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Population Genetics andEvolution
74-6540
TEACHER’S MANUAL
World-Class Support for Science & Math
ADVANCED PLACEMENT® BIOLOGY
Laboratory 8
©2004 Carolina Biological Supply Company Printed in USA
This protocol has been adapted from the Advanced Placement® Biology Laboratory Manual with permission from theCollege Entrance Examination Board. The AP designation is a registered trademark of the College Board. Theselaboratory materials have been prepared by Carolina Biological Supply Company, which bears sole responsibility forthe kit contents.
In this laboratory, students will
• learn about the Hardy-Weinberg law of genetic equilibrium
• study the relationship between evolution and changes in allele frequency byusing the class as a sample population
Before beginning this laboratory, students should understand
• the process of meiosis and its relationship to the segregation of alleles
• the basics of Mendelian Genetics
• the Hardy-Weinberg equation and its use in determining the frequency ofalleles in a population
• that natural selection can alter allelic frequencies in a population
• the effects of allelic frequencies of selection against the homozygous recessiveor other genotypes
At the completion this laboratory, students should be able to
• calculate the frequencies of alleles and genotypes in the gene pool of apopulation using the Hardy-Weinberg equations
• discuss natural selection and other causes of microevolution as deviations fromthe conditions required to maintain Hardy-Weinberg equilibrium
Exercise Description Approximate Time Requirement
8A Estimating allele frequencies for a 15 minutesspecific trait within a sample population
8B A Test of Hardy-Weinberg Equilibrium 30 minutes
8C Selection 30 minutes
8D Heterozygote Advantage 30 minutes
8E Genetic Drift (Optional) 30 minutes
Photocopy the Student Guide from this manual for your class.
Mark index cards with the letters A and a (see Setup for Each Student, below, andExercise 8B, Introduction, in the Student Guide).
For Exercise 8A, draw Table 8.2 on the board or overhead for use in collecting classdata.
For Exercises 8B, 8C, and 8D (and optional Exercise 8E), draw Table 8.3 on the boardor overhead. Data collection for the class is best performed by asking for a show ofhands.
TEACHER’S MANUAL LABORATORY 8
3
Time Requirements
Required
Knowledge
Expectations
Objectives
Preparation
LABORATORY 8. POPULATION GENETICS AND EVOLUTION
LABORATORY 8 TEACHER’S MANUAL
4
Following is a list of the materials needed for one student to perform the exercises inthis lab. Prepare as many setups as needed for your class.
*Item not included in kit.
Note: Each student should begin with four cards: two labeled A and two labeled a.Additional cards may be needed for the exercises that come later.
Safety
Some school systems ban the placement of anything in the mouth during a science class.Some school systems ban the use of PTC taste papers. Abide by your system’s state andlocal regulations. If you cannot do Exercise 8A with PTC taste paper, any readilyobservable trait that is controlled by a single gene locus can be substituted. See FurtherInvestigations for some suggested traits.
Exercise 8A: Estimating Allele Frequencies for a Specific Trait Within aSample PopulationNotice that we had to calculate a value for q before we could we could determine p.Why is this true?
1. Use your class data and the Hardy-Weinberg equations to complete Table 8.2.
strip of control paper
strip of PTC paper
3 × 5″ index cards labeled A
3 × 5″ index cards labeled a
extra index cards labeled A
extra index cards labeled a
*calculator with a square root function
Exercise8B
Exercise8C
Exercise8D
Exercise8E
2
2
4
4
1
2
2
4
4
1
2
2
4
4
1
2
2
4
4
1
Setup for Each Student
Student Materialsand Equipment
Exercise8A
1
1
1
Answers toQuestions in theSudent Guide
Phenotypes
Tasters Nontasters Frequency of(p2 + 2pq) (q2) the Alleles (%)
Count % of Total Count % of Total p qClass Data
North AmericanPopulation
Table 8.2*
Phenotypic Proportions of Tasters and Nontasters and Frequency of the
Determining Alleles
SG pg.13
TEACHER’S MANUAL LABORATORY 8
5
Exercise 8B: A Test of Hardy-Weinberg EquilibriumStudents must understand that the four cards used in this exercise represent the allelespresent in the representative gametes produced as a result of the process of meiosis.They should also understand that the values for p and q are estimates of allelefrequencies and are not normally derived from direct counts, as is done in theseexercises. The Hardy-Weinberg equations give the most reliable results when applied tovery large samples. The sample sizes derived in these simulations are too small for theHardy-Weinberg equations to give accurate results; thus the need to use direct counts.
