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POPULATION GENETICS

The goal of the POPULATION GENETICIST is to understand the genetic composition of a population and the forces that determine and change that composition, sometimes leading to reproductive isolation between populations that were formerly the same species.
POPULATION GENETICISTS study evolution by devising mathematical models to describe how gene and allele frequencies change in populations.

In order for this to work for a particular trait the population geneticist has decided to study, GENOTYPE MUST BE REFLECTED BY PHENOTYPE. (Now that molecular techniques allow us to analyze DNA sequences almost as if they were phenotypes, this definition is expanding.)

  • This is fairly straightforward for traits controlled by one locus
  • It's incredibly complex with polygenic traits, and traits that express variable penetrance, expressivity and expression due to environmental factors.
    Most often, the relative frequency of alleles is studied, rather than the changing frequency of diploid genotypes (AA, Aa and aa).

    Since every diploid individual in a population has two alleles for every gene, the number of alleles for any given gene in the population will be double the number of individual organisms.

    Let's pick ONE GENE with TWO ALLELES (one dominant, one recessive) and examine how the relative frequency of those two alleles can either remain constant or change over generations in a hypothetical population.

    HARDY-WEINBERG EQUILIBRIUM

    1908 - G.H. Hardy, a British mathematician, and W. Weinberg, a German physician, independently reported a mathematical rule that describes allele frequencies of a particular gene in question in a population IF EVOLUTION IS NOT OCCURRING IN THE POPULATION. It's an expansion of the binomial equation,

    (p + q)2

    THE RULES:

  • For a population segregating two alleles (in our case, one dominant and one recessive) of a particular gene in which A is the dominant allele and a is the recessive allele, the total frequency of all alleles, A + a = 1.0.

  • The total number of either allele is equal to:

    # of the allele (A or a)/total # of alleles in the population

    Hence, p + q = 1.0 (100% of the alleles of this gene in this population)

    Hardy and Weinberg independently noted that at any given point in time, and at any given relative "starting" frequency of A and a, the genotype frequencies in a idealized, model population would be equal to

    p2 + 2pq + q2

    ...in which

    Let's examine this in our population of beetles.

    We have a population of 1000 beetles in which A codes for black elytra (wing covers) and a codes for red elytra. If 50% of the alleles for elytra color are A, and 50% are a, then

    If the gene for elytra color is not undergoing any change in relative frequency of its alleles (i.e., if there's no evolution going on in this population with respect to elytra color), then the frequencies of genotypes in our idealized population of beetles are predicted by the Hardy-Weinberg model to be:

    0.52 + 2(0.5)(0.5) + 0.52

    or

  • 25% homozygous dominant individuals
  • 50% heterozygous individuals
  • 25% homozygous recessive individuals

    • So if this population consists of 1000 individuals, and this particular gene is not undergoing any kind of evolutionary change, then you would expect 250 of the beetles to be AA, 500 to be Aa and 250 to be aa.

    If you start with allele frequencies different from 50% each, then simply take the relative frequencies and plug into the equation to get the expected genotype frequencies in your population (for example, 10% A and 90% a, or whatever your population might have).

    The distribution of genotypes in a population in Hardy-Weinberg equilibrium can be graphically expressed in this way.

    If genotype frequencies in your study population deviate from those predicted by the HW equation (i.e., from your calculation), then something EVOLUTIONARY is going on.

    HARDY-WEINBERG IS AN IDEALIZED MODEL TO WHICH ACTUAL POPULATIONS CAN BE COMPARED. This mathematical model assumes that...

  • the population is infinitely large
  • mating is random
  • there is no mutation of the gene in question
  • there no migration into or out of this population
  • there is no natural selection acting on the gene in question ...and this is not always the case! If any of these conditions is NOT met, then the allele frequencies may vary from those predicted by HW, and this means the population is EVOLVING, at least with respect to the particular gene you are studying.

    MATERIAL FOR EXAM III ENDS HERE