Background Information:
How can a sex-linked trait be identified?
Sex-linked traits are characteristics carried on genes of the sex chromosomes. Thomas
Hunt Morgan was one of the first scientists known to observe sex linkage. In his
experiments with the common fruit fly, Drosophila melanogaster, he analyzed the results
of the following Drosophila mating:
1. A white-eyed male crossed with a red-eyed female produced all red-eyed F1
offspring, as expected according to Mendelian laws of inheritance. An F1 red-eyed male
crossed with an F1 red-eyed female would result in 3 red-eyed F2 offspring (male or
female): 1 white-eyed F2 offspring (male or female) according to Mendelian laws of
inheritance. But Morgan found that all of the F2 female offspring and half of the F2 male
offspring had red eyes, which is not expected according to Mendelian laws of inheritance.
A white-eyed female crossed with a red-eyed male resulted in all of the F1 female
offspring having red eyes and all of the F1 male offspring having white eyes, which is
expected according to Mendelian laws of inheritance. An F1 red-eyed female crossed
with an F1 white-eyed male would result in 3 red-eyed F2 offspring (male or female): 1
white-eyed F2 offspring (male or female) according to Mendelian laws of inheritance.
But Morgan found that half of the F1 offspring of each sex had red eyes and the other
half had white eyes, which is not expected according to Mendelian laws of inheritance.
2. Morgan's results proved that more complex laws of inheritance exist. The alleles for
Drosophila eye-color are not simply dominant or recessive. Because of the ratios that
resulted from his Drosophila crosses, Morgan concluded that the gene for eye color is
carried by, or linked to, the X chromosome. He also found that the Y chromosome does
not carry a gene for this characteristic.
Fruit flies are often used to study genetics because of their short life cycle (about 2
weeks), their small size (several hundred can be housed in a small jar), and their
fecundity (a single pair may produce more than a hundred offspring).
English biologist Reginald Punnett developed a simple method for finding the expected
proportions of genotypes and phenotypes in the offspring of a genetic cross. It is called a
Punnett square. Punnett squares can be used to predict the traits of offspring in genetic
crosses. When using Punnett squares to analyze the results of crosses involving eye-color
in Drosophila, XR represents the dominant allele for red-eye color and Xr represents the
recessive allele for white-eye color.
Fisher
Lab Procedure:
Virtual Lab: Sex-Linked Traits
Genetics Chapter 7
Purpose: Explore sex-linked gene for eye color in Drosophila flies.
Objective: Construct and Analyze Punnett square for monohybrid genetic crosses
involving sex-linked traits.
When completing post lab questions please refer to your textbook for further
explanation. Textbook pages 201-203 McDougal
Instructions:
 Go to my website
 Go to Biology
 Go to Virtual Labs
 Go to McGraw-Hill Virtual Labs Link
 Go to Sex-linked Traits
 Go to Laboratory Exercise
Procedure:
1. Click on the T.V. screen found in your virtual lab. Watch the short video.
2. Next, complete the Punnett square activity by clicking on the laboratory
notebook.
Complete the table below:
Female, red eyes
Female, red eyes
Female, white eyes
Male, red eyes
Male, white eyes
When you have completed the Punnett square activity, return to the laboratory
scene to begin the actual laboratory activity.
3. In this exercise, you will perform a Drosophila mating in order to observe
sex-linked trait transmission. Click on the top shelf. There are four types of
fruit flies that can be used as parents for your initial mating. Each vial is
labeled with the sex and phenotype of the flies it contains. Click and drag a
male and a female parent to the empty vial on the second shelf. Use the
Punnett square below to predict the genotypes/phenotypes of the offspring
(Note: refer to the genotype table you created above if needed):
Parental generation
Genotype:
Genotype: Parental
generation
Phenotype:
Phenotype:
Offspring:
Offspring:
Genotype:
Genotype:
Phenotype:
Phenotype:
____Female, red eye
_____ Female, white eye
_____Male, red eye _____ Male, white eye
When you are finished, click “Mate and Sort”.
4. You will now see information appear in the vials sitting on the next shelf
below; shelf three. These are the offspring of the parent flies you selected
above, and they represent the first filial (F1) generation. In your “Data Table”
please input the numbers of each sex and phenotype combination for the F1
generation. These numbers will be placed into the first row marked “P
generation Cross”.