When taking the class data, remember that the totals for all of the genotypes (A/A + A/a+ a/a) should remain constant; that is, population size remains constant. If not, you havemissed counting someone or have counted someone twice. This condition appliesthroughout these simulations.
Exercise 8C: SelectionStudents may expect that the a allele will be rapidly eliminated; it will persist “hidden”in the heterozygote for many generations and the value of p will increase only slightlyafter five or ten generations (for humans, approximately 200 years).
Genotype Totals
Generation A/A A/a a/a
1
2
3
4
5
Table 8.3 Class Totals for
Exercise 8B SG pg. 15
Genotype Totals
Generation A/A A/a a/a
1
2
3
4
5
Table 8.4 Class Totals for
Exercise 8C SG pg. 17
LABORATORY 8 TEACHER’S MANUAL
6
Exercise 8D: Heterozygote Advantage
Genotype Totals
Generation A/A A/a a/a
1
2
3
4
5
6
7
8
9
10
Table 8.5 Class Totals for
Exercise 8D
SG pg. 19
TEACHER’S MANUAL LABORATORY 8
7
Other kinds of forces that affect allele frequencies in a population, e.g., genetic drift,gene flow, changing the value of p, or changing the extent of selection, can also besimulated.
For further reference see “Evolution—More Than a Game,” by A.H. Markart III and P.Hyland, 1977, A.I.B.S. Education Review, Vol.6 (No.3).
The distributions of many other human genetic traits can be investigated. Following aresome suggestions taken from Corriher, Charles M., “Mendelian Inheritance in Humans,”Carolina Tips, Vol. 48, No. 11, November 1, 1985.
Earlobe Attachment: A dominant allele produces a free, or unattached, earlobe (Figure 1). The homozygous recessive condition is a direct attachment of the earlobe tothe head. Other genes affect the size and shape of the ear.
Tongue Rolling: The ability to roll the edges of the tongue upward from the sides toform a tube is due to a dominant allele (Figure 2). Nonrollers are homozygous recessivefor the trait.
Widow’s Peak: The widow’s peak is a distinct downward point of the frontal hairlineand is due to a dominant allele. Homozygous recessive individuals have a straighthairline. Of course, male pattern baldness, a sex-influenced trait, can complicate scoringfor this phenotype.
Middigital Hair: the presence of midphalangeal hair, hair between the second and thirdknuckles (going distally from the hand) is due to a dominant allele (Figure 3). Studentsmay require a hand lens to determine their phenotypes, as the hairs can be quite small.
Students can report on studies of the distribution of human blood groups and othertraits. Population studies of left-handedness are among the most fascinating andcontroversial of all. Many studies indicate that left-handed people are more prone todeath from various diseases and accidents. Since left-handedness seems to be inherited,there should be selection pressure against it, but the percentage of the human populationthat is left-handed seems not to have changed over thousands of years.
