ADVANCED PLACEMENT BIOLOGY
®
Laboratory 8
Population Genetics and
Evolution
74-6540
TEACHER’S MANUAL
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This protocol has been adapted from the Advanced Placement® Biology Laboratory Manual with permission from the
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the kit contents.
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TEACHER’S MANUAL
LABORATORY
LABORATORY 8
8. POPULATION GENETICS AND EVOLUTION
Objectives
In this laboratory, students will
• learn about the Hardy-Weinberg law of genetic equilibrium
• study the relationship between evolution and changes in allele frequency by
using the class as a sample population
Required
Knowledge
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 of
alleles in a population
• that natural selection can alter allelic frequencies in a population
• the effects of allelic frequencies of selection against the homozygous recessive
or other genotypes
Expectations
At the completion this laboratory, students should be able to
• calculate the frequencies of alleles and genotypes in the gene pool of a
population using the Hardy-Weinberg equations
• discuss natural selection and other causes of microevolution as deviations from
the conditions required to maintain Hardy-Weinberg equilibrium
Time Requirements
Exercise
8A
8B
8C
8D
8E
Preparation
Description
Approximate Time Requirement
Estimating allele frequencies for a
15 minutes
specific trait within a
sample population
A Test of Hardy-Weinberg Equilibrium
30 minutes
Selection
30 minutes
Heterozygote Advantage
30 minutes
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, and
Exercise 8B, Introduction, in the Student Guide).
For Exercise 8A, draw Table 8.2 on the board or overhead for use in collecting class
data.
For Exercises 8B, 8C, and 8D (and optional Exercise 8E), draw Table 8.3 on the board
or overhead. Data collection for the class is best performed by asking for a show of
hands.
3
LABORATORY 8
Student Materials
and Equipment
TEACHER’S MANUAL
Following is a list of the materials needed for one student to perform the exercises in
this lab. Prepare as many setups as needed for your class.
Setup for Each Student
Exercise Exercise Exercise Exercise Exercise
8C
8D
8E
8B
8A
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
1
1
1
2
2
4
4
2
2
4
4
2
2
4
4
2
2
4
4
1
1
1
1
*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 and
local regulations. If you cannot do Exercise 8A with PTC taste paper, any readily
observable trait that is controlled by a single gene locus can be substituted. See Further
Investigations for some suggested traits.
Answers to
Questions in the
Sudent Guide
Exercise 8A: Estimating Allele Frequencies for a Specific Trait Within a
Sample Population
Notice 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.
Table 8.2*
Phenotypic Proportions of Tasters and Nontasters and Frequency of the
Determining Alleles
SG pg.13
Phenotypes
Tasters
(p2 + 2pq)
Count
Class Data
North American
Population
4
% of Total
Nontasters
(q2)
Count
% of Total
Frequency of
the Alleles (%)
p
q
TEACHER’S MANUAL
LABORATORY 8
Exercise 8B: A Test of Hardy-Weinberg Equilibrium
Students must understand that the four cards used in this exercise represent the alleles
present 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 allele
frequencies and are not normally derived from direct counts, as is done in these
exercises. The Hardy-Weinberg equations give the most reliable results when applied to
very large samples. The sample sizes derived in these simulations are too small for the
Hardy-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 have
missed counting someone or have counted someone twice. This condition applies
throughout these simulations.
Table 8.3 Class Totals for
Exercise 8B SG pg. 15
Generation
1
2
3
4
5
Genotype Totals
A/A
A/a
a/a
Exercise 8C: Selection
Students 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 slightly
after five or ten generations (for humans, approximately 200 years).
Table 8.4 Class Totals for
Exercise 8C SG pg. 17
Generation
1
2
3
4
5
Genotype Totals
A/A
A/a
a/a
5
LABORATORY 8
TEACHER’S MANUAL
Exercise 8D: Heterozygote Advantage
Table 8.5 Class Totals for
Exercise 8D
SG pg. 19
Generation
1
2
3
4
5
6
7
8
9
10
6
Genotype Totals
A/A
A/a
a/a
TEACHER’S MANUAL
Further
Investigations
Figure 1:
Left, a free earlobe;
right, an attached
earlobe
LABORATORY 8
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 be
simulated.
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 are
some suggestions taken from Corriher, Charles M., “Mendelian Inheritance in Humans,”
Carolina Tips, Vol. 48, No. 11, November 1, 1985.
