (emigration) the population. Some genes may migrate more readily than others.

  • 6 ) Bottleneck Effect occurs when a part of the population is eliminated by chance.

  • 7) Founder Effect - occurs when the founders of a new population have a specific genotype. Ex. polydactyl hands in Amish in Pennsylvania.

  • 8) Natural Selection Selective Advantage: the most important reason for changes to H-W equilibrium

    New mutations may arise that give the organism an advantage over others of the same species

  • These alleles become more common with timeMeans that some alleles are helping individuals to survive and reproduce

  • I. Stabilizing Selection: atypical phenotypes are eliminated, and an average is favored. Ex. birth weight or color.

  • II. Directional Selection an atypical phenotype is selected for because of a progression of change in the environment. Ex. horse evolution, peppered moth.

  • III. Disruptive Selection two or more phenotypes are selected due to different characteristics within a habitat. Ex. fish that feed on bottom vs fish that feed on top.

  • SpeciationDivergence producing new species, two types:Allopatric speciation: physical separation of species drives the splitting of one species into two (or more)Eg. Grand Canyon SquirrelsDarwins Finches

  • May not be immediately obviousEg. Anole lizards in Cuba not physically separated now, but were 5 million years agoAllopatry Animation

  • 2. SympatryDivision of one species into two or more in absence of physical barriersDisputed by some

  • H-W Equilibrium - SummaryDoes not change unless a force is acting upon itThis force is often natural selection leads to evolution