Download Allele Frequency Lab

Allele Frequency Lab
Purpose:
In this lab you will be attempting to show how giraffe necks may become longer in response to selective
pressures in their environments. Assume that during a long drought food has become scarce where the giraffes
are living. Genes (symbolized by beans) control the length of the giraffe neck: black beans represent long necks
and white beans represent short necks.
Guidelines:
1. A giraffe receiving two “long neck” alleles (two black beans) from his parents has a longer neck than the
medium necked giraffe. These giraffes would be able to feed on the choice of leaves higher up on the trees.
2. A giraffe receiving one “long” allele and one “short” allele (one black bean and one white bean) would have
a medium neck length and would only be able to feed on the old, tough lower branches that have been
picked over by the other giraffes.
3. A giraffe receiving two “short” alleles (two white beans) would have a shorter neck than the medium
necked giraffe. This giraffe would not be able to reach over the lowest branches. A giraffe with this neck
length variation would die from starvation.
Materials:
Black beans, white beans, a paper bag.
Procedure:
1. Count out 60 white beans and 40 black beans and put them in a paper bag. The beans in the bag will
represent the gene pool of the giraffe population: these are all of the possible alleles in the population. We
will study the offspring from those giraffes by removing beans two at a time from the bag. Each pair
removed represents a young giraffe.
2. Gently shake the bag before each choice. Select 25 pairs of beans at random from the bag. DO NOT peek.
Set aside these beans, keeping them in pairs. These beans represent the giraffes in the first generation.
3. Correctly record the number of pairs in each group under “Generation 1”.
4. Place all of the “short neck” combinations (two white beans) in the morgue- DO NOT return them to the
paper bag.
5. Return all of the remaining beans to the paper bag.
6. Starting at step 2, repeat the process for another 7 generations. Record the results in your data table as seen
below. Towards the end you may or may not have 25 giraffes in each generation.
7 . When all of your data is collected construct a line graph. Label the x axis “Generation” and the y axis
“Number of giraffes”. Use a different color for each giraffe neck length and don’t forget to include a key.
Then create two pie graphs depicting the percentage of each allele for generation 1 and generation 8. Don’t
forget your key.
Data Table:
Generation
Long Neck
Medium Neck
Short Neck
1
Total number of giraffes in each generation
2
3
4
5
6
7
8
Gene Pool & Allele Frequency for each generation
Total # of Alleles
% Long allele
% Short allele
Number of Giraffes by Generation
Key
Long
neck
Medium
neck
Short
neck
% Alleles in Generation 1
% Alleles in Generation 8
Key
Long
Allele
Short
Allele
Discussion Questions:
1. What was your initial allele frequency for each allele in generation 1? _____% long _____% short
2. What was your final allele frequency for each allele in generation 8? _____% long _____% short
3. How do you think the allele for “long necks” originally appeared in the giraffe population? _____________
4. Why was the homozygous white bean combination taken out of the gene pool? _______________________
5. What was the selective pressure in the environment acting on the giraffe population? __________________
6. How did that selective pressure affect the gene pool and allele frequency? ___________________________
_______________________________________________________________________________________
7. Use your data for generation 1and notes pg. 11 to draw a distribution graph showing the original allele
frequency.
Key
Percentage of Population
Original
Allele
Frequency
Final
Allele
Frequency
Short
Medium
Long
8. Use your data for generation 8 and notes pg. 11 to add a graph to the above axes showing the final allele
frequency. Please draw your new allele frequency in another color.
9. What kind of selection did the population undergo? ____________________
10. What is your evidence for #9? ______________________________________________________________
11. Now look at your data collectively and compare it to at least 3 other groups in class. Does your data align
with the other groups? _______
a. If no, what happened to your data to make it so different? _______________________________
______________________________________________________________________________
12. Black mice in the desert exist in a lower frequency of the population. The genotype for Black fur coat is
dominant for mice. The recessive trait for desert mice is a brown coat. A volcano erupted in the same
desert that changed the landscape into a blackened lava field of of black ash and lava flows that have since
hardened. Answer the following questions and design an experimental design of how to accurately access
the change in allele frequencies due to the volcanic explosion.
a.
Why are the Black mice currently in the lower percent of the population even though the
genotype for Black is dominant?
b. Design an experiment to observe the outcomes of this volcanic eruption and explain any changes
in allele frequency do to the eruption.