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BIOLOGY Population Genetics Lab

www.njctl.org Evolution

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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 and heterozygous advantage on allele frequencies in a simulated mating exercise.

Purpose Slide 4 / 25

1 strip of PTC Paper 1 strip of Control Paper 4 Index Cards 1 Coin Calculators

Materials Slide 5 / 25 Procedure 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. Step 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. Step 2 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.

Slide 6 / 25 Procedure A:

Using the Hardy-Weinberg Principle to Calculate Allele Frequencies Step 3 Obtain the results for the entire class and enter the results in Table 2. Step 4 Using the Hardy-Weinberg equation, calculate the frequencies of each allele. You must show your work.

Slide 7 / 25 Data

Taster Non Taster PTC Control

Phenotypes Allele Frequency Tasters Non Tasters p q Class Population # % # %

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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.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 1 An Ideal Population

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Step 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.

Step 2 Find a random “mate”. You can pair off with anyone. Step 3 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.

Step 4 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.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 1 An Ideal Population

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Step 5 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. Step 6 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. Step 7 Repeat steps 3 and 4 for three more generations for a total

• f 5 generations.

Step 8 Tally the genotypes of the 5

th generation of the entire class

and record the results under Case 1. Step 9 Calculate the frequency of “A” and of “a” after five generations of random mating in your population.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 1 An Ideal Population

Slide 11 / 25 Data

Initial Class Frequencies: AA: _____ Aa: _____ aa: _____ Your Initial Genotype: _____ F1 Genotype: _____ F2 Genotype: _____ F3 Genotype: _____ F4 Genotype: _____ F5 Genotype: _____

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Data

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

= _________

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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”.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 2 Selection

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Step 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). Step 2 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. Step 3 Tally the genotypes of the 5

th generation of the entire class

and record the results under below. Step 4 Calculate the frequency of “A” and of “a” after five generations of random mating in your population.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 2 Selection

Slide 15 / 25 Data

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: ____

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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

• ver the homozygote dominant genotype and is selected. As in case

2, you will need extra cards for this exercise.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 3 Heterozygous Advantage

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Step 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. Step 2 Repeat the procedure for a total of five generations and record the genotypes of every generation below. Step 3 Calculate the frequency of “A” and of “a” after five generations

• f random mating in your population.

Step 4 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.

Step 5 Calculate the allele frequencies after ten generations of random mating.

Procedure B:

Testing the Hardy-Weinberg Principle: Case 3 Heterozygous Advantage

Slide 18 / 25 Data

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: ____

Slide 19 / 25 Data

F6 Genotype: _____ F7 Genotype: _____ F8 Genotype: _____ F9 Genotype: _____ F10 Genotype: _____

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Population Genetics Lab Analysis

Grade:«grade» Subject:«subject» Date:«date»

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1 In Case 1, what would the expected values of p and q be after five generations?

Students type their answers here

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2 How close were your class results to an ideal population?

Students type their answers here

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3 What happened, if anything, to the allele frequencies after selection was added to the simulation?

Students type their answers here

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4 Explain the difference in your results from the selection and heterozygous advantage simulations.

Students type their answers here

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5 Why is the heterozygote genotype important in maintaining genetic

variation within a population?

Students type their answers here