Figure 2:
Tongue roller
Figure 3:
Middigital hair
Earlobe Attachment: A dominant allele produces a free, or unattached, earlobe
(Figure 1). The homozygous recessive condition is a direct attachment of the earlobe to
the 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 to
form a tube is due to a dominant allele (Figure 2). Nonrollers are homozygous recessive
for the trait.
Widow’s Peak: The widow’s peak is a distinct downward point of the frontal hairline
and is due to a dominant allele. Homozygous recessive individuals have a straight
hairline. Of course, male pattern baldness, a sex-influenced trait, can complicate scoring
for this phenotype.
Middigital Hair: the presence of midphalangeal hair, hair between the second and third
knuckles (going distally from the hand) is due to a dominant allele (Figure 3). Students
may 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 other
traits. Population studies of left-handedness are among the most fascinating and
controversial of all. Many studies indicate that left-handed people are more prone to
death 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 population
that is left-handed seems not to have changed over thousands of years.
7
LABORATORY 8
LABORATORY
STUDENT GUIDE
8. POPULATION GENETICS AND EVOLUTION
Objectives
In this activity, you will
• learn about the Hardy-Weinberg law of genetic equilibrium
• study the relationship between evolution and changes in allele frequency by
using your class as a sample population
Required
Knowledge
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 of
alleles in a population
• that natural selection can alter allelic frequencies in a population
• the effects of allelic frequencies of selection against the homozygous recessive
or other genotypes
Expectations
At the completion of this laboratory, you should be able to
• calculate the frequencies of alleles and genotypes in the gene pool of a
population using the Hardy-Weinberg formula
• discuss natural selection and other causes of microevolution as deviations from
the conditions required to maintain Hardy-Weinberg equilibrium
Background
The Hardy-Weinberg Theorem states that the frequencies of alleles in a sexually
reproducing population remain constant (in equilibrium) from generation to generation
unless acted upon by outside factors. That is, if we consider two alleles, A and a, in a
population, the reshuffling of alleles that occurs due to meiosis and recombination does
not change the numbers of these alleles in the population. Hardy and Weinberg argued
that a population’s allele and genotype frequencies would remain statistically constant
as long as five conditions were met:
1. The breeding population is very large. In a small population, chance events can
greatly alter allele frequency. For example, if only five individuals of a small
population of deer carry allele a and none of the five reproduce, allele a is
eliminated 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 and
reproducing.
The Hardy-Weinberg Theorem provides a yardstick by which we can measure changes
in allele frequency, and therefore, in evolution. If we can determine the frequency of a
pair of alleles in a population, we can sample that population over several generations
and answer the question, “Is the population evolving with respect to these particular
alleles?” The Hardy-Weinberg equations can be applied to estimate the frequencies of
specified alleles within a population at any given time.
8
STUDENT GUIDE
LABORATORY 8
Consider the following data on Rh blood type from a hypothetical human population:
Table 8.1 Results of Testing for Human Rh Blood Type
Blood
Type
Number of
People
Rh+
4,680
Rh–
1,320
Total
6,000
Assume that Rh blood type is inherited through two alleles: D, a dominant allele for
Rh+ 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 be
expanded 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 homozygous
recessive:
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 is
evolving with respect to the Rh factor alleles.
9
LABORATORY 8
STUDENT GUIDE
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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Exercise 8A: Estimating Allele Frequencies for a Specific Trait Within a
Sample Population
Introduction
Procedure
Using the class as a sample population, you will estimate the allele frequency of a gene
controlling the ability to taste the chemical PTC (phenylthiocarbamide). A bitter-taste
reaction to PTC is evidence of the presence of a dominant allele in either the
homozygous condition (A/A) or the heterozygous condition (A/a). The inability to taste
the 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 has
completely 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. If
you sense little more than the taste of the paper itself, you are a nontaster. Record
the class data in Table 8.2.
Questions
1. Use your class data and the Hardy-Weinberg equations to complete Table 8.2.
Table 8.2 Phenotypic Proportions of Tasters and Nontasters and
Frequencies of the Determining Alleles
Phenotypes
Tasters
(p2 + 2pq)
Count
% of Total
Nontasters
(q2)
Count
% of Total
Frequency of
the Alleles (%)
p
q
Class Data
North American
Population
70
30
2. What percentage of your class are heterozygous tasters? ______________________
10
STUDENT GUIDE
LABORATORY 8
3. What are some reasons that the values for p and q for your class might differ from
the 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 to
remain constant for North America over the next 200 years? State your answer in
terms of the five conditions that Hardy and Weinberg stated must be fulfilled for
allele frequencies to remain constant.
