Download Allelic Frequency

Change Over Time
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State Standard:
H.2L.4- Explain how biological evolution is the
consequence of the interactions of genetic variation, reproduction and inheritance,
natural selection, and time.
Response
Scoring Rubric
10 Exceeds
Evaluate the impact of the interactions of genetic variation, reproduction and inheritance, natural
(excellent)
selection and time on biological evolution (change)
8
Meets
Explain how biological evolution is the consequence of the interactions of genetic variation,
(proficient)
reproduction and inheritance, natural selection, and time, using multiple lines of scientific evidence.
6
Nearly Meets
Describe the process of biological evolution through natural selection
Does Not Meet
4
Recognize that species have changed over time
1
Incomplete
Did not participate
Score
Allelic Frequencies and Sickle- Cell Anemia
ickle-cell anemia, a potentially fatal disease, results from a mutant allele for hemoglobin, the oxygencarrying protein on red blood cells. There are two alleles for the production of hemoglobin.
Individuals with two Hemoglobin A alleles (AA) have normal red blood cells. Those with two mutant
Hemoglobin S alleles (SS) have abnormal sickle- shaped red blood cells and suffer from sickle-cell anemia.
Heterozygous (AS) individuals carry the mutant allele but do not suffer from its debilitating effects. They
have both normal and sickle-shaped red blood cells.
In the United Sates, about 1 in 500 African- Americans develops sickle-cell anemia. But in Africa,
about 1 in 100 individuals develops the disease. Why is the frequency of a potentially fatal disease so much
higher in Africa?
The answer is related to another potential fatal disease, malaria. Individuals with an AA hemoglobin
genotype have a significant greater risk of contracting malaria and may die from the disease. This results in
the removal of Hemoglobin A alleles from the gene pool. The SS genotype, which results in sickle-cell
anemia, is usually fatal before the age of twenty. This results in the removal of Hemoglobin s alleles from
the population. A person with an AS genotype does not develop sickle-cell anemia and has less chance of
contracting malaria. Such a person is better able to survive and reproduce in a malaria-infected region.
Therefore, both the individual’s A allele and S allele remain in the population. The frequency of the
Hemoglobin s allele in malaria-infected regions of Africa is 16%. But in the United States, where malaria
has been eradicated, the allelic frequency is 4%.
Simulate
You will now simulate the fusion of gametes and record the resulting genotypes of each offspring,
using all of the beans in the unlabeled container. You will also simulate the effect of being homozygous or
heterozygous for the hemoglobin gene in a malaria-infected region. One team member will select two beans
from the unlabeled container 50 times. After each selection, a second team member will indentify and record
the genotype of the “offspring” in Table 1 by making a slash under the appropriate column head. The same
team member then places each pair of beans in container AA, AS, or SS, depending on the genotype. During
the periods when the blindfold team member is making a selection, the recorder will randomly call out the
word “malaria” a total of 25 times. (This represents a 50% malaria infection rate.) if the genotype of the
selected pair of gametes is AA, that offspring will contact malaria and die. Therefore, place that pair of
alleles in the container labeled “Non-Surviving Alleles” and put a circle around the slash (that is, the
recorded genotype) in table 1. If the genotype is AS, the individual will survive. Put a circle around the
slash in Table 1, and place the pair of beans in the container labeled AS. If the genotype is SS, the individual
will die. As with the individual homozygous for the normal Hemoglobin gene, place the beans in the
container labeled “Non-Surviving Alleles” and draw a circle around the recorded genotype.
S
Procedure
1. Place 75 red beans in the unlabeled container (this is the 1st generation gene pool). These beans
represent gametes carrying the Hemoglobin A allele.
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2. Add 25 white beans to the unlabeled container (this is the 1st generation gene pool). These beans
represent gametes carrying the Hemoglobin S allele.
3. Without looking, one partner should draw two beans out of the “gene pool”. The other partner
records in Table 1. REMEMBER: 1 TICK MARK = 1 BEAN or "Allele" !!!!!!!!!!!!!
