Genetics of Drosophila Lab Report
Brianna Horn
Genetics – Fourth Block
April 21, 2008
ABSTRACT
The purpose of this experiment was to learn about Mendelian genetics through the study
of Drosophila melanogaster. The Drosophila were altered to show one or more mutated trait,
including eye color, body color, or wing length. When crossed, the type of mutation, whether
monohybrid, dihybrid, or sex-linked, would become apparent and an excellent teaching method
of the different laws of genetics. The flies were bred through three generations and recorded at
each generation. The flies were put to sleep, separated by sex, and sorted into groups based on
the traits they exhibited. The number of flies, as well as their traits were recorded thoroughly.
Punnett squares showed that the cross was dihybrid. Chi-squares proved that the null hypothesis
was valid. This proves that the experiment was a success. The flies varied through chance alone,
and proved how dihybrid crosses function.
INTRODUCTION
In 1851, Gregor Mendel started working with garden pea plants, being the first to work
be involved in the findings of genetics. Through his work, he formed three Laws of Inheritance –
the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.
Though Mendel worked with peas and not flies, genetics works the same way with both
organisms. Genetics, or the study of inheritance, applies the same way to all organisms that
perform meiosis to in order to produce offspring. This means that the laws still apply to this
study of Drosophila melanogaster.
There were three different types of crosses available during this experiment. Each was
basic, uncomplicated by other variations. There was only three things that could have been
altered with each of the flies. The wild type of Drosophila has a tan body color, normal wing
length and shape, and normal red eyes. The results would tell which cross was performed. If the
cross was monohybrid, only one trait on either the female of male parents was mutated. An
example of this would be that the eye color of all the females was sepia, while the males were
still pure wild-types. The Law of Dominance is shown here in the simplest expression. The Law
of Dominance states that the dominant allele will be shown over the recessive allele. This means
that the dominant allele will be expressed instead of the recessive allele if they are both included
in the heterozygous genotype. If S means normal eyes and s means sepia eyes, then there are
three possible outcomes. If the alleles cross to form a combination of SS, the genotype is
homozygous dominant, and the phenotype shown is the normal eye color. If the cross forms an
Ss combination, the genotype is heterozygous, but as the Law of Dominance states, the S
overpowers the s, and the phenotype shown is the normal eyes. The only way the sepia eyes can
be expressed is if the combination formed is homozygous recessive, ss. A Punnett square is
shown to prove the point of the parent generation.
Monohybrid Parent Generation Cross
♂
♀
S
S
s
Ss
Ss
s
Ss
Ss
The results of the Punnett square show that all of the offspring of the parent generation
are heterozygous and all show the phenotype of normal eye color. However, when the next
generation, the F1 generation, is crossed, the results will be different.
Monohybrid F1 Generation Cross
♂
♀
S
s
S
SS
Ss
s
Ss
ss
In the F1 cross, both parents are heterozygous. This results in various phenotypes in the
offspring. These results show the genotypes of the F2 generation. While the genotype ratios as
1:2:1 (SS:Ss:ss), the phenotype is 3:1 (normal color : sepia color). Seventy-five percent of the
offspring will have the normal color eyes, while the twenty-five percent with the ss genotype
express the sepia eyes.
The Law of Segregation is shown in its basic form as well. The Law of Segregation states
that alleles separate in gamete formation. The separated alleles in the Punnett square is a perfect
example of this. The alleles separate in order to form a diploid organism with another parent.
Another type of cross is a dihybrid cross. In a dihybrid cross, two different traits are
mutated. For example, one parent could have an ebony body and normal wings (EEVV), while
the other parent could have a normal body with vestigial wings (eevv). The traits do not affect
each other. Both traits are separate and the alleles are sorted independently. This is a prime
example of the Law of Independent Assortment, where different traits are inherited
independently.
Dihybrid Parent Generation Cross
♂
♀
ev
ev
ev
ev
EV
EV
EV
EV
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
All of the F1 generation will have the same phenotype and genotype. They all have the
heterozygous genotype EeVv, which produces a phenotype of ebony bodies and normal wings.
When the F1 generation crosses, it will be crossing heterozygotes with heterozygotes. This will
form a very different outcome.
Dihybrid F1 Generation Cross
♂
EV
♀
EV
Ev
eV
ev
EEVV EEVv EeVV EeVv
EEVv EEvv EeVv Eevv
EeVV EeVv eeVV eeVv
EeVv Eevv eeVv eevv
Ev
eV
ev
Out of these results, nine out of sixteen have a genotype of E_V_, expressing a phenotype of
ebony bodies and normal wings. Three out of sixteen have a genotype of E_vv, expressing ebony
bodies and vestigial wings. Three out of sixteen have a genotype of eeV_, expressing normal
bodies and normal wings. One out of the sixteen is homozygous recessive for both traits, eevv,
expressing normal bodies and vestigial wings. These results form a genotypic and phenotypic
ratio of 9:3:3:1 (E_V_, ebony body normal wings : E_vv, ebony body vestigial wings : eeV_,
normal body normal wings : eevv, normal body vestigial wings).
The last possible type of cross that could be encountered during this experiment was sexlinked cross. The sex-linked cross is a little more complicated, as the sex of the Drosophila
makes a difference. Sex-linked crosses mean that the allele is on the X chromosome. While
females have two X chromosomes, males have one X and one Y. This means that if males have
the allele on the X chromosome, they express that trait. Females, however, have two X
chromosomes. This means they can carry the trait without necessarily showing it, so long as the
trait is recessive. This gives females an advantage over males if the trait is a disease or harmful
mutation.