5. You will next need to select one of the F1 female flies and one of the F1 male
flies from shelf three and drag them to the empty vial on shelf four to create
the second filial (F2) generation. After you have dragged your selections
down to the empty vial on shelf four fill in the Punnett square below to
predict the offspring:
Genotype:
Genotype:
Phenotype:
Phenotype:
Genotype:
Genotype:
Phenotype:
Phenotype:
____Female, red eye ______ Female, white eye
____Male, red eye ____ Male, white eye
After clicking “Mate and Sort”, you will now have information on their
offspring (the F2 generation) to input into your “Data Table.” This
information will be placed into the second row marked “F1 generation Cross”
under Trial #1.
NOTE: there are additional lines remaining to use if your instructor requires
the analysis of additional crosses. Click reset to begin the activity with a new
P generation, follow the above instructions to complete a second trial.
6.
When you have completed this exercise you may begin answering the post
lab questions.
When completing post lab questions please refer to your textbook and/or notes
for further explanations. Textbook pages 201-203 McDougal
Data/Observations:
Cross
Type
Phenotype
of Male
Parent
Phenotype
of Female
Parent
Number
of Red
eye, Male
Offspring
Number
of White
eye, Male
Offspring
Number
of Red
eye,
Female
Offspring
Number
of White
eye,
Female
Offspring
P
Generation
Cross
Trial
#1
F1
Generation
Cross
P
Generation
Cross
F1
Generation
Cross
1. In a mating between a red-eyed male fruit fly and a red-eyed heterozygous
female, what percentage of the female offspring is expected to be carriers? How
did you determine the percentage?
X
X
X
r
R
r
R
Y
2. In a mating between a red-eyed male fruit fly and a white-eyed female fruit fly,
what percentage of the male offspring will have white eyes? Describe how you
determined the percentage.
X
X
X
R
Y
r
r
r
3. Colorblindness also results from a sex-linked recessive allele on the X
chromosome in humans. Determine the genotypes of the offspring that result
from a cross between a color-blind male and a homozygous female who has
normal vision. Describe how you determined the genotypes of the offspring.
X
X
X
c
Y
C
C
4. Explain why sex-linked traits appear more often in males than in females.
Post-laboratory Questions:
1.
Through fruit fly studies, geneticists have discovered a segment of DNA called
the homeobox which appears to control:
a. Sex development in the flies
b. Life span in the flies
c. Final body plan development in the flies
2.
The genotype of a red-eyed male fruit fly would be:
a. XRXR
b. XRXr
c. XrXr
d. A or B
e. None of the above
3.
Sex-linked traits:
a. Can be carried on the Y chromosome
b. Affect males more than females
c. Can be carried on chromosome 20
d. A and B
e. None of the above
4.
A monohybrid cross analyzes:
a. One trait, such as eye color
b. Two traits, such as eye color and wing shape
c. The offspring of one parent
5.
A female with the genotype “XRXr”:
a. Is homozygous for the eye color gene
b. Is heterozygous for the eye color gene
c. Is considered infected
d. A and B
e. B and C
6.
In T.H. Morgan’s experiments:
a. He concluded that the gene for fruit fly eye color is carried on the X
chromosome
b. He found that his F1 generation results always mirrored those predicted by
Mendelian Laws of Inheritance
c. He found that his F2 generation results always mirrored those predicted by
Mendelian Laws of Inheritance
d. A and B
e. All of the above
7.
In this laboratory exercise:
a. The Punnett square will allow you to predict the traits of the offspring created
in your crosses
b. XR will represent the recessive allele for eye color, which is white
c. Xr will represent the dominant allele for eye color, which is red
d. All of the above
8.
In a cross between a homozygous red-eyed female fruit fly and a white-eyed
male, what percentage of the female offspring is expected to be carriers?
a. 0%
b. 25%
c. 50%
d. 75%
e. 100%
9.
In a cross between a white-eyed female and a red-eyed male:
a. All males will have white eyes
b. 50% of males will have white eyes
c. All females will have red eyes
d. 50% of females will have white eyes
10.
In human diseases that are X-linked dominant, one dominant allele causes the
disease. If an affected father has a child with an unaffected mother:
a. All male offspring are unaffected
b. Some but not all males are affected
c. All females are unaffected
d. Some but not all females are affected