Population Genetics and Evolution Laboratory

PSIBiologyName______

Objective

Students will investigate a genetically inherited trait and apply the Hardy-Weinberg Principle to a population. They will demonstrate the stability of allele frequencies over five generations in an ideal Hardy-Weinberg population and they will then examine the effects of natural selection, heterozygous advantage, and genetic drift on allele frequencies in a simulated mating exercise.

Materials

Each Student needs:

1 strip of PTC Paper

1 strip of Control Paper

4 Index Cards

1 Coin

Also needed: Calculators

Time Requirements:

Pre-lab prep (For teachers): None

Lab activity: approximately 60 min.

Procedure

Part A – Using the Hardy-Weinberg Principle to Calculate Allele Frequencies

You will test your ability to taste PTC (phenylthiocarbamide). The ability to taste this bitter chemical is governed by a dominant allele. You will determine the allele frequencies for this trait in your class population. DO NOT share PTC and control strips with other students. Use them once and dispose of them in the trash when you are done.

  1. Obtain a piece of PTC test paper and place it on your tongue. Notice if you can detect a bitter taste or not. If you can, you possess the trait to taste PTC. Record your results in Table 1.
  1. Obtain a piece of control paper and place it on your tongue. Compare your results with the PTC paper to your results with the control paper to help you determine if you indeed can taste the PTC or not. Record your results in Table 1. Dispose of both test strips in the trash.
  1. Obtain the results for the entire class and enter the results in Table 2
  1. Using the Hardy-Weinberg equation, calculate the frequencies of each allele. You must show your work.

Table 1

Taster / Non Taster
PTC
Control

Table 2

Phenotypes / Allele Frequency Based on the Hardy-Weinberg Equation
Tasters
(p2 + 2pq) / Non Tasters
(q2) / p / q
Class
Population / # / %

Part B – Testing the Hardy-Weinberg Principle

Case 1: Testing and Ideal Population

Your class will serve as the population for this and the following exercises. Each index card represents a haploid chromosome. Each student will have two “A” cards and two “a” cards. Each parent will begin with the genotype Aa; therefore, initial genotype frequencies will be as follows:

AA: 0.25

Aa: 0.50

Aa: 0.25

Record these initial frequencies under Case 1 below.

  1. Obtain four index cards. Label two cards with an “A” and the other two with an “a”. These will serve as your haploid chromosomes. Under Case 1, record “Aa” as your initial genotype.
  1. Find a random “mate”. You can pair off with anyone.
  1. Turn you cards upside down and shuffle them. Turn over the top card in your pile. Your partner should do the same. These two cards will represent the genotype of your first offspring.

Repeat this by each of you turning over a second card and pairing them. This will represent the genotype of your second offspring.

  1. Your partner and yourself will now assume the genotypes of the two offspring that you produced in number 3. One of you assumes the genotype of the first offspring and the second partner assumes the genotype of the second offspring. Each partner will record his or her genotype next to “F1 Genotype” under Case 1.
  1. Depending on which genotype you are assuming, you may have to obtain different cards. For example if the two offspring you produced in number 3 are AA and Aa, one partner begins the next generation with 4 “A” cards and the other partner will retain 2 “A” and 2 “a” cards.
  1. Randomly choose another classmate to pair off with for the next generation and repeat steps 3 and 4. Each partner will record his or her genotype next to “F2 Genotype” under Case 1.
  1. Repeat steps 3 and 4 for three more generations for a total of 5 generations.
  1. Tally the genotypes of the 5th generation of the entire class and record the results under Case 1.
  1. Calculate the frequency of “A” and of “a” after five generations of random mating in your population.

Initial Class Frequencies: AA: _____Aa: _____aa: _____

Your Initial Genotype: _____

F1 Genotype: _____

F2 Genotype: _____

F3 Genotype: _____

F4 Genotype: _____

F5 Genotype: _____

Final Class Frequencies: AA: _____Aa: _____aa: _____

p: ___ q: ____

Number of “A” alleles present at the fifth generation

Number of offspring with genotype AA ______X 2 = ______A alleles

Number of offspring with genotype Aa______X 1 = ______A alleles

Total = ______A alleles

p = TOTAL number of A alleles

TOTAL number of alleles in population= ______

Number of “a” alleles present at the fifth generation

Number of offspring with genotype aa ______X2 = ______a alleles

Number of offspring with genotype Aa ______X1 = ______a alleles

Total = ______a alleles

q = TOTAL number of a alleles

Total number of alleles in the population = ______

Case 2: Selection

In this exercise, we will be adding selection to make it a more realistic situation. In this case, there is 100% selection against homozygous recessive offspring. The recessive allele in this case is mutated making an “aa” individual non-viable. “Aa” and “AA” are viable and will be able to reproduce. You will need to have some extra index cards for this exercise since selection can lead to elimination of certain alleles if an offspring dies due to being “aa”.

  1. Follow the same procedure as in Case 1 except that if you produce an offspring with the “aa” genotype, it does not survive; therefore, you must eliminate these two alleles from the population. I order to maintain population size, you must produce two surviving offspring. If you have to eliminate alleles due to the death of an offspring, you must draw two new alleles from the extra cards. (You can randomly write “A” or “a” on the extra cards).
  1. Repeat the above procedure for a total of five generations, selecting against any “aa” offspring in each generation. Record the genotypes after each generation below.
  1. Tally the genotypes of the 5th generation of the entire class and record the results under below.
  1. Calculate the frequency of “A” and of “a” after five generations of random mating in your population.

Initial Class Frequencies: AA: _____Aa: _____aa: _____

Your Initial Genotype: _____

F1 Genotype: _____

F2 Genotype: _____

F3 Genotype: _____

F4 Genotype: _____

F5 Genotype: _____

Final Class Frequencies: AA: _____Aa: _____aa: _____

p: ___ q: ____

Case 3: Heterozygote Advantage

Another type of selection is in certain diseases where a homozygous dominant individual is more severely affected than a heterozygote. One example is malaria. In this case, the heterozygote is favored over the homozygote dominant genotype and is selected. As in case 2, you will need extra cards for this exercise.

  1. Follow the same procedure as in Case 2, eliminating any “aa” individuals and their alleles. Additionally, if an “AA” offspring is produced, flip a coin. If the coin lands on “heads”, the offspring does not survive. If the coin lands on “tails”, the offspring lives.
  1. Repeat the procedure for a total of five generations and record the genotypes of every generation below.
  1. Calculate the frequency of “A” and of “a” after five generations of random mating in your population.
  1. Continue the procedure for five more generations for a total of ten generations. Start with the genotypes from the end of the fifth generation. Record your results below.
  1. Calculate the allele frequencies after ten generations of random mating.

Initial Class Frequencies: AA: _____Aa: _____aa: _____

Your Initial Genotype: _____

F1 Genotype: _____

F2 Genotype: _____

F3 Genotype: _____

F4 Genotype: _____

F5 Genotype: _____

Final Class Frequencies: AA: _____Aa: _____aa: _____

(After five generations)

p: ___ q: ____

F6 Genotype:_____

F7 Genotype:_____

F8 Genotype:_____

F9 Genotype:_____

F10 Genotype:_____

Analysis

  1. In Case 1, what would the expected values of p and q be after five generations?
  1. How close were your class results to an ideal population?
  1. What happened, if anything, to the allele frequencies after selection was added to the simulation?
  1. Explain the difference in your results from the selection and heterozygote advantage simulations.
  1. Why is the heterozygote genotype important in maintaining genetic variation within a population?