AP Biology
Ms. Ellis
Name_______________________
Lab 8: Population Genetics and Evolution1
Overview
In this lab you will:
1.
2.
learn about the Hardy-Weinberg law of genetic equilibrium, and
study the relationship between evolution and change in allele frequency by using your class to
represent a sample population.
Objectives
Before doing this lab you should understand:
1. how natural selection can alter allelic frequencies in a population
2. the Hardy-Weinberg equation and its use in determining the frequency of alleles in a population,
and
3. the effects on allelic frequencies of selection against the homozygous recessive or other genotypes.
After doing this lab you should be able to:
1. calculate the frequencies of alleles and genotypes in the gene pool of a population using the
Hardy-Weinberg formula, and
2. discuss natural selection and other causes of microevolution as deviations from the conditions
required to maintain Hardy-Weinberg equilibrium.
Introduction
In 1908 G.H. Hardy and W. Weinberg independently suggested a scheme whereby evolution could
be viewed as changes in the frequency of alleles in a population of organisms. They argued that if a
population met five conditions that there would be no changes in allele frequencies from generation to
generation. Those five conditions are as follows:
1. The breeding population is large, reducing the effect of chance on changes in allele frequencies
2. Mating is random with respect to the gene in question. Individuals show no mating preference for
a particular phenotype.
3. There is no mutation of the alleles.
4. No individuals enter or leave the population by migration (no immigration or emigration).
5. There is no selection. In other words, the alleles of the gene in question do not affect mating or
survival, so every genotype has an equal chance of surviving and reproducing.
Hardy and Weinberg also described the distribution of alleles and genotypes within a population
mathematically. The following variables are used, and each is expressed in decimal form:
p = frequency of the dominant allele
q = frequency of the recessive allele
Note that p + q must always equal 1. The analysis does not work if there are more than 2 alleles of
the gene being studied. (It can be done, but the math is much more complicated.)
In a population, p is also the likelihood of a dominant allele being passed on to an offspring, and q
is the likelihood of a recessive allele being passed on. Thus, we end up with the following conclusions:
p2 = frequency of homozygous dominant individuals in the population
2pq = frequency of heterozygous individuals in the population
1
Adapted from the College Board AP Biology Student Lab Manual, 2001 edition.
q2 = frequency of homozygous recessive individuals in the population
Note that p2 + 2pq + q2 should always equal 1.
How can this rule be applied to real populations? One cannot see the frequency of alleles in a
population; one can only see phenotypes. Nonetheless, the rules can allow one to determine allele
frequencies. The following steps allow one to determine the values of p and q in an observed population.
1.
2.
3.
4.
Determine the frequency of the recessive phenotype. The only way to have the recessive
phenotype is to be homozygous recessive. Thus, the frequency of the recessive phenotype = q 2.
Taking the square root of the value determined in step 1 gives the value of q.
Since q + p must = 1, the value of p can now be determined.
If desired, the values of 2pq and p2 can also now be determined.
The Hardy-Weinberg equation describes an existing situation. If the five conditions are met, then no
change will occur in either allele or genotype frequencies in the population. Of what value is such a rule?
It provides a yardstick by which changes in allele frequency, and therefore evolution, can be measured.
One can look at a population and ask: Is evolution occurring with respect to a particular gene locus? Since
evolution is difficult if not impossible to observe in most natural populations, we will model the
evolutionary process using the class as a simulated population. The purpose of this simulation is to provide
an opportunity to test some of the basic tenets of population genetics and evolutionary biology.
Part A: Estimating Allele Frequencies for a Specific Trait within a Sample Population
Using the class as a sample population, the allele frequency of a gene controlling the ability to
taste the chemical PTC (phenylthiocarbamide) can be estimated. A bitter-taste reaction to PTC is evidence
of the presence of one or more dominant alleles (i.e., the individual is either AA or Aa). The inability to
taste the chemical at all indicates the homozygous recessive condition (aa).
Before coming to lab:
1.
2.
Read through the procedure, and write out the procedure in your own words or in pictures in your
lab notebook.
Copy the table below into your lab notebook. Be sure to give it a title.
Procedure
1.
2.
Using the PTC taste-test papers provided, tear off a short strip and press it to your moist tongue
tip. PTC tasters will sense a bitter taste. Those people are considered to be tasters.
A decimal number representing the frequency of tasters (p2 + 2pq) should be calculated by
dividing the number of tasters in the class by the total number of students in the class. A decimal
number representing the frequency of non-tasters (q2) can be obtained by dividing the number of
non-tasters by the total number of students. Record these numbers in your table.