Sex-Linked Parent Generation Cross
♂
Xo
Y
X
XXo
XY
X
XXo
or
XY
X
Y
Xo
XXo
XoY
Xo
XXo
XoY
♀
♂
♀
In both set of results, the females are only carriers of the trait. In order for them to
express the trait, both of their parents would need to carry or express the trait. In the first Punnett
square, all of the females are carriers and all the males are normal. In the second Punnett square,
all of the females are carriers and all of the males express the trait. The mother must have or
carry the trait for the son to express the trait. It does not matter what the father has, as he gives
his son only the Y chromosome.
Sex-Linked F1 Generation Cross
♂
♀
X
Y
X
XX
XY
Xo
XXo
or
XoY
Xo
Y
Xo
XoXo
XoY
X
XXo
XY
♂
♀
In the first Punnett square, fifty percent of the females are carriers and fifty percent are
homozygous dominant. Fifty percent of the males express the trait and fifty percent do not. In the
second Punnett square, fifty percent of the females express the trait, and fifty percent are carriers;
fifty percent of the males are normal, and fifty percent express the trait.
As for this experiment, the female parents had normal eyes and wings, but had ebony
bodies, and the males had normal eyes and bodies, but had vestigial wings. Based on these facts,
there is a strong possibility that the cross for this set of flies is dihybrid. The F1 generation should
come out with ebony bodies and normal wings, as ebony bodies is dominant over normal bodies
and normal wings are dominant over vestigial wings. If this is the case, the F2 generation will
have a 9:3:3:1 outcome.
MATERIALS AND METHODS
There are many parts to this experiment. The homozygous parents were paired in a vial
full of food on March 16, 2008. They were given about a week to mate and lay eggs, then were
removed from the vial on March 25, 2008. In order to remove the flies without allowing them to
escape, a cotton swab soaked in Anesthefly was slipped between the foam stopper and the vial.
The cotton swab was left in for about two minutes, but not longer, as it would kill the flies. The
vial was help upside down while the anesthefly was putting the flies to sleep so the flies did not
fall into their food and drown. When all flies were asleep, the foam was removed and all the flies
were dumped into a Petri dish. They were then separated based on sex and sorted based on what
traits they expressed that differed from the wild type. All data was recorded. The parents were
then placed in the fly morgue. The F1 generation, still larva were set in a warm environment to
ensure healthy and speedy development. After the larva became flies, five males and five
females were anesthetized and transferred into a new vial on March 31, 2008. The new vial had
been made earlier. It had a mixture of equal amounts of food and water mixed together at the
bottom. It also had a plastic net placed in the middle, and a small amount of coffee paper rolled
into a cone shape placed on the net. When more of the F1 generation hatched into flies, they were
anesthetized, sexed, sorted, and recorded, then placed in the fly morgue. On April 7, 2008, the
parents of the F2 generation were anesthetized and placed in the fly morgue. On April 16, 2008,
all of the hatched F2 generation flies were anesthetized, sexed, sorted, and recorded, then placed
in the fly morgue.
RESULTS
Table 1: Phenotypes of the Parental Generation
Phenotype
No. of Males No. of Females Total
Ebony bodies, normal wings, normal eyes
0
7
10
Vestigial wings, normal bodies, normal eyes
3
0
Table 2: Phenotypes of the F1 Generation
Phenotype
No. of Males No. of Females
Ebony bodies, normal wings, normal eyes
127
129
Table 3: Phenotypes of F2 Generation
Phenotype
No. of Males No. of Females
Ebony bodies, normal wings, normal eyes
65
69
Ebony bodies, vestigial wings, normal eyes
29
22
Normal bodies, normal wings, normal eyes
27
33
Normal bodies, vestigial wings, normal eyes
10
13
Total
256
Total
273
Table 4: Punnett Square of Parent Generation Forming Possible F1 Genotypes
♂
♀
ev
ev
ev
ev
EV
EV
EV
EV
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
EeVv
Genotype: 100% EeVv
Phenotype: 100% ebony bodies, normal wings
Table 5: Punnett Square of F1 Generation Forming Possible F2 Genotypes
♂
♀
EV
Ev
eV
ev
EV
Ev
eV
ev
EEVV EEVv EeVV EeVv
EEVv EEvv EeVv Eevv
EeVV EeVv eeVV eeVv
EeVv Eevv eeVv eevv
Genotype: 9:3:3:1 :: E_V_ : E_vv : eeV_ : eevv
Phenotype: 9:3:3:1 :: ebony, normal : ebony,
vestigial : normal, vestigial : normal, normal
Table 6: Chi Square Test for F2 Generation
Phenotype
# Observed (o)
# Expected (e)
E_V_
E_vv
eeV_
eevv
134
56
60
23
153
51
51
17
Total
273
273
-19
5
9
6
361
25
81
36
2.36
0.49
1.59
2.12
6.56
DISCUSSION
My results showed about a 9:3:3:1 ratio, which is typical of a dihybrid cross. The ebony
bodies proved to be dominant, and the vestigial wings proved to be recessive, as the
heterozygous F1 flies all had ebony bodies and normal wings. This supports the hypothesis made.
The Chi square test had a result of 6.56. As there were four different phenotypes, the number of
degrees of freedom was three. As the probability of variations happening on chance alone was
ninety-five percent, the critical value was 7.82. As 6.56 is less than 7.82, the null hypothesis is
accepted. Any variations in numbers were due to chance alone. The results accurately supported
the dihybrid cross explained in the introduction, and follow the pattern of the ratio expected.