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Exercise 8B: A Test of Hardy-Weinberg Equilibrium
Introduction
Procedure
In this activity, your entire class will simulate a population of randomly mating
individuals. Choose another student at random (for this simulation, assume that gender
and genotype are irrelevant to mate selection.) The population begins with a frequency
of 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: each
represents a chromosome. Two cards (chromosomes) will have allele A and two cards
will have allele a. The four cards represent the products of meiosis. Each “parent” will
contribute 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 the
card on top to contribute to the production of the first offspring. Your partner should
do the same. 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 as the
Generation 1 Genotype on his or her Data Sheet.
2. Retrieve your cards and reshuffle them. Repeat Step 1 to produce a second
offspring. The second partner records the genotype of this offspring on his or her
Data Sheet. The very short reproductive career of this generation is over.
11
LABORATORY 8
STUDENT GUIDE
3. You and your partner now become the next generation by assuming the genotypes
of the two offspring you have produced. That is, Student 1 assumes the genotype of
the first offspring and Student 2 assumes the genotype of the second offspring as
you 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, all
marked a. If you have the genotype A/A, you will need four cards all marked A. If
you 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 the
offspring of the next generation. The sex of your mate does not matter, nor does the
genotype. Repeat Steps 1–3, being sure to record your new genotype, after each
generation, 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 your
hand to report your genotype. Record the class totals in Table 8.3
Table 8.3 Class Totals for Exercise 8B
Generation
1
2
3
4
5
Data Analysis
12
Genotype Totals
A/A
A/a
a/a
Compare your data for Generation 5 in Table 8.3 with the class data for PTC tasting in
Table 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? ___________________________________
STUDENT GUIDE
LABORATORY 8
The Hardy-Weinberg equations are used to give estimates of allele frequencies for large
populations. Here, we have a small population and there is no need to estimate allele
frequencies, 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 =
p=
Total number of A alleles
Total number of alleles in the population
=
_________ A alleles
________
In this case, the total number of alleles in the population is equal to the number of
students 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 =
q=
Questions
Total number of a alleles
Total number of alleles in the population
=
_________ a alleles
________
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 at
equilibrium?
____________________________________________________________________
If not, why not?
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13
LABORATORY 8
STUDENT GUIDE
4. Look back at the five conditions that must be met for allele frequencies to remain
constant. Which, if any, of these conditions might not have been met in this
simulation?
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Exercise 8C: Selection
Introduction
In nature, not all genotypes have the same rate of survival; that is, the environment
might favor some genotypes while selecting against others. An example is human
sickle-cell anemia, a disease caused by a single gene mutation. Individuals who are
homozygous recessive (a/a) often do not survive to reach reproductive maturity. In this
simulation, you will assume that the homozygous recessive individuals never survive
(100% selection against), and that heterozygous and homozygous dominant individuals
survive 100% of the time.
Procedure
Everyone begins with the heterozygous genotype; thus, the initial frequency of each of
the alleles is again 0.5 (50%). Follow the procedure in Exercise 8B, with the following
modification: every time your “offspring” is a/a, assume that it does not survive to
reproduce. Because you want to maintain a constant population size, the same two
parents must try again until they produce two surviving offspring.
Proceed through five generations, selecting against the homozygous recessive offspring
100% of the time. Then, calculate the new p and q frequencies using the method from 8B.
Table 8.4 Class Totals for Exercise 8C
Generation
1
2
3
4
5
Questions
Genotype Totals
A/A
A/a
a/a
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? _____________________________
14
STUDENT GUIDE
LABORATORY 8
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 the
values you just calculated for Exercise 8C. How do the new frequencies of p and q
compare 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 another
five generations.
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5. In a large population, would it be possible to completely eliminate a deleterious
recessive allele? Explain your answer.
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Exercise 8D: Heterozygote Advantage
Introduction
From Exercise 8C, it is easy to see that the lethal recessive allele rapidly decreases in
the population. However, studies show an unexpectedly high frequency of the sickle-cell
allele in some human populations. These populations exist in areas where malaria is (or
until recently was) killing many people. It seems that individuals who are heterozygous
for sickle-cell anemia are slightly more resistant to a deadly form of malaria than are
homozygous dominant individuals. In malaria-ridden areas, there is a slight selection
against homozygous dominant individuals as compared to heterozygotes. This fact is
easily incorporated into our simulations.