Phenotypes
Tasters
Class
population
North
American
population
#
Non-tasters
%
0.55
Allele frequency based on
H-W equation
p
q
#
%
0.45
3.
Use the Hardy-Weinberg equation to determine the frequencies (p and q) of the two alleles.
Calculate the values for both the class and for the North American population, and record the
values in the table.
Questions
Answer these questions in your lab notebook. Label them as part of Part A.
1.
2.
What is the percentage of heterozygous tasters in your class?
What percentage of the North American population is heterozygous for the taster trait?
Part B: Test of an Ideal Hardy-Weinberg Population
The entire class will represent a breeding population, so find a large open space for this
simulation. In order to ensure random mating, choose another student at random. In this simulation, we
will assume that gender and genotype are irrelevant to mate selection.
The class will simulate a population of randomly mating heterozygous individuals with an initial
gene frequency of 0.5 for both the dominant allele A and the recessive allele a. This gives starting
genotype frequencies of 0.25 AA, 0.5 Aa, and 0.25 aa. Each student will start with the genotype Aa for
simplicity. Record this on the data page. Each member of the class will receive four index cards. Two
cards will have “A” written on them and two will have “a” written on them. The four cards represent the
products of meiosis.
Before coming to lab
1.
2.
3.
4.
Read through the procedure so you know what you will be doing. You do not need to copy down
the procedure.
Set up a data table in which you can record your starting genotype and your genotype after each
mating. Label the table as part of Part B (Ideal Population).
Set up a place to record the initial class frequencies of AA, Aa, and aa genotypes.
Set up a place to record the class data after the fifth mating (numbers and frequencies of AA, Aa,
and aa), and leave a place to calculate and record the values of p and q.
Procedure
1.
2.
3.
4.
5.
6.
7.
Turn the four cards over so that the letters do not show, shuffle them, and take the card on top to
contribute to the production of the first offspring. Your randomly-selected mating partner should
do the same. Put the two cards together. The two cards represent the alleles of the first offspring.
One (and only one) of you should record the genotype of this offspring in the Part B section on the
data page.
Still working with the same mating partner, repeat step one. The genotype of the second offspring
should be recorded by the student who did not record the genotype of the first offspring.
The very short reproductive career of this generation is over. You and your partner now become
the next generation by assuming the genotypes of the offspring that you recorded. Each student
should obtain, if necessary, new cards representing the products of meiosis that he or she is now
capable of producing. For example, if you are now AA, then you need four “A” cards.
Randomly seek out another student with whom to mate in order to produce offspring for the next
generation. Remember, the sex of your mate does not matter, nor does the genotype.
Follow the same mating procedure as you did for the first generation, being sure to record your
new genotype after each generation. At the end of each generation, remember to record the
genotype that you assumed.
Class data will be collected for each generation. Record the class frequencies of each genotype in
the fifth generation.
Allele Frequency: The allele frequencies, p and q, should be calculated for the entire population
after five generations of simulated random mating. Do this by actually counting number of alleles,
not by using the Hardy-Weinberg equation. For example, if a class of 20 students had 6 AA
individuals, 10 Aa individuals, and 4 aa individuals, there would be 22 A alleles (2 from each AA
individual and 1 from each Aa individual) and 18 a alleles (1 from each Aa and 2 from each aa).
To get p, we would divide the number of A alleles by the total number of alleles in the gene pool.
In this example, that would be 22 divided by 40, which means p = 0.55 (in this example).
Questions
Answer the following questions in your lab notebook. Make it clear that your answers are in response
to the Part B questions.
1.
2.
3.
What does Hardy-Weinberg predict should be the values for p and q after 5 generations?
Do the results in this simulation agree with the prediction? If not, how do they differ?
What major assumptions of Hardy-Weinberg equilibrium were not followed in this simulation?
Part C: Selection
In this case, you will modify the simulation to make it more realistic. In the natural environment,
not all genotypes have the same rate of survival; that is, the environment might favor some phenotypes
while selecting against others. An example is the human condition of sickle-cell anemia. This is a disease
caused by a mutant allele, and individuals who are homozygous recessive often do not survive to reach
reproductive maturity. For simplicity 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.
Before coming to lab
1.
2.
3.
4.
Read through the procedure so you know what you will be doing. You do not need to copy down
the procedure.
Set up a data table in which you can record your starting genotype and your genotype after each
mating. Label the table as part of Part C (Selection).
Set up a place to record the initial class frequencies of AA, Aa, and aa genotypes.
