Mendelian Genetics in Fruit Flies
Gregor Mendel studied the genetics of inheritance by crossing individuals with
variations in specific traits and observing the occurrence of these traits in the offspring.
His conclusions led to his first law: the Law of Segregation. This law states that a trait
is determined by two factors that separate during meiosis and unite during fertilization.
These factors remain as independent units and do not blend in the offspring. You will be
studying the inheritance of certain characteristics by performing your own crosses
between some specific variations in traits in the common fruit fly, Drosophila
melanogaster.
Objectives
Review the basic concepts of genetics studied in the Human Genetics lab.
When you have finished this lab, you will be able to:
1. summarize the life cycle of Drosophila melanogaster and distinguish the
egg, larva (instar), pupa, and adult.
2. determine the sex and strain of fruit flies.
3. predict the progeny expected from experimental crosses by using a punnett
square.
4. analyze the data collected from actual crosses and compare it to the expected
outcome.
5. draw conclusions based on these data.
6. define and use intelligently all words that appear in boldface print.
The Life Cycle of Drosophila melanogaster
Drosophila melanogaster, the common fruit fly, normally feeds on sugars and yeast
produced by rotting fruit. The fruit fly has been extensively used in genetic experiments
primarily because it is grown easily in the lab, reaches maturity in 10-14 days, and one
female can produce hundreds of offspring. Also, many different mutations are known
and have been studied.
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Fruit flies have a fairly complicated life cycle that is completed within 10-14 days.
They spend the first day or two of their lives as eggs. These hatch into larvae (instars)
that eat and grow for 6 to 8 days. Next the pupa emerges and lives on the walls of the
culture vessel for 3-4 days before hatching into the winged, free-flying adult form. These
adults mate 6-8 hours after hatching. See Figure 1 for a summary of this life cycle.
Note: This lab will occasionally require you to come in on your own time to work with
the flies.
Figure 1. Life Cycle of Drosophila melanogaster.
Where in the culture bottle will you find the following life stages of the fruit fly?
eggs? _____________________________ larvae? ___________________________
pupae? ____________________________ adults? ___________________________
From your observations, draw each of these stages in Drosophila melanogaster
development. (Do not simply copy the above figures.)
Egg
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Larva
Pupa
Adult
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Genetics of Drosophila melanogaster
Drosophila melanogaster is a diploid organism, having two alleles for each gene
located on homologous chromosomes. It has four pairs (2n = 8) of chromosomes, three
pairs of autosomes and one pair of sex chromosomes. As in humans, the female is XX
and the male is XY. The cultures provided for you are pure breeding, that is, they are
homozygous, the two alleles being the same for the characteristic to be studied. Wild
type Drosophila melanogaster may be designated as ++ or by capital letters for the traits
to be examined. Examples of wild type traits are: red eye color, indicated by “R”; long
(normal) wing length, indicated by “L”; and gray body color, indicated by “G”.
Examining flies:
1. The use of ether to anesthetize flies allows them to sleep for 50 minutes without being
killed. Follow directions in Figure 2 for the anesthetizing directions.
2. Mark a white card with a line drawn in the middle. Label the two sides “male” and
“female”. Place the sleeping flies in the center.
3. Closely examine ++ type flies and especially note the following characteristics:
eye color ______________________
wing length ____________________
body color _____________________
4. Using a soft brush separate the males from the females by the following
characteristics. Have your instructor check that the flies have been sexed correctly.
See Figure 1 for males and females.
Size:
males: usually smaller than females.
Body shape:
males: the tip of the abdomen is black and rounded
females: the tip of the abdomen is pointed and has light and
dark bands.
Forelegs:
males: have dark spots called sex combs
females: have no sex combs
5. In preparation for your first cross, place three males into a new culture vial and three
females into another new culture vial. See Figure 3. Label the vial to indicate the sex
strain of flies, the date, and your name and lab session. Remember to leave the vial on
its side until the flies recuperate. Otherwise, they will get stuck in the sticky medium.
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Figure 2. Use of ether on Drosophila melanogaster
Figure 3. Transferring anesthetized flies to a vial.
Fill in Table 1.
Table 1. External characteristics observed in male and female fruit flies.
Characteristic observed
Male
Female
Overall size
Color of abdomen
Shape of abdomen tip
Are sex combs present?
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Crosses
Monohybrid crosses involve following the inheritance pattern of one genetic trait or
characteristic through several generations. These crosses will be set up between three
wild type female flies and three males from one of the mutant types. Reciprocal crosses
involving the same traits will also be made between wild type males and mutant females.
We will be working with several mutant types: white-eye = r, vestigial (short) wings = 1,
and ebony (black body) = g.
The wild type characteristics are dominant and designated by a capital letter. The
phenotypes (observed characteristics) and genotypes (alleles present) for the wild type
flies have been filled in on Table 3. Eye color alleles are indicated as superscripts on
chromosome X (for example, Xr) because this is a sex-linked trait. The mutant alleles are
recessive and designated by a small letter.
Your instructor will assign the crosses for your lab group to perform.
Possible crosses:
Monohybrid, autosomal crosses
Wild males x vestigial wing female
Wild females x vestigial wing male (reciprocal cross)
Wild male x ebony/black body female
Wild female x ebony/black body male
(reciprocal cross)
Monohybrid, sex-linked
Wild female x white-eye male
Wild male x white-eye female
(reciprocal cross)
Procedures
1. Collect and anesthetize mutant flies of the two appropriate types as indicated by
the instructor.
