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Transcript of Population Genetics and Hardy-Weinberg Populations Lab ... · PDF filePopulation Genetics and...

  • Population Genetics and Hardy-Weinberg Populations Lab

    General Biology 2

    Learning Objectives

    1. You will determine if a sample random population meets the definition of an Ideal Population for a

    non-apparent trait.

    2. You will simulate a randomly mating population to see if this maintains an Ideal Population and

    discuss reasons that may result in it moving away from an Ideal Population.

    3. You will model a large, randomly mating population and investigate the effects of mutation, selection

    and population size.

    4. You will formulate a hypothesis about the effect of various environments on a model population and

    then design an experiment to test this hypothesis.

    Introduction

    Mendels work on mechanisms of inheritance was not sufficient to determine the effect that genetics

    had on evolution. Later work by various scientists undertook the question of how genes and alleles

    interacted on a population-level scale and how the changes in this distribution led to observable

    evolution of species. The study of genes as a function of the entire population, rather than in

    individuals, is referred to as population genetics, and it is easiest to explain using the work of G.H. Hardy

    (a mathematician) and Wilhelm Weinberg (a physician who independently developed the concept

    contemporary to Hardy).

    Hardy-Weinberg Genetic Equilibrium

    The concept of genetic equilibrium is that the allele and genotype frequencies of a population will

    remain constant (in equilibrium) unless a disturbing force is introduced into the population. A

    population in genetic equilibrium is considered an Ideal Population and has a number of distinct

    characteristics.

    1. The population is very large.

    2. The population randomly mates

    3. The alleles of the population have identical chances of success

    4. There is no immigration out of or emigration into the population

    5. There is no mutation

    Obviously, very few populations meet these criteria and therefore very few populations are in genetic

    equilibrium. This is a good thing, since genetic equilibrium implies an unchanging genetic population

    (remember though this will be a dynamic, not static, equilibrium) and evolution is by definition a change

    in the population gene allele frequencies.

    Genetic equilibrium can be mathematically described in the following manner. For a gene with two

    alleles, their frequencies (q and p, expressed as decimals) would be mathematically related by the

    following equation.

  • q2 + 2pq + p2 = 1 Equation 1

    For a gene with simple dominant/recessive inheritance, where q is the dominant allele you get the

    following.

    q2 + 2pq = frequency of dominant phenotype Equation 2

    p2 = frequency of recessive phenotype Equation 3

    It should be apparent from these equations that 2pq represents the frequency of the heterozygote

    while q2 and p2 represent the homozygotes. The equations can be drawn out to include even more

    alleles, but quickly become mathematically tedious.

    Testing a Random Population

    We can learn to work with the basic mathematics of Hardy-Weinberg and analyze some real data with a

    very simple experiment on the class. We will test whether the class population is in agreement with the

    population average distribution for a specific genetic trait. For this to work we need a trait that easily

    measured, demonstrates simple dominant/recessive inheritance, and is non-apparent in the overall

    population (so as to limit bias). The trait will use is the ability to test the chemical phenylthiocarbamide

    (PTC). Being able to taste the compound is a dominant trait in humans and allows you to detect a very

    bitter taste from this compound (dont ask me to describe it any better, since I cannot taste it).

    Nontasters possess the recessive phenotype.

    Question 1

    If you use A and a to designate the alleles for tasting and nontasting, what would the genotype(s) of

    a taster and nontaster be?

    Procedure

    1. Take a piece of the PTC test paper and place it on your tongue.

    2. Record whether you are a taster or nontaster on the lab data sheet.

    3. Determine the percentage of the population that are tasters and nontasters from pooled lab

    numbers.

    4. Since nontasters are homozygote recessives, the allele frequency for the recessive gene can be

    determined by taking the square root of the decimal percentage for nontasters (p2 in equation

    3).

    5. The total distribution of alleles is p+q = 1, therefore the allele frequency for q (dominant allele)

    can be calculated.

    6. Determining the frequency of homozygous dominant individuals is determined by squaring q.

    7. The frequency of heterozygotes is determined by 2pq.

  • 8. Record your results in provided table on the lab data sheet.

    9. Compare to the provided values for North American populations.

    Question 2

    What factors could play into our sample being different from the North American average.

    An Ideal Population

    In this experiment the entire class will represent an entire breeding population. In order to ensure

    random mating, choose another student at random. The class will simulate a population of randomly

    mating heterozygous individuals with an initial gene frequency of .5 for the dominant allele A and the

    recessive allele a and genotype frequencies of .25AA, .50 Aa and .25 aa. Your initial genotype is Aa.

    Record this in your notebook. Each member of the class will receive four cards. Two cards have a and

    two cards have A. The four cards represent the products of meiosis. Each parent contributes a haploid

    set of chromosomes to the next generation.

    Procedure

    1. Begin the experiment by turning over the four cards so the letters are not showing, shuffle

    them, and take the card on top to contribute to the production of the first offspring. Your

    partner should do the same.

    2. Put the two cards together. The two cards represent the alleles of the first offspring. One of you

    should record the genotype of this offspring in your notebook.

    3. Each student pair must produce two offspring, so all four cards must be reshuffled and the

    process repeated to produce a second offspring. Then, the other partner should record the

    genotype. The very short reproductive career of this generation is over.

    4. Now you and your partner need to assume the genotypes of the two new offspring.

    5. Next, the students should obtain the cards requires to assume their new genotype.

    6. Each person should then randomly pick out another person to mate with in order to produce

    the offspring of the next generation. Follow the same mating methods used to produce

    offspring of the first generation.

  • 7. Record your data. Remember to assume your new genotype after each generation.

    8. We will collect class data after each generation.

    Analysis

    1. Determine the allele frequencies of each allele in each of the generations.

    2. Compare these to the initial values for the alleles.

    Question 3

    Is this population in genetic equilibrium, if not, why not?

    Return of the Dotties

    You guys remember the Dotties from last semester in BIO161. For those that have blocked that memory

    out, they are the hypothetical little creatures that we use to mimic populations. In BIO161 we simply

    black boxed the equations for how these populations bred, but now we will elaborate a little on their

    genetics.

    Dotties come in six colors and this is governed by three alleles for a single locus. The phenotypes and

    genotypes are as follows:

    Color Genotype

    Blue Purple

    Red Orange Yellow Green

    BB BR RR RY YY BY

    Question 4

    From the preceding information, what type of inheritance pattern is most likely for the Dotties with

    regards to color?

  • We will be starting with initial populations in equilibrium (even distributed frequencies of alleles) and

    looking at the effects of various changes on the equilibrium. For the work I have provided a spreadsheet

    to calculate the allele frequencies and successive generations. Each population should start with sixty

    (60) total Dotties.

    Initial Population Behavior

    We are going to look at an initial population of Dotties in a neutral background to note their behavior.

    This population will be fairly large (60 total), not subject to mutation or emigration/immigration and

    randomly mate (all Dotties randomly mate by definition). This means that we are only limited by the

    success of the alleles. We will provide a random selection and see if the resultant population is in

    equilibrium.

    Procedure

    1. Count out ten (10) of each color Dotty and place them in the small beaker.

    2. Shake the Dotties in the beaker and then pour the across the neutral grey fabric background.

    3. Starting at the upper left corner of the background scan across the fabric from left to right, top

    to bottom removing every third Dotty you come across.

    4. Recover the remaining forty (40) Dotties and record their phenotype distribution in your

    notebook.

    Analysis

    1. Determine the allele frequencies for the three alleles.

    2. Determine the expected phenotype distributions given an Ideal Population.

    3. Construct a table with three columns (Genotype, Observed Frequency, Expecte