Further
Investigations
Figure 3:Middigital hair
Figure 2:Tongue roller
Figure 1:Left, a free earlobe;right, an attachedearlobe
LABORATORY 8 STUDENT GUIDE
8
In this activity, you will
• learn about the Hardy-Weinberg law of genetic equilibrium
• study the relationship between evolution and changes in allele frequency byusing your class as a sample population
Before beginning this laboratory, you should understand
• the process of meiosis and its relationship to the segregation of alleles
• the basics of Mendelian Genetics
• the Hardy-Weinberg equation and its use in determining the frequency ofalleles in a population
• that natural selection can alter allelic frequencies in a population
• the effects of allelic frequencies of selection against the homozygous recessiveor other genotypes
At the completion of this laboratory, you should be able to
• calculate the frequencies of alleles and genotypes in the gene pool of apopulation using the Hardy-Weinberg formula
• discuss natural selection and other causes of microevolution as deviations fromthe conditions required to maintain Hardy-Weinberg equilibrium
The Hardy-Weinberg Theorem states that the frequencies of alleles in a sexuallyreproducing population remain constant (in equilibrium) from generation to generationunless acted upon by outside factors. That is, if we consider two alleles, A and a, in apopulation, the reshuffling of alleles that occurs due to meiosis and recombination doesnot change the numbers of these alleles in the population. Hardy and Weinberg arguedthat a population’s allele and genotype frequencies would remain statistically constantas long as five conditions were met:
1. The breeding population is very large. In a small population, chance events cangreatly alter allele frequency. For example, if only five individuals of a smallpopulation of deer carry allele a and none of the five reproduce, allele a iseliminated from the population.
2. Mating is random. Individuals show no preference for a particular phenotype.
3. There is no mutation of the alleles.
4. No differential migration occurs. No immigration or emigration.
5. There is no selection. All genotypes have an equal chance of surviving andreproducing.
The Hardy-Weinberg Theorem provides a yardstick by which we can measure changesin allele frequency, and therefore, in evolution. If we can determine the frequency of apair of alleles in a population, we can sample that population over several generationsand answer the question, “Is the population evolving with respect to these particularalleles?” The Hardy-Weinberg equations can be applied to estimate the frequencies ofspecified alleles within a population at any given time.
LABORATORY 8. POPULATION GENETICS AND EVOLUTION
Objectives
Required
Knowledge
Background
Expectations
STUDENT GUIDE LABORATORY 8
9
Consider the following data on Rh blood type from a hypothetical human population:
Assume that Rh blood type is inherited through two alleles: D, a dominant allele forRh+ blood type, and a recessive allele d for Rh– blood type.
Using the Hardy-Weinberg equations,
let p = frequency of the dominant allele (D, in this case)
and q = frequency of the recessive allele (d)
Each person is either Rh+ or Rh–
therefore,
p + q = 1 or 100% of the population
Since humans are diploid, individuals may be homozygous dominant (D/D),heterozygous (D/d), or homozygous recessive (d/d). Thus, the basic equation must beexpanded to represent all the genotype frequencies.
Thus, (p + q) (p + q) = 1
or p2 + 2pq + q2 = 1
where p2 = frequency of the homozygous dominant (D/D),
2pq = frequency of the heterozygous condition (D/d),
and q2 = frequency of the homozygous recessive (d/d).
Notice that p2 + 2pq = frequency of the Rh+ phenotype in the population.
From the data in Table 8.1, we can now calculate q2, the frequency of the homozygousrecessive:
q2 = 1320/6000 = 0.22
then q = √ 0.22 = 0.47
p + q = 1
p = 1 – q
p = 1 – 0.47 = 0.53
This tells us that 53% of the population tested has the allele D and 47% has the allele d.
If we repeated our sample over several generations, we could tell if this population isevolving with respect to the Rh factor alleles.
Table 8.1 Results of Testing for Human Rh Blood Type
Blood Number ofType People
Rh+ 4,680
Rh– 1,320
Total 6,000
LABORATORY 8 STUDENT GUIDE
10
Notice that we had to calculate a value for q before we could we could determine p.Why is this true?
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Using the class as a sample population, you will estimate the allele frequency of a genecontrolling the ability to taste the chemical PTC (phenylthiocarbamide). A bitter-tastereaction to PTC is evidence of the presence of a dominant allele in either thehomozygous condition (A/A) or the heterozygous condition (A/a). The inability to tastethe chemical at all depends on the presence of homozygous recessive alleles (a/a).
1. Press a strip of control taste paper to your tongue tip. Wait until your saliva hascompletely saturated the paper, then note the taste of the paper.
2. Now repeat Step 1 using the PTC taste paper. PTC tasters will sense a bitter taste.For the purposes of this exercise, these individuals are considered to be tasters. Ifyou sense little more than the taste of the paper itself, you are a nontaster. Recordthe class data in Table 8.2.
1. Use your class data and the Hardy-Weinberg equations to complete Table 8.2.
2. What percentage of your class are heterozygous tasters? ______________________
Introduction
Exercise 8A: Estimating Allele Frequencies for a Specific Trait Within a Sample Population
Procedure
Questions
Phenotypes
Tasters Nontasters Frequency of(p2 + 2pq) (q2) the Alleles (%)
Count % of Total Count % of Total p qClass Data
North American70 30Population
Table 8.2 Phenotypic Proportions of Tasters and Nontasters and
Frequencies of the Determining Alleles
STUDENT GUIDE LABORATORY 8
11
3. What are some reasons that the values for p and q for your class might differ fromthe same frequencies reported for the North American population?
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4. Would you expect the frequencies of the alleles for PTC tasting and nontasting toremain constant for North America over the next 200 years? State your answer interms of the five conditions that Hardy and Weinberg stated must be fulfilled forallele frequencies to remain constant.
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In this activity, your entire class will simulate a population of randomly matingindividuals. Choose another student at random (for this simulation, assume that genderand genotype are irrelevant to mate selection.) The population begins with a frequencyof 0.5 (50%) for the dominant allele A and also 0.5 (50%) for the recessive allele a.Your initial genotype is A/a. Record this on the Data Sheet. You have four cards: eachrepresents a chromosome. Two cards (chromosomes) will have allele A and two cardswill have allele a. The four cards represent the products of meiosis. Each “parent” willcontribute a haploid set of chromosomes to the next generation.
1. Turn the four cards over so the letters are not showing, shuffle them, and take thecard on top to contribute to the production of the first offspring. Your partner shoulddo the same. Put the two cards together. The two cards represent the alleles of thefirst offspring. One of you should record the genotype of this offspring as theGeneration 1 Genotype on his or her Data Sheet.
2. Retrieve your cards and reshuffle them. Repeat Step 1 to produce a secondoffspring. The second partner records the genotype of this offspring on his or herData Sheet. The very short reproductive career of this generation is over.
Exercise 8B: A Test of Hardy-Weinberg Equilibrium
Introduction
Procedure
LABORATORY 8 STUDENT GUIDE
12
3. You and your partner now become the next generation by assuming the genotypesof the two offspring you have produced. That is, Student 1 assumes the genotype ofthe first offspring and Student 2 assumes the genotype of the second offspring asyou have recorded them on your Data Sheets. Obtain additional cards if necessary.For example, if you now have the genotype a/a, you will need four cards, allmarked a. If you have the genotype A/A, you will need four cards all marked A. Ifyou have the genotype A/a, keep the original four cards.
4. Now, randomly seek out another person with whom to mate in order to produce theoffspring of the next generation. The sex of your mate does not matter, nor does thegenotype. Repeat Steps 1–3, being sure to record your new genotype, after eachgeneration, on your Data Sheet. Repeat this exercise to produce five generations.
5. Your teacher will collect class data for Generation 5 by asking you to raise yourhand to report your genotype. Record the class totals in Table 8.3
Compare your data for Generation 5 in Table 8.3 with the class data for PTC tasting inTable 8.2. What information do you have for Generation 5 that you do not have in Table 8.2?
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From Table 8.3, what is the population size? ___________________________________
Genotype Totals
Generation A/A A/a a/a
1
2
3
4
5
Table 8.3 Class Totals for Exercise 8B
Data Analysis
STUDENT GUIDE LABORATORY 8
13
The Hardy-Weinberg equations are used to give estimates of allele frequencies for largepopulations. Here, we have a small population and there is no need to estimate allelefrequencies, because we have actual counts. Calculate p and q as follows:
Number of A alleles present at the fifth generation
Number of offspring with genotype A/A _________ × 2 = _________ A alleles
Number of offspring with genotype A/a _________ × 1 = _________ A alleles
Total = _________ A alleles
p = Total number of A alleles = ________Total number of alleles in the population
In this case, the total number of alleles in the population is equal to the number ofstudents in the class × 2.