15
LABORATORY 8
Procedure
STUDENT GUIDE
Begin again with the genotype A/a. Follow the procedure in Exercise 8B, with the
following modifications: if your offspring is A/A, flip a coin. If heads, the offspring does
not survive. If tails, the offspring does survive. The genotype a/a never survives. Parents
must produce two surviving offspring each generation. This time, simulate ten
generations. Total the class genotypes and then calculate the p and q frequencies for
Generation 5 and for Generation 10. If time permits, the results from another five
generations would be extremely informative.
Table 8.5 Class Totals for Exercise 8D
Generation
1
2
3
4
5
6
7
8
9
10
Questions
Genotype Totals
A/A
A/a
a/a
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 Exercise
8C or Exercise 8D? Explain your answer.
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16
STUDENT GUIDE
LABORATORY 8
4. What is the importance of heterozygotes (the heterozygote advantage) in
maintaining 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/a
individuals reproduce and all of the A/A individuals reproduced. How would you
expect this to change the allele frequencies for Generation 10?
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Exercise 8E: Genetic Drift
Introduction
It is possible to use our simulation to look at the phenomenon of genetic drift in detail.
Procedure
Divide the lab into several smaller, isolated populations. For example, a class of 30
could be divided into 3 separate populations of 10 individuals each. Individuals from
one population do not interact with individuals from other populations. Follow the
procedure in Exercise 8B through five generations. Record the new genotypic
frequencies and then calculate the new frequencies of p and q for each population.
Questions
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 an
evolutionary force?
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17
LABORATORY 8
Hardy-Weinberg
Problems
STUDENT GUIDE
1. In Drosophila, the allele for normal length wings is dominant over the allele for
vestigial 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 the
Hardy-Weinberg equations to estimate the number of homozygous dominant and
heterozygous individuals.
2. The allele for the ability to roll the tongue into a tube is dominant over the allele for
the lack of this ability. In a population of 500 individuals, 25% show the recessive
phenotype. Use the Hardy-Weinberg equations to estimate the number of
homozygous dominant and heterozygous individuals.
3. The allele for the hair pattern called “widow’s peak” is dominant over the allele for
no widow’s peak. In a population of 1,000 individuals, 510 show the dominant
phenotype. About how many individuals would have each one of the three possible
genotypes?
4. In the United States, about 16% of the population is Rh–. The allele for Rh– is
recessive 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 the
time. What is the frequency of the dominant allele?
18
Name ____________________
LABORATORY 8
DATA SHEET
Exercise 8B
Hardy-Weinberg Equilibrium
Initial Class Frequencies
p = 0.5
q = 0.5
My Initial
Genotype
Exercise 8C
Selection
Initial Class Frequencies
p = 0.5
q = 0.5
My Initial
Genotype
A/a
Generation 1
Genotype
Generation 1
Genotype
Generation 2
Genotype
Generation 2
Genotype
Generation 3
Genotype
Generation 3
Genotype
Generation 4
Genotype
Generation 4
Genotype
Generation 5
Genotype
Generation 5
Genotype
Final Class Frequencies
p=
q=
p=
A/a
Generation 1
Genotype
Generation 6
Genotype
Generation 2
Genotype
Generation 7
Genotype
Generation 3
Genotype
Generation 8
Genotype
Generation 4
Genotype
Generation 9
Genotype
Generation 5
Genotype
Generation 10
Genotype
Final Class Frequencies,
Generation 5
q=
My Initial
Genotype
A/a
Generation 1
Genotype
Generation 2
Genotype
Generation 3
Genotype
Generation 4
Genotype
Final Class Frequencies,
Generation 6
p=
q=
Exercise 8E
Genetic Drift
Initial Class Frequencies
p = 0.5
q = 0.5
Initial Class Frequencies
p = 0.5
q = 0.5
My Initial
Genotype
A/a
Final Class Frequencies
Exercise 8D
Heterozygote Advantage
p=
Date _____________________
q=
Generation 5
Genotype
Final Class Frequencies
p=
q=
19
LABORATORY 8
20
Carolina Biological Supply Company
2700 York Road, Burlington, North Carolina 27215
Phone: 800.334.5551 • Fax: 800.222.7112
Technical Support: 800.227.1150 • www.carolina.com
CB251450405