Download Gene Linkage in Fruit Flies

Name ________________________________ Class _______ Date _________________
Gene Linkage in Fruit Flies
Use the virtual Genetics lab room to
determine how the genes are linked in fruit fly
chromosomes.
Lab Bench Used
Drosophila melanogaster, commonly known as the fruit fly, is a classic model organism
for genetics research. In this virtual lab activity, you will investigate patterns of
inheritance in different Drosophila traits, including some that are affected by gene
linkage.
Enter the Virtual Bio Lab and select the title of this lab activity from the “Heredity”
menu on the whiteboard. You will be taken to the virtual Genetics lab room.
Part A: Patterns of Inheritance
In the Genetics lab, select the fruit fly (“Fly, Fruit”) in the Species Selector. Select “Body Color
1” as the trait to examine. First, cross a male that is homozygous for “Brown” with a female that
is homozygous for “Ebony.” Make sure the “Include Male/Female” box in the lower left corner
of the offspring results box is unchecked.
1. What body color phenotypes emerge in the F1 generation? Record the percentage
of the offspring that display each phenotype.
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2. Click on “Next Generation.” Use “Generate Random Crosses” to produce the F2
generation from a random pairing from the F1 generation. Cross them enough
times to produce 1000 offspring. What phenotypes show up in the F2 generation?
What is the distribution of phenotypes?
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3. Explain why “incomplete dominance” is an accurate name for the type of
inheritance at work in these crosses.
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Heredity:
Gene Linkage in Fruit Flies
Name ________________________________ Class _______ Date _________________
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Part B: Testing Mendel
Reset the fruit fly experiment again. This time, select “Eye Color” as the trait and run a
heterozygote-heterozygote cross enough times to produce 1000 offspring.
4. What is the pattern of inheritance for eye color trait? Provide evidence to support your
conclusion.
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5. Reset the experiment and select “Eye Shape” as the trait. Again, run a heterozygoteheterozygote cross enough times to produce a sample size of 1000 offspring. What is the
pattern of inheritance? Explain.
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6. According to Mendel’s principle of independent assortment, the patterns of
inheritance for two different traits should not affect each other. Use the two-factor
Punnett square below to determine the results you would expect from a cross of
fruit flies that are heterozygous for both traits, assuming Mendel’s principle holds
true. R = red, r = white-sepia; B = bar, b = wildtype.
Heterozygous Female (both traits)
Heterozygous
RB
Rb
rB
rb
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Gene Linkage in Fruit Flies
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Male
(both traits)
RB
Rb
rB
rb
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Heredity:
Gene Linkage in Fruit Flies
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7. Based on the patterns of inheritance you determined in Questions 4 and 5, what is the expected
phenotype ratio for the offspring?
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Set up the fruit fly experiment so that both eye color and eye shape are analyzed in the same
cross. (To select two traits, hold the control or command button as you click on both traits in the
list.) Set up the alleles so that both parents are heterozygous for both traits, as in the Punnett
square above. Again, click “Cross” enough times to produce 1000 offspring.
8. Record the percentage of offspring that have each phenotype combination. (Hint: To convert
decimals to percentages, multiply by 100.)
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9. Convert the percentages to a ratio like the one in Question 7. (Hint: Find an approximate
common factor for all four numbers, such as 3 or 5, and divide each percentage by that factor.
Round the results to the nearest whole number.)
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10. How do the actual results of the virtual two-factor cross compare to your expected results
from the two-factor Punnett square? Be specific in your description.
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Stop for a moment to think about why a fruit fly has two copies of its genes. Like other diploid
organisms, it has two copies of every chromosome—one from its father, one from its mother.
However, in order to reproduce, a fly needs to produce a gamete that has only one copy of every
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Heredity:
Gene Linkage in Fruit Flies
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chromosome. Through meiosis, every gamete gets a mix of the fly’s mother’s and father’s
chromosomes. Crossing-over between the homologous chromosomes makes the mix of genes in
each gamete even more random.
However, some genes are located very close together on a chromosome. During meiosis,
unless a crossing-over event occurs right between the two genes, they will tend to be found
together in the offspring. These are called linked genes.
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Heredity:
Gene Linkage in Fruit Flies
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11. What combinations of eye color and eye shape alleles appeared more frequently than you
expected in the F1 offspring?
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12. The genes for eye color and eye shape are linked, both located on the same chromosome in
fruit flies. Why would the red-bar and white-sepia-wildtype allele combinations be more
prevalent in the F1 generation than the other combinations? (Hint: Think about the parents’
chromosomes.)
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Heredity:
Gene Linkage in Fruit Flies