Josh Ye
Ms. Buckley
AP Biology
1/15/2019
Lab Report - ​D. melanogaster a​ nd inheritance
Purpose:
The purpose of this experiment is to investigate the inheritance pattern of the gene by
performing crosses between two ​parent​ flies and observing their ​offspring​ (F​2​ generation). In our
specific scenario, the gene that is being investigated is that which controls the ​sepia eye​ trait.
Background:
One topic integral to the study of genetics is inheritance. The study of genetic inheritance
describes how traits are passed down from one generation to the next. For this to be studied,
however, living organisms must be crossed, and their offspring observed. While this process is
often not difficult to perform in small animals and plants, a dilemma that arises is the time and
space that it often takes to cultivate plants or grow animals. Observing multiple generations of
plants or animals takes time, space, and resources such as water and food.
Drosophila melanogaster​, colloquially known as the fruit fly, however, can be used in
these genetic experiments easily. These organisms occupy little space, require little care, have
simple food requirements and a short life cycle, making the organism indispensable to modern
genetic studies, and have been used in these studies since at least 1907. In addition, its genetic
traits are easily observable, and many offspring are produced from few parents. One female fly,
over the course of ten days, can lay up to 500 eggs. The life cycle of ​D. melanogaster​ consists of
three stages: Larval, Pupa, and Adult. In the larval stage, the flies climb out of the food medium
and stop moving. In the pupal stage, the cuticle hardens, and the larval body shortens. Larval
tissues are broken down to provide energy for this phase. Later in the pupal stage, the eyes,
wings, and legs become readily visible. In the adult stage, these organisms are also easily sexed,
making identification quite easy.
By observing physical traits of the parent and offspring (F​1​ generation), the genetic
makeup of both the parents and the offspring be identified. Simple genetic traits in fruit flies
such as eye color, wing shape, and others, follow the inheritance patterns described by Gregor
Mendel. Mendel’s genetic studies were not conducted with ​D. melanogaster,​ but instead with
pea plants. His studies set the foundation of modern genetics.
From his studies, Mendel discovered three laws of inheritance. First, the ​Law of
Dominance.​ This law describes that when two homozygous parents with contrasting alleles for
the same character are crossed, only the ​dominant​ allele will be expressed in the offspring. This
allele determines the physical appearance of the offspring. For instance, in ​D. melanogaster​, if
wild type eyes are dominant over sepia type eyes, the offspring of a pure wild male and a pure
sepia female will show wild type eyes. Mendel’s second law is the ​Law of Segregation.​ The Law
of segregation states that the two alleles for a heritable character ​segregate​ during gamete
formation and end up in ​different gametes​. The gametes of a fruit fly will contain only one allele
for eye color. The other allele comes from the other parent. Mendel's third law is the ​Law of
Independent assortment​. This law describes that genes for different traits can segregate
independently during the formation of gametes. This process occurs during Metaphase I of
meiosis, where the orientation of homologous chromosomes in relation to the poles of the cell
are random. This means that the biological selection for one trait has nothing to do with the
selection of another.
When performing genetic crosses between two organisms, it is often essential to know if
a plant exhibiting the dominant trait is pure (homozygous) or a hybrid (heterozygous). Because
of the law of dominance, both plants will exhibit the same phenotype. In order to determine this,
a ​test cross ​must be performed. To perform a test cross, the organism showing the dominant
phenotype is crossed with an organism that is pure for the recessive phenotype. If the parent
organism showing a dominant phenotype is heterozygous, we can expect that the about ½ of the
offspring will show the dominant phenotype, while the other half of the offspring will show the
recessive phenotype. However, if the parent organism showing a dominant phenotype is
homozygous, we can expect that all of the offspring will show the dominant phenotype.
Hypothesis:
Because all offspring of a sepia female and a wild male showed the wild phenotype, the
sepia trait is inherited recessively autosomally. We can thus conclude that all the flies in the F​1
generation (the flies we received in the vials) were heterozygous for the trait. The results,
therefore, of the cross sp+ sp × sp+ sp , will be ¼ sp+ sp+ , 2/4 sp+ sp , and ¼ spsp . This means
that ¾ of the flies will show the dominant phenotype, wild type, while ¼ of the flies will show
the recessive phenotype, sepia.
Materials and Methods:
Materials:
Certain materials are necessary for this procedure.
● Vial of flies (offspring of pure sepia
and pure wild)
● Flynap for the anesthetizing of the
flies
● Water
● Foam stopper
● Paintbrush
● Petri Dish
● Banana extract powder (food source)
Methods:
1. Obtain an empty vial. Quickly open the vial containing the flies, and transfer them to the
empty vial.