Number of a alleles present at the fifth generation
Number of offspring with genotype a/a _________ × 2 = _________ a alleles
Number of offspring with genotype A/a _________ × 1 = _________ a alleles
Total = _________ a alleles
q = Total number of a alleles = ________Total number of alleles in the population
1. What are the frequencies of the alleles in Generation 5?
a. the frequency (q) of allele a in Generation 5? _____________________________
b. the frequency (p) of allele A in Generation 5? _____________________________
2. Are the values for p and q in Generation 5 different from the beginning values?____________________________________________________________________
3. Is your answer to Question 1, above, consistent with the alleles being atequilibrium?____________________________________________________________________
If not, why not?
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Questions
LABORATORY 8 STUDENT GUIDE
14
4. Look back at the five conditions that must be met for allele frequencies to remainconstant. Which, if any, of these conditions might not have been met in thissimulation?
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In nature, not all genotypes have the same rate of survival; that is, the environmentmight favor some genotypes while selecting against others. An example is humansickle-cell anemia, a disease caused by a single gene mutation. Individuals who arehomozygous recessive (a/a) often do not survive to reach reproductive maturity. In thissimulation, you will assume that the homozygous recessive individuals never survive(100% selection against), and that heterozygous and homozygous dominant individualssurvive 100% of the time.
Everyone begins with the heterozygous genotype; thus, the initial frequency of each ofthe alleles is again 0.5 (50%). Follow the procedure in Exercise 8B, with the followingmodification: every time your “offspring” is a/a, assume that it does not survive toreproduce. Because you want to maintain a constant population size, the same twoparents must try again until they produce two surviving offspring.
Proceed through five generations, selecting against the homozygous recessive offspring100% of the time. Then, calculate the new p and q frequencies using the method from 8B.
1. What are the frequencies of the alleles in Generation 5?
a. the frequency (q) of allele a in Generation 5? _____________________________
b. the frequency (p) of allele A in Generation 5? _____________________________
Exercise 8C: Selection
Introduction
Procedure
Genotype Totals
Generation A/A A/a a/a
1
2
3
4
5
Table 8.4 Class Totals for Exercise 8C
Questions
STUDENT GUIDE LABORATORY 8
15
2. Are the values for p and q in Generation 5 different from the beginning values?Explain your answer.
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3. Compare the values for p and q calculated for Generation 5, Exercise 8B with thevalues you just calculated for Exercise 8C. How do the new frequencies of p and qcompare to the initial frequencies in Exercise 8B?
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4. Predict what would happen to the frequencies of p and q if you simulated anotherfive generations.
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5. In a large population, would it be possible to completely eliminate a deleteriousrecessive allele? Explain your answer.
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From Exercise 8C, it is easy to see that the lethal recessive allele rapidly decreases inthe population. However, studies show an unexpectedly high frequency of the sickle-cellallele in some human populations. These populations exist in areas where malaria is (oruntil recently was) killing many people. It seems that individuals who are heterozygousfor sickle-cell anemia are slightly more resistant to a deadly form of malaria than arehomozygous dominant individuals. In malaria-ridden areas, there is a slight selectionagainst homozygous dominant individuals as compared to heterozygotes. This fact iseasily incorporated into our simulations.
Exercise 8D: Heterozygote Advantage
Introduction
LABORATORY 8 STUDENT GUIDE
16
Begin again with the genotype A/a. Follow the procedure in Exercise 8B, with thefollowing modifications: if your offspring is A/A, flip a coin. If heads, the offspring doesnot survive. If tails, the offspring does survive. The genotype a/a never survives. Parentsmust produce two surviving offspring each generation. This time, simulate tengenerations. Total the class genotypes and then calculate the p and q frequencies forGeneration 5 and for Generation 10. If time permits, the results from another fivegenerations would be extremely informative.
1. What are the frequencies of the alleles in Generation 10?
a. the frequency (q) of allele a in Generation 10? ____________________________
b. the frequency (p) of allele A in Generation 10? ____________________________
2. Account for the differences in p and q frequencies from Exercise 8C to Exercise 8D.
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3. Do you think the recessive allele will be completely eliminated in either Exercise8C or Exercise 8D? Explain your answer.
