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Genetics of Organisms: Lab Quiz Answers
You have been given a vial containing the fruit flies shown below.
After two weeks, you collect the offspring from this pair and obtain
the results shown in the following table. Use this table to answer
Questions 1 – 5.
1. On the basis of the results shown in the table, which statement is
most likely true?
a. The genes for red eyes and normal wings are linked.
b. The gene for no wings is sex-linked.
c. The gene for red eyes and the gene for no wings are both
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parents.
2. If the parents were both heterozygous for eye color and wings,
what phenotypic ratio would you predict for the offspring if these
traits are not linked?
a. 1 red eyes/no wings
1 red eyes/normal wings
1 sepia eyes/normal wings
1 sepia eyes/no wings
b. 9 red eyes/normal wings
3 red eyes/no wings
3 sepia eyes/normal wings
1 sepia eyes/no wings
c. 3 red eyes/normal wings
1 sepia eyes/no wings
d. 1 red eyes/normal wings
2 sepia eyes/normal wings
1 red eyes/no wings
2 sepia eyes/no wings
The correct response is b. If there were linkage, choice a would be
correct, but when two traits sort independently, the offspring of
heterozygous parents will show the traits in a 9:3:3:1 ratio.
3. Based on the hypothesis that this is a dihybrid cross, with the two
genes unlinked, calculate
using the data in the
table of observed phenotypes.
a. 6.04
b. 7.81
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= the sum of
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
4. How many degrees of freedom are there for this data?
a.
b.
c.
d.
1
2
3
4
The correct answer is c. There are 4 categories minus 1 = 3 degrees
of freedom.
5. Compare the chi-square value obtained above with the
Critical Values Table below for p = 0.5. Which of the following
statements would be true?
Degrees of freedom (df)
1
2
3
0.05
3.84
5.99
7.82
0.01
6.64
9.21 11.3
Probability
(p)
0.001
10.8
13.8
16.3
4
5
9.49 11.1
13.2
15.1
18.5
20.5
a. Since the calculated value for chi-square is less than
7.815, the results support the hypothesis that the parents
are heterozygous for two unlinked traits.
b. Since the calculated value for chi-square is less than
7.815, the results support the hypothesis that eye color
and wings are linked.
c. Since the calculated value for chi-square is less than
7.815, the results are inconclusive. The experiment
should be repeated.
d. Since the calculated value for chi-square is so large, the
variations from the expected are excessive. The
experiment should be repeated.
The correct response is a. The expected results are based on
obtaining a 9:3:3:1 ratio from two heterozygous parents. Since
the chi-square value is below 7.815, the results are statistically
significant.
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings