Download Hardy weinberg lab

Population Genetics and Evolution
In 1908, G. H. Hardy and W. Weinberg independently suggested a scheme whereby
evolution could be viewed as changes in frequency of alleles in a population of
organisms. In this scheme, if A and a are alleles for a particular gene locus and each
diploid individual has two such loci, then p can be designated as the frequency of the A
allele and q as the frequency of the a allele. For example, in a population of 100
individuals (each with two loci) in which 40 % of the alleles are A, p would be 0.40. The
rest of the alleles (60%) would be a and q would be equal to 0.60. p+q=1. These are
referred to as allele frequencies. The frequency of the possible diploid combinations of
these alleles (AA, Aa, and aa) is expressed as p2+2pq+q2=1.0. Hardy and Weinberg also
argued that if 5 conditions are met, the population’s alleles and genotype frequencies will
remain constant from generation to generation. These conditions are as follows:
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The breeding population is large.
Mating is random
There is no mutation of the alleles.
No differential migration occurs.
There is no natural selection
The Hardy-Weinberg equation describes an existing situation. Of what value is such a
rule? It provides a yardstick by which changes in allelic frequencies can be measured. If
a population’s alleles frequencies change, it is undergoing microevolution.
OBJECTIVES
In this experiment, you will
 Use PTC paper to calculate the allele frequencies for a specific trait in a real
population.
 Use a mating simulation to simulate how various features affect allele frequencies
in a simulated population from one generation to the next.
 Practice using the Hardy-Weinberg equation through a series of activities and
problems.
MATERIALS
strips of PTC paper
a index cards
A index cards
PROCEDURE
Part I: PTC Paper
1. Using the class as a sample population, the allele frequency of a gene controlling
the ability to taste the chemical PTC (phenylthiocarbamide) could be estimated.
A bitter taste reaction is evidence of the presence of a dominant allele in either a
homozygous (AA) or heterozygous (Aa) condition. The inability to taste the PTC
is dependent on the presence of two recessive alleles (aa).
2. Tear off a small piece of PTC paper. Put the piece of PTC paper into your mouth.
PTC tasters will detect a bitter taste as the paper touches their tongues.
3. A decimal 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 the non-tasters (q2) can be
obtained by dividing the number of non-tasters by the total number of students.
You should then record these numbers in Table 1.
4. Use the Hardy-Weinberg equation to determine the frequencies (p and q) of the
two alleles. The frequency q can be calculated by taking the square root of q2.
Once q has been determined, p can be determined because 1-q=p. Record these
values in Table 1 for the class and also calculate and record values of p and q for
the North American population.
Part II: Case I: An Ideal Hardy-Weinberg Population
5. 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 as your mating partner. In this simulation, we will assume that gender
and genotype are irrelevant to mate selection.
6. The class will simulate a population of randomly mating heterozygous individuals
with an initial gene frequency of 0.5 for the dominant allele A and for the
recessive allele a and the genotype frequencies of 0.25 AA, 0.50 Aa, and 0.25 aa.
Record this information in Table 2.
7. Each member of the class will receive eight cards. Four cards will have A on the
them and four cards will have a on them. Each student will select the four cards
from their eight that represent the products that he/she could form from meiosis.
At this point, each student will put two A cards and two a cards into his/her hand.
8. Turn the four cards in your hands over so that the letters are now 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 cards together. The two
cards represent the alleles of the first offspring. One of you should record the
genotype of this offspring in Table 2. Each student pair must produce two
offspring, so all four cards must be reshuffled and the mating process repeated to
produce a second offspring to serve as the mating partners’ new genotype to
record in Table 2.
9. The two mating partners should use the genotypes of these produced offspring to
adjust the four cards in his/her hands. A student whose new genotype is Aa will
keep the two A and the two a cards in his/her hands. A student whose new
genotype is AA should have four A cards in his/her hands. A student whose new
genotype is aa should have four a cards in his/her hands.
10. Now, select a new mating partner and repeat the process. Record these new
genotypes in Table 2. Continue repeating this process until you have gotten F5
genotypes.
11. Class data will be collected of the F5 genotypes and allele frequencies can be
calculated. Record these calculations in Table 2.
Part III: Case II: Effects of Selection
12. 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 genotypes while selecting against others. For this
simulation, you will assume that the homozygous recessive individuals never
survive and that the heterozygous and homozygous dominant individuals survive
100% of the time.
13. Start as you did with Case I. Use the same initial allele and genotype frequencies.
Record that information in Table 3. This time there is one big difference.
Whenever one of the offspring receive the genotype of aa, that individual will die
and not persist in the population. However, to mainstain a constant population
size, parents do not assume the genotype of the homozygous recessive condition.
Rather, they repeat the mating process until a heterozygous or homozygous
dominant genotype is achieved. No one should record homozygous recessive as
any of his/her genotypes.
14. Record that genotype in Table 3. Find a new mating partner and repeat the
process. Continue repeating the process until F5 offspring are achieved. Stop
when you have an F5 genotype and wait for class data.
15. With the class data, calculate the genotype and allele frequencies and record them
in Table 3.
DATA
Table 1
Class
Population
North
American Pop
Phenotype Frequencies
% Tasters
% Non-Tasters
0.55
Table 2
AA =
Initial Genotype
F1 Genotype
F2 Genotype
F3 Genotype
F4 Genotype
F5 Genotype
AA =
Final Allele Frequencies
Table 3
AA =
Initial Genotype
F1 Genotype
F2 Genotype
F3 Genotype
F4 Genotype
F5 Genotype
AA =
Final Allele Frequencies
p
Allele Frequencies
q
0.45
Initial Class Frequencies
Aa =
aa =
Final Class Frequencies
Aa =
p=
aa =
q=
Initial Class Frequencies
Aa =
aa =
Final Class Frequencies
Aa =
p=
aa =
q=
QUESTIONS
1. From the PTC data, what is the percentage of heterozygous tasters in the class?
What is the percentage of heterozygous tasters in the North American population?
2. How do the two heterozygous frequencies in question #1 compare? What could
account for any differences between the values?
3. Is the trait for tasting PTC display smooth or discontinuous variation? Is the trait
due to monogenic or polygenic inheritance? How do you know?
4. In Part II, what does the Hardy-Weinberg equation predict that the final p and q
value should equal? Why?
5. Is your final p and q value in Part II consistent with the Hardy-Weinberg
predictions? Why or why not?
6. Were any of the major assumptions not adhered to in this simulation? Which one
or ones? How can this be corrected?
7. In Part III, how do the final p and q values compare to the initial p and q values?
8. What can account for the differences between the final and starting p and q values
in this simulation?
9. Predict what would happen to the values of p and q if this simulation was repeated
for another 5 generations. Explain your prediction.
10. In a large population would it be possible to completely eliminate a deleterious
allele? Why or why not?
11. In Drosophila, the allele for normal-length wings is dominant over the allele for
vestigial wings (vestigial wings are small and incapable of flight). In a population
of 1,000 individuals, 360 have vestigial wings. How many individuals would you
expect to be homozygous dominant for this trait? How many individuals would
you expect to be heterozygous for this trait?
12. In the Unites 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 to be
homozygous recessive? Heterozygous? Homozygous dominant?
13. In certain African countries 4 % of the newborn babies have sickle-cell anemia,
which is due to two recessive alleles. Out of a random population of 1,000
babies, how many would you expect to have sickle cell, be heterozygous carriers,
and to not have any sickle-shaped cells?