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2. Note the distinguishing characteristics of mutant flies. How do you distinguish
the mutants from the wild type?
3. Separate according to sex.
4. Place three females or three males of the appropriate mutant type in the vial with
three wild-type flies of the opposite sex. Remember to keep the vial on its side
until the flies have recuperated.
5. Label the vials.
Complete Table 2 filling in the information about the phenotypes and genotypes of the
wild and mutant flies.
Table 2. Phenotypes and genotypes of wild and mutant flies.
Trait
Wild type
Eyes:
Phenotype
Red eyes
Genotype
XRXR, XRY
Wing length:
Phenotype
long wings
Genotype
LL
Body:
Phenotype
Gray (normal) body
Genotype
GG
White Eye
Vestigial Wing
Ebony Body
Do a punnett square for the crosses performed and fill in expected ratio of phenotypes
expected in the offspring of the F1, first filial generation.
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Punnet square of monohybrid, autosomal cross.
P1
Female
Genotype
Male
X
Phenotype
Predicted F1 generation:
Female
Genotype
Male
and
Phenotype
-----------------------------------------------------------------------------------------------------------Punnet square of monohybrid, sex-linked cross.
P1
Female
Genotype
Male
x
Phenotype
Predicted F1 generation:
Female
Genotype
Male
and
Phenotype
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Week 2
After seven days, remove the adult flies from the culture and place in the morgue.
This prevents incestuous breeding between the parent generation and their offspring.
(Some flies just have no morals at all!) Return the culture vial to its place for 5-7 days
more of incubation.
Week 3
Flies that hatch are your F1, first filial generation. Anesthetize the flies as
previously, and place them in the center of the card and separate them according to sex.
Monohybrid, autosomal cross:
Are both sexes alike? ______________________________________________________
Do the offspring look like the wild type parent or the mutant parent? ________________
Find the group in the lab that performed the reciprocal cross.
Do their results agree with yours? ____________________________________________
Place three females or three males of the appropriate mutant type in the vial with three
wild-type flies of the opposite sex. Remember to keep the vial on its side until the flies
have recuperated. Label the vials. The remaining flies should be placed in the morgue.
Monohybrid, sex-linked cross:
Are both sexes alike? ______________________________________________________
What is the phenotype of the F1 female? _______________________________________
What is the phenotype of the F1 male? ________________________________________
Find the group in the lab that performed the reciprocal cross.
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Do their results agree with yours? ____________________________________________
If they are not the same, how do they differ? ___________________________________
Week 4
After seven days, remove the adult flies from the culture and place in the morgue.
Return the culture vial to its place for 5–7 days more of incubation.
Flies that hatch are your F2, second filial generation.
Monohybrid, autosomal cross:
Anesthetize the flies as previously, and place them in the center of the card and separate
them according to phenotype. Count the number of flies of each phenotype and enter in
Table 3.
Are both sexes alike? _____________________________________________________
Do the offspring look like the wild type parent or the mutant parent? ________________
Find the group in the lab that performed the reciprocal cross.
Do their results agree with yours? ___________________________________________
Punnet square of F1 monohybrid, autosomal cross.
Actual F1 (Is it the same as you predicted?)
Female
Genotype
Male
x
Phenotype
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Predicted F2 generation:
Female
Genotype
Male
and
Phenotype
-----------------------------------------------------------------------------------------------------------Table 3. F2 generation results of monohybrid, autosomal cross in Drosophila.
F1 Phenotypes
Number of Flies
(Sex-linked only)
(Sex-linked only)
Total
Total
Give the actual phenotype ratio that you have observed _________________________
Compare the actual phenotype ratio with the expected ratio from your punnett square.
Are they the same? _______________________________________________________
Exactly the same? __________________Why might the actual results differ from what
you predicted? ___________________________________________________________
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Monohybrid, sex-linked cross:
Anesthetize the flies as previously, and place them in the center of the card and separate
them according to sex and phenotype. Count the number of flies of each category and
enter in Table 4.
Are both sexes alike? ______________________________________________________
What is the phenotype of the F2 female? _______________________________________
What is the phenotype of the F2 male? ________________________________________
Find the group in the lab that performed the reciprocal cross.
Do their results agree with yours? ____________________________________________
If they are not the same, how do they differ? ___________________________________
Punnet square of F1 monohybrid, sex linked cross
Actual F1 (Is it the same as you predicted?)
Female
Genotype
Male
x
Phenotype
Predicted F2 generation:
Female
Genotype
Male
and
Phenotype
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Table 4. F2 generation results of monohybrid, sex-linked cross in Drosophila.
Number of Flies
F2 Phenotypes
(Sex-linked only)
(Sex-linked only)
Total
Total
Give the actual phenotype ratio that you have observed ___________________________
Compare the actual phenotype ratio with the expected ratio from your punnett square.
Are they the same? ________________________________________________________
Exactly the same? _______________________Why might the actual results differ from
what you predicted? _______________________________________________________
Extra Credit:
As demonstrated by your instructor, perform a Chi-squared analysis to evaluate your
results.
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