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
dominant.
d. The gene for eye color is inherited independently of the gene
for wings.
e. The F1 mates were both homozygous for both eye color and
wings.
The correct response is d. There is no evidence for any type of
linkage, since both males and females show the traits in
approximately equal proportions, and eye color and wings appear to
sort independently. If the parents were homozygous for these traits,
the offspring would not show different phenotypes from both
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.
b.
c.
d.
6.04
7.81
4.977
24.0
The correct answer is c. This is obtained by using the formula
= the sum of
Red
eyes/normal
wings
Red eyes/no
wings
Sepia
eyes/normal
wings
o
e
o-e
98
90
8
0.7111
22
30
8
2.1333
26
30
4
0.5333
Sepia eyes/no 14
wings
10
4
1.6
= 4.977
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.