Week 5 - Virtual Results of Drosophila Crosses
General
By now you have become familiar with the complexities that arise in analyzing the
inheritance of traits. You are also aware that the inheritance pattern depends on 1) interactions
among alleles (dominance), 2) number of loci that determine the trait, 3) interactions among loci
(epistasis), and 4) whether the gene is located on a sex chromosome. Finally, you know that the
principle of independent assortment is violated when genetic loci are located near each other on the
same chromosome.
In today's exercise, you are simulating the results of genetic crosses among Drosophila
mutant strains. Normally, these experiments would require over one month to complete, because the
generation time for Drosophila is about two weeks. Using the simulation, you can accomplish a
number of crosses with simulated scores of 10,000 flies within three hours. The large number of
flies means that sampling error is relatively unimportant, so we can use phenotypic ratios to develop
a greater understanding of the process of inheritance.
Virtual Fly Lab
This program allows you to cross flies with different phenotypes and obtain simulated F1
offspring. You can either self-cross these F1 offspring or you can back cross one of them to one of
the parents. The F2 offspring can also be self crossed or back crossed to one of its parents. The
program does NOT give you the genotype of your flies. You must infer this from your results.
Mutations belong to nine groups: Bristle (5 mutations), Body Color (5 mutations), Antennae
(1 mutation), Eye Color (4 mutations), Eye Shape (4 mutations), Wing Size (3 mutations), Wing
Shape (4 mutations), Wing Vein (2 mutations), Wing Angle (1 mutation). You can only cross
individuals that possess mutations belonging to different groups. In other words, you cannot cross a
male with sepia eyes with a white-eyed female.
The authors of Virtual Fly lab caution you to focus on at most two traits at a time. You can
imagine that the results get exceedingly complicated with more than two traits, and in some cases,
the computer will refuse to complete the analysis.
All of these traits are determined by one locus with two alleles: Wild-type and mutant. The
mutants may be dominant or recessive. They may also be autosomal, sex-linked, or lethal. For
some combinations of two loci, you will obtain the 9:3:3:1 phenotypic ratio in the F2 offspring, but
for others, you will obtain very different results. It is up to you to infer the cause of these deviations
from independent assortment.
Rules of the software
1) If you select a mutation, the fly is made homozygous (or hemizygous) for that mutation, unless
the mutation is lethal. If the mutation is lethal, the fly is made heterozygous. A homozygote for a
lethal mutation would be dead.
2) Crossing over occurs only in females. This means that if you want to study inheritance of linked
genes, you need to cross wild-type female with a male that shows the mutant phenotype for all traits.
Bio 122
Virtual Fly Lab
Spring 2007
Using the software
•
Change the setting 'Each Mating Creates' from '1000 Offspring' to '10,000 Offspring.'
•
Click on the 'Design' button below the 'ghost-like' picture of each fly to choose the phenotype of
that fly. The mutations are listed on the left side.
•
When you click on one of them, you will see a pictured list of mutations to choose from. When
finished selecting mutations, click the 'Select' button on the bottom in the middle of the screen.
After completing your selections for the female fly, do the same for the male fly.
•
Click the large button in the middle of the screen 'Mate' to perform the cross.
•
Scroll down the results screen to see pictures of the offspring phenotypes. Click 'Analyze
Results' to see the numerical results of the cross. When you are done examining the numbers,
click 'Return to Lab' on the upper left to return to the screens where you can perform crosses.
•
To cross the F1 offspring, press 'select' beneath them. Notice that the offspring fly at the bottom
row on the screen now moves up to the top row, indicating that it will now be used as the parent
in the subsequent cross. To perform a test cross, select one of the F1 offspring and select the
recessive genotype for the fly with which you will mate your F1 fly.
SPECIFICS: Students will work in groups of two
Load Firefox. Either enter the following URL in the 'Open Location' part of the 'File' menu:
http://biologylab.awlonline.com/. You or your lab partner must purchase access to Flylab before
using it the first time.
Activity 1- Perform the following crosses and record the phenotype(s) of the F1 offspring. Then
note whether the mutant was dominant or recessive, and whether the gene is on the X chromosome
(sex-linked). Finally, perform two crosses using one mutant phenotype of your choice.
Parental Female X
Parental Male
vestigial wings
X
wild-type
wild-type
X
vestigial wings
yellow body
X
wild-type
wild-type
X
yellow body
lobed eyes
X
wild-type
wild-type
X
lobed eyes
dichaete wings
X
wild-type
wild-type
X
dichaete wings
5a__________
X
____________
5b__________
X
____________
Offspring phenotype
F1
F1
Female
Male
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Locus characteristics
mutant
sex-linked?
dominant?
Bio 122
Virtual Fly Lab
Spring 2007
Activity 2- Now perform crosses A-E below between wild-type females and male double mutant
flies. After crossing the parental generation,
1. Self cross the F1 flies and click on 'Analyze Results' at the bottom left of your screen.
2. Fill in the F2 'self' phenotypic ratios in Table 2b and click on 'return to lab' on the upper left
corner of the window.
3. Click on 'New Mate' and cross the wild-type females with male double mutants again.
4. Mate the F1 female with a male double mutant.
5. Fill in the 'test' phenotypic ratios in Table 2b and click 'return to lab' on the upper left corner.
6. Make a reasonable interpretation of the results you just tabulated. What genetic processes cause
the observed phenotypic ratios? If two loci appear to be linked, indicate the map distance
between them.
Table 2a
Cross Parental Female X
Parental Male
wild-type
X
1. purple eyes
A.
2. ebony body
wild-type
X
1.
apterous
(no wings)
B.
2. incomplete wing veins
wild-type
X
1. purple eyes
C.
2. black body
wild-type
X
1.
white eyes
D.
2. crossveinless wings
E.
wild-type
X
1. ______________
2. ______________
Table 2b. Results of self crosses and test crosses.
fem male fem male fem male
cross fem
male
wild wild mut1 mut1 mut2 mut2
A ratio self
F1 X F1
A ratio test
F1 X
fem
both
male
both
P
Interpretation
_________________________________________________________________________
_________________________________________________________________________
B ratio self
F1 X F1
B ratio test
F1 X
P
Interpretation
_________________________________________________________________________
__________________________________________________________________________
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Bio 122
Virtual Fly Lab
Spring 2007
Table 2b. Results of self crosses and test crosses. Continued
fem male fem male fem male
cross fem
male
wild wild mut1 mut1 mut2 mut2
C ratio self
F1 X F1
C ratio test
F1 X
fem
both
male
both
P
Interpretation
_________________________________________________________________________
_________________________________________________________________________
D ratio self
F1 X F1
D ratio test
F1 X
P
Interpretation
_________________________________________________________________________
_________________________________________________________________________
E ratio self
F1 X F1
E ratio test
F1 X
P
Interpretation
_________________________________________________________________________
_________________________________________________________________________
Hints for interpretation- If phenotypic ratios are similar for both sexes, this suggests that the locus
is not sex linked. When sex linkage is present, you should see phenotypic differences within the F1
generation, while the recessive allele does not appear in the F1 for autosomal loci. When both loci
code for different traits and are on autosomes, you would expect the phenotypic ratios to conform to
Mendel’s Rule of independent assortment, unless they are linked. If they are linked, you expect to
see an excess of parental phenotypes in the F2 generation. If one locus codes for one aspect of a
feature, and another codes for another aspect of the same feature, you might expect to see epistasis
(i.e. variation at one locus masks expression of variation at another), and this shows up in the crosses
as a ‘missing’ phenotype (normally we expect to see four phenotypes in the F2 generation)
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