Set up a place to record the class data after the fifth mating (numbers and frequencies of AA, Aa,
and aa), and leave a place to calculate and record the values of p and q.
Procedure
The procedure is similar to that for Part B.
1.
2.
Start again with the two “A” and two “a” cards, and produce your offspring through random
matings as you did in Part B. This time, however, there is one important difference. Each time
your offspring is aa, it does not reproduce. Since we want to maintain a constant population size,
the same two parents must try again until they produce two surviving and fertile offspring. If
necessary, get new allele cards so that you can continue through five generations.
After completing five generations, selecting against the aa genotype in every generation, add up
the genotype frequencies that exist in the population and calculate the new p and q frequencies in
the same way you did for Part B.
Questions
Answer the following in your lab notebook. Indicate that your answers are to the Part C questions.
1.
2.
How do the new frequencies of p and q compare to the initial frequencies in Part B?
What major assumption(s) of Hardy-Weinberg equilibrium were not strictly followed in this
simulation?
3.
4.
Predict what would happen to the frequencies of p and q if you simulated another five generations.
In a large population, would it be possible to completely eliminate a deleterious recessive allele,
assuming no mutations produce new copies of the deleterious allele? Explain.
Part D: Heterozygote Advantage
From Part C, it is easy to see what happens to the lethal recessive in a population. However, data
from many human populations show an unexpectedly high frequency of the sickle-cell allele. Thus, our
simulation does not accurately reflect the real situation. This is because individuals who are heterozygous
are slightly more resistant to a deadly form of malaria than homozygous dominant individuals. In other
words, there is a slight selection against homozygous dominant individuals as compared to heterozygotes.
This fact is easily incorporated into our simulation.
Before coming to lab
1.
2.
3.
4.
5.
Read through the procedure so you know what you will be doing. You do not need to copy down
the procedure.
Set up a data table in which you can record your starting genotype and your genotype after each
mating. Label the table as part of Part D (Heterozygote Advantage).
Set up a place to record the initial class frequencies of AA, Aa, and aa genotypes.
Set up a place to record the class data after the fifth mating (numbers and frequencies of AA, Aa,
and aa), and leave a place to calculate and record the values of p and q.
Set up a place to record the class data after the tenth mating (numbers and frequencies of AA, Aa,
and aa), and leave a place to calculate and record the values of p and q after the tenth mating.
Procedure
1.
2.
3.
4.
5.
In this round, keep everything the same as it was in Part C, except that if your offspring is AA,
you must flip a coin. If the coin lands heads up, the individual does not survive, and you must try
to mate again. If the coin lands tails up, the offspring does survive.
Simulate five generations, starting with everyone as Aa. Offspring with the genotype aa never
survive and offspring with the genotype AA only survive if the coin toss comes up tails. Since we
want to maintain a constant population size, parents who have an offspring that dies must try again
until they produce two surviving offspring. If you need new allele cards as you move through the
simulation, get them.
Record your results. Also record the class totals and calculate the values of p and q.
Starting with the genotype you had at the end of the F5 generation, go through five more
generations using the same rules.
Record your data, along with the class totals. Calculate values for p and q after 10 generations.
Questions
Answer the following in your lab notebook. Label your answers as part of Part D.
1.
2.
3.
Explain how the changes in p and q in Part C compare with the changes in Part B and Part D.
Do you think the recessive allele will ever be completely eliminated in the situation presented in
Part C or Part D? (Assume that one would go through many generations, not just five or 10.)
What is the importance of heterozygotes in maintaining genetic variation in populations?
Hardy-Weinberg Problems
Answer these questions in your lab notebook. Show all of your work if you want full credit for a correct
answer.
1.
In Drosophila, the allele for normal-length wings is dominant over the allele for vestigial wings.
(Vestigial wings are stubby curls that can not be used for flight.) In a population of 1,000
individuals, 360 show the recessive phenotype. How many individuals would you expect to be
homozygous dominant and how many would you expect to be heterozygous for the wing gene?
2.
In humans, the allele for unattached earlobes is dominant over the allele for attached earlobes. In
a population of 500 individuals, 25% show the recessive phenotype. How many individuals
would you expect to be homozygous dominant, and how many would you expect to be
heterozygous for this trait?
3.
The allele for the hair line 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. How
many individuals would you expect for each of the three possible genotypes for this trait?
4.
In the United States, about 16% of the population is Rh negative. The allele for Rh negative is
recessive to the allele for Rh positive. If the student population of a high school in the U.S. is
2,000, how many students would you expect for each 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, how many would you expect
for each 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?