2. Use the Flynap to anesthetize the flies. Insert the wand containing Flynap into the vial.
Keep the wand in the vial until all the flies fall to the bottom.
3. Remove the anesthetized flies from the vial and place them into a petri dish. Using a
paintbrush, sort the flies by sex.
4. Prepare the food mixture for the flies in two new vials. Combine one tablespoon of
banana extract powder and one tablespoon of water in the vials.
5. After the food mixtures have coagulated, turn the vials onto its side. Then, place at least
five flies of each sex into each of the vials.
6. After the flies have awoken, turn the vials into the upright position.
7. After a few days, when the flies have mated and larvae and/or pupa are visible, release
the parent flies.
Results:
It was hypothesized that the F​2​ generation of flies would have the ratio 3 dominant (wild
type) to one recessive (sepia type). This was due to the fact that it was determined the F​1
generation of flies were all heterozygous dominant, and the results of this monohybrid cross
would result in the 3:1 ratio of dominant to recessive in the F​2​ generation. In addition, it was
hypothesized that the sepia trait is inherited in an autosomal manner, meaning it is inherited
through the organism’s somatic cells and is not sex-linked. This was due to the fact that in the F​2
generation the ratio of wild to sepia was 3:1 regardless of sex. If the trait was x-linked recessive,
we would expect to see every male with the sepia phenotype, and all females with the wild
phenotype. The following punnett square demonstrates this idea .
F​1​ generation
sp​+
sp+
sp
sp​+​sp
sp​+​sp
sp
sp​+​sp
sp​+​sp
Where sp is the sepia phenotype and sp+ is the wild phenotype. As stated earlier, every one of
the offspring in the F1 generation should be heterozygous dominant, given that there are no ​de
novo​ mutations.
Given these results for the F​1​ generation, the F​2​ generation should follow:
F​2​ generation
sp+
sp
sp+
sp​+​sp​+
sp​+​sp
sp
sp​+​sp
spsp
This punnett square thus affirms that the F2 generation has a 3:1 ratio of dominant to recessive,
and also in accordance with Gregor Mendel’s findings.
The actual numbers of flies are shown in these tables for both generations.
F​1​ generation
Male
Female
Wild Type
34
27
Sepia Type
0
0
Due to our hypothesis that the trait tested was not sex linked, the distinction between sex was not
recorded.
F​2​ generation
Number
Wild Type
298
Sepia Type
89
To further demonstrate the validity of these results, a ​chi-square​ analysis can be performed to
test the “goodness of fit”.
Phenotype
Observed
Expected
(​o-e​)
(​o-e​)2​​ /e
Wild Type
298
290.25
7.75
0.2069
Sepia Type
89
96.75
-7.75
0.2069
2
Since χ2 = ∑ (o−e)
, we must sum the (o − e)2 /e values. Thus, our chi-square value is
e
χ2 = 0.2069 + 0.2069 =0.4138.
Discussion:
The ​p​ value can be calculated from the χ2 value using the Ti-89 calculator. Since for one
degree of freedom the ​p ​value is ​p=0​ .5200, at the critical level α = 0.05 , the results are
significant. Thus, our prediction was a good fit for our actual outcome, and our null hypothesis
can be accepted.
One error that can account for differences between the expected and observed ratios is
length of time that the flies were left in the vial. Because the F​2​ generation of flies were left in
the vials for long enough for mating to occur and the F​3​ generation to appear, some F​3​ flies could
have been counted alongside the F​2​ generation of flies. This is problematic because it is much
more likely for the offspring of two F​2​ generation of flies to show the wild phenotype than the
sepia phenotype. Other deviations from the expected values could be due to flies escaping the
vial, or simply chance, because genetic crosses are based on probability and processes of
probability are never certain.
This experiment showed the significance and reliability of the genetic models first
discovered by Gregor Mendel, and also demonstrates his ​laws of inheritance,​ most importantly,
the laws of dominance and independent assortment. In both the F​1​ and F​2​ generations, the
dominant allele was always expressed when it was present. In addition, each of the flies received
one eye color allele from each parent.
One further experiment that could be explored is to test an x-linked or sex-linked trait.
This would be interesting to investigate to determine if x-linked traits in ​D. melanogaster​ are
inherited the same way as x-linked traits in humans.
Acknowledgements:
https://www.khanacademy.org/science/high-school-biology/hs-classical-genetics/hs-introduction
-to-heredity/a/the-law-of-segregation
http://researchguides.library.vanderbilt.edu/c.php?g=156859&p=1161911