AP Biology
Population Genetics and Evolution
Case I (Hardy-Weinberg)
Your entire class represents a breeding population. As a class, come up with a universal
mating dance/call and find a large open space in your classroom for this simulation. In order
to insure random mating, choose any of the students in the class and confidently approach
them using the chosen mating dance/call. The sex of your mate does not matter, nor does the
genotype. You will produce offspring by placing your two allele cards behind your back and
randomly shuffling them. Each parent contributes a haploid set of chromosomes to the next
generation. Each couple must have two offspring.
To maintain a constant population size, the parent genotype dies. You assume the genotype
of one of your two offspring, and your partner then assumes the other offspring’s genotype.
To do this, you may have to go to the central allele supply and pick up a new card. Be sure to
record this new genotype in the F1 slot on the data sheet. Now thank your partner and
proceed to a new individual. Follow the exact same mating procedures, being sure to record
the new genotype after each generation. After five generations, return to your seats. Your
teacher will help you total the genotypic frequencies and calculate the new p and q.
Allele Frequency: The allele frequencies, p and q should be calculated for the population
after five generations of random mating.
Answer the following questions using complete sentences:
1. What does Hardy–Weinberg equation predict for the new p and q?
2. Do the results you obtained in this simulation agree? ______ If not, why not?
3. What major assumption(s) were not strictly followed in this simulation?
4. What is a genetic drift?
CASE II (Selection)
Now that you have the facts of life well in hand, we can begin to modify our simulation,
making it more realistic and enable us to investigate some basic questions about selection and
gene frequencies. In humans, there are several genetic conditions that have been thoroughly
investigated. One good example is sickle-cell anemia. This is a single allele trait; the
homozygous recessive genotype is lethal, and the afflicted individual usually dies prior to
reaching reproductive maturity. Both the homozygous dominant and the heterozygote
survive. At this point, we will consider both the homozygous dominant and the heterozygote
phenotypically identical. In other words, we are selecting against the homozygote recessive
100%of the time.
The procedure is similar to Case I. Start again with your initial genotype and choose the
genotype of your offspring is in Case I. This time, however, there is one important difference.
AP Biology
Population Genetics and Evolution
Every time your offspring is aa it dies. Since we want to maintain a constant population size,
the same two parents must try again until they produce a surviving offspring.
Proceed through five generations, selecting against the homozygous recessive offspring.
Return to your seat. Now, add up the genotype frequencies that exist in the population and
calculate the new p and q.
Answer the following questions using complete sentences:
1. How do the new p and q compare to the values in Case I?
2. Has the population changed?
3. What would happen to p and q if we went another five generations?
4. In a large population, what are the chances of eliminating completely a deleterious
recessive allele?
CASE III (Heterozygote Advantage)
From Case II, it is easy to see that the lethal recessive allele rapidly decreases in the
population. However, data from many human populations show an unexpectedly high
frequency of the sickle-cell allele present in some populations. In other words, our simulation
does not accurately reflect the real situation. The heterozygote is slightly more resistant to a
deadly form of malaria than the homozygous dominant individual. In other words, there is
slight selection against the homozygous dominant individual as compared to the heterozygote.
This fact is easily incorporated into our simulation. In this round, keep everything the same
as in Case II, except that if your offspring is AA, flip a coin. Heads he lives, tails he dies of
malaria, and the same parents must mate again until they get a viable offspring. Remember
our firm belief in ZPG.
Go through five generations, starting again with your initial genotype. The genotype aa dies
100% of the time. After five generations, return to your seat. Total the class genotypes and
calculate p and q. Now, starting with the genotype you last had, go through five more
generations, and again total the genotypes and calculate p and q. If time permits, another five
generations is extremely informative.
Answer the following questions using complete sentences:
1. How do p and q in Case II compare with Case I and Case III?
2. Do you think the recessive allele will be completely eliminated in either Case II or Case
III?
3. What is the importance of heterozygous advantage in maintaining genetic variation in
populations?
AP Biology
Population Genetics and Evolution
CASE IV (Genetic Drive, optional)
In Case I, the role of small populations in genetic drift was suggested. It is possible to use our
simulation to look at this phenomenon in more detail. Divide the lab into several smaller
populations (for example, a class of 30 could be divided into 3 populations of 10 each.) When
the populations are reproductively isolated by mountain ranges, oceans, or imaginary walls,
total the genotypic and allelic frequencies in each population. Now go through five
generations as in Case I. Record the new genotypic frequencies, and calculate the new p and
q for each population.
Answer the following questions using complete sentences:
1. How did the initial genotype frequencies of the populations compare?
2. What do your results indicate about the importance of populations size as an evolutionary
force?
Hardy-Weinberg Problems
1. Dark eyes are dominant to light eyes. In a population of 1000 individuals, 360 show the
recessive phenotype. How many individuals would you expect to be homozygous
dominant and heterozygous for this trait?
2. The ability to roll one’s tongue is dominant to a lack of ability. In a population of 500
individuals 25% show the recessive phenotype. How many individuals would you expect
to be homozygous dominant and heterozygous for this trait?
3. Having a Widow’s peak is dominant to lacking a widow’s peak. In a population of 1000
individuals 510 show the dominant phenotype. How many individuals would you expect
of each of the possible three genotypes for this trait?
4. In the U.S. about 16% of the population is Rh-. Rh- is recessive to Rh+. If the student
population of a high school is 2000, how many students would you expect for each of the
three possible genotypes?
5. In Nigeria, 4% of the newborn babies have sickle-cell anemia which is a recessive trait.
Out of a random population of 1000 newborn babies, how many would you expect for
each of the three possible genotypes?
AP Biology
Population Genetics and Evolution
DATA PAGE
Case I
(Hardy-Weinberg Equilibrium)
Case III
(Heterozygote Advantage)
Initial: AA ____ Aa ____ aa ____
Initial: AA ____ Aa ____ aa ____
My Initial Genotype: _____
My Initial Genotype: _____
F1 ____
F1 ____
F6 ____
F2 ____
F2 ____
F7 ____
F3 ____
F3 ____
F8 ____
F4 ____
F4 ____
F9 ____
F5 ____
F5 ____
F10____
Final: AA ____ Aa ____ aa ____
p____ q ____
Final: (After 5 generations)
AA ____ Aa ____ aa ____
p____ q ____
Final: (After 10 generations)
AA ____ Aa ____ aa ____
p____ q ____
CASE II
(Selection)
Case IV
(Genetic Drift)
Initial: AA ____ Aa ____ aa ____
Initial: AA ____ Aa ____ aa ____
My Initial Genotype: _____
My Initial Genotype: _____
F1 ____
F1 ____
F2 ____
F2 ____
F3 ____
F3 ____
F4 ____
F4 ____
F5 ____
Final: AA ____ Aa ____ aa ____
p____ q ____
F5 ____
Final: AA ____ Aa ____ aa ____
p____ q ____