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Procedure
Questions
Genotype Totals
Generation A/A A/a a/a
1
2
3
4
5
6
7
8
9
10
Table 8.5 Class Totals for Exercise 8D
STUDENT GUIDE LABORATORY 8
17
4. What is the importance of heterozygotes (the heterozygote advantage) inmaintaining genetic variation in populations?
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5. Suppose you repeated this activity, but you did the coin toss to determine if the A/aindividuals reproduce and all of the A/A individuals reproduced. How would youexpect this to change the allele frequencies for Generation 10?
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It is possible to use our simulation to look at the phenomenon of genetic drift in detail.
Divide the lab into several smaller, isolated populations. For example, a class of 30could be divided into 3 separate populations of 10 individuals each. Individuals fromone population do not interact with individuals from other populations. Follow theprocedure in Exercise 8B through five generations. Record the new genotypicfrequencies and then calculate the new frequencies of p and q for each population.
1. Explain how the initial genotypic frequencies of the populations compare.
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2. What do your results indicate about the importance of population size as anevolutionary force?
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Exercise 8E: Genetic Drift
Introduction
Procedure
Questions
LABORATORY 8 STUDENT GUIDE
18
1. In Drosophila, the allele for normal length wings is dominant over the allele forvestigial wings (vestigial wings are stubby, little curls that cannot be used for flight).In a population of 1,000 individuals, 360 show the recessive phenotype. Use theHardy-Weinberg equations to estimate the number of homozygous dominant andheterozygous individuals.
2. The allele for the ability to roll the tongue into a tube is dominant over the allele forthe lack of this ability. In a population of 500 individuals, 25% show the recessivephenotype. Use the Hardy-Weinberg equations to estimate the number ofhomozygous dominant and heterozygous individuals.
3. The allele for the hair pattern called “widow’s peak” is dominant over the allele forno widow’s peak. In a population of 1,000 individuals, 510 show the dominantphenotype. About how many individuals would have each one of the three possiblegenotypes?
4. In the United States, about 16% of the population is Rh–. The allele for Rh– isrecessive to the allele for Rh+. If a high school has a population of 2,000 students,about how many students would have each one of the three possible genotypes?
5. In certain African countries, 4% of the newborn babies have sickle-cell anemia,which is a recessive trait. Out of a random population of 1,000 newborn babies,about how many babies would have each one of the three possible genotypes?
6. In a certain population, the dominant phenotype of a certain trait occurs 91% of thetime. What is the frequency of the dominant allele?
Hardy-WeinbergProblems
19
LABORATORY 8 DATA SHEET
My InitialA/a
Genotype
Generation 1
Genotype
Generation 2
Genotype
Generation 3
Genotype
Generation 4
Genotype
Generation 5
Genotype
Final Class Frequencies
p = q =
Exercise 8B
Hardy-Weinberg Equilibrium
Initial Class Frequencies
p = 0.5 q = 0.5
My InitialA/a
Genotype
Generation 1
Genotype
Generation 2
Genotype
Generation 3
Genotype
Generation 4
Genotype
Generation 5
Genotype
Final Class Frequencies
p = q =
Exercise 8C
Selection
Initial Class Frequencies
p = 0.5 q = 0.5
My InitialA/a
Genotype
Generation 1
Genotype
Generation 2
Genotype
Generation 3
Genotype
Generation 4
Genotype
Generation 5
Genotype
Final Class Frequencies
p = q =
Exercise 8E
Genetic Drift
Initial Class Frequencies
p = 0.5 q = 0.5My Initial
A/aGenotype
Generation 1
Genotype
Generation 2
Genotype
Generation 3
Genotype
Generation 4
Genotype
Generation 5
Genotype
Final Class Frequencies,
Generation 5
p = q =
Exercise 8D
Heterozygote Advantage
Generation 6
Genotype
Generation 7
Genotype
Generation 8
Genotype
Generation 9
Genotype
Generation 10
Genotype
Final Class Frequencies,
Generation 6
p = q =
Initial Class Frequencies
p = 0.5 q = 0.5
Name ____________________
Date _____________________
20
LABORATORY 8
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