4. If two reds are selected (AA), place them in the AA container and write two tick marks in column
AA in Table 1.
5. If one red and one white are selected (AS), place them in the AS container and write two tick marks
in column AS in Table 1.
6. If two whites are selected (SS), place them in the SS container and write two tick marks in column SS
in Table 1.
7. Randomly (every other time) the recorder should call “malaria” to simulate the malaria infection rate
of 50%. Circle the two tick marks in Table 1 showing a “bite”. AA's that are bit show a high
probability of death so let's put them in the non-surviving alleles container.
8. Count the number of surviving AA alleles (red beans) remaining in the population and record that
number below. Do the same for AS (remember to add all the surviving A's together and the
surviving S's together. The combined total represents the total number of alleles for hemoglobin in
the population. Calculate the allelic frequencies as shown and record your results.
AA genotype
Table 1 “Second Generation
AS genotype
SS genotype
a. How many Red beans are remaining in the population?
____________A alleles
b. How many White beans are remaining in the population?
____________S alleles
c. What is the total number of alleles in the population? A+S=
_________________
d. What is the frequency (percent) of the A allele?
_________________
_ A__ x 100 =
A+S
e. What is the frequency (percent) of the S allele?
_ S__ x 100 =
_________________
A+S
9. After you have completed the cycle for one generation, you are ready to begin the next. All of the
beans in the containers labeled AA and AS should be emptied into the unlabeled container. However,
place all the beans from the container labeled SS into the container for “Non-Surviving Alleles”
(Since individuals who are homozygous SS will not usually live long enough to have children, you
will not use the SS gametes for tallying the next generation.)
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10. To determine the 50% malaria infection rate for the generation of the population, take the total
number of A and S remaining in the population and divide it by 4. This is the number of times you
will randomly call out “malaria” in the next round of fusion of gametes. (For example, if you had 47
A and 17 S alleles in part “c” above, you would have a total of 64 alleles in the population. Divide
this by 4 and you would get the number 16. This would be 50% of the next generation of people
produced by the gamete fusions.)
11. Repeat the procedures you followed in step 3 through 7. Use Table 2 to record your data.
12. Repeat the procedures you followed in step 8. Record your data and calculation in the spaces
provided under Table 2.
AA genotype
Table 2 Third Generation
AS genotype
SS genotype
f. How many Red beans are remaining in the population?
____________A alleles
g. How many White beans are remaining in the population?
____________S alleles
h. What is the total number of alleles in the population? A+S=
_________________
i. What is the frequency (percent) of the A allele?
_ A__ x 100 =
A+S
_________________
j. What is the frequency (percent) of the S allele?
_ S__ x 100 =
A+S
_________________
Analysis
1. a. What was the frequency of the A allele in the original population?
___75 %_______
b. What was the frequency of the S allele in the original population?
_________________
c. What was the frequency of the A allele in the second population?
_________________
d. What was the frequency of the S allele in the second population?
_________________
e. What was the frequency of the A allele in the third population?
_________________
f. What was the frequency of the S allele in the third population?
_________________
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g. Explain your findings.
2. Since few people with sickle-cell anemia are likely to survive to have children on their own, why
hasn’t the Hemoglobin S allele been eliminated by natural selection?
3. Why is the frequency of the Hemoglobin S allele so much lower in the United States than in Africa?
4. Scientists are working on a vaccine against malaria. What impact would the vaccine have on the
frequency of the Hemoglobin S allele in Africa?
5. Using your chapter 15 notes, which one of the 3 types of natural selection pressures are demonstrated
in the lab?
Circle one:
Stabilizing
Directional
Disruptive
Only answer the following if you’re going for a 10 
6. Gradualism is the idea that species originate through a gradual change of adaptations over time. For this
to work, some mutations must be favorable. What does “natural selection” say about favorable traits?
7. Adaptive radiation is the idea that an ancestral species can evolve into an array of species to fit a
number of diverse habitats. Using the example of a donkey and a horse, why is the mule NOT a new
species?
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