Kulsum Musani
November 27, 2012
Fly Project Report
Introduction:
Austrian monk Gregor Mendel formed the foundation of the science of genetics through his
work, was able to apply basic principles of heredity to humans and animals. He worked with
common pea plants and through selective cross breeding, observed seven traits and discovered
that some traits show up in offspring without the blending of parental characteristics. Mendel
worked with pea plants because they can grow easily in large number, and be reproductively
manipulated and have both male and female reproductive organs. This way, Mendel could
selectively cross pollinate plants with particular traits and view the outcome over many
generations.
The parental generation (P1) plants were homozygous, meaning that both alleles were the same.
Their offspring (F1 generation) were heterozygous, meaning that they inherited one allele from
each parent, with all the offspring in a 3:1 ratio. As a result, when the F1 plants breed, each has
an equal chance of passing on either allele to each offspring. From this, we now see that when
one trait is crossed (monohybrid cross), separation of alleles during meiosis and random
fertilization of gametes results in a 3:1 ratio of offspring. Additionally, when two traits are
crossed (dihybrid cross), genes assort independently, producing a 9:3:3:1 ratio of offspring.
Mendel made three important based on his results: the inheritance of each trait is determined by
“units” called genes, an individual inherits one such unit from each parent for each trait, and that
a trait might not show up in an individual, but it can still be passed on to the next or future
generations. His observations also led to two important principles: the law of segregation and the
principle of independent assortment. The law of segregation states that the pair of alleles of each
parent separate and only one allele passes from each parent on to an offspring, while the
principle of independent assortment states that different pairs of alleles are passed to offspring
independently of each other.
Mendel laid the foundation of genetics, and his work was expanded upon by Thomas Hunt
Morgan, who bred thousands of fruit flies (Drosophila Melanogaster). He confirmed Mendel’s
laws of inheritance and the hypothesis that genes are located on chromosomes. One important
discovery that he made was that eye color in Drosophila Melanogaster was a sex-linked trait. He
came up with several important theories: genes reside on chromosomes, each gene resides on a
specific chromosome, the trait for eye color is on the sex chromosome, and recombination
frequency, which he used to create a genetic map.
In this experiment, we confirmed firsthand the discoveries of Mendel and Morgan by breeding
Drosophila Melanogaster over the course of several weeks. We used fruit flies because they are
easy and relatively cheap to handle, produce a large progeny, have their entire genome mapped
out, and are sexually dimorphic, to name a few. They also have a short life cycle (approximately
10 days). Females first lay eggs and after one day, the eggs hatch, producing small, white larvae.
Between days six and seven, larvae reach full size and pupate. Mature flies then emerge between
day ten and twelve.
Males and females can be easily distinguished in the sense that females are larger than males.
Males also have a darker abdomen which is narrow and cylindrical, while females have a
spherical abdomen. The posterior end of male flies are round and blunt, while females have a
sharp and protruding posterior. Males also have small bristles on their first pair of legs, called a
sex comb, which are absent in females.
Virgin flies were used for the experiment in order to have controlled mating with male flies,
since non-virgin female flies store sperm from previous mating with males. For our specific
experiment, we crossed a wild type female (X+X+) with a white male (XwY). We expected to
obtain an F1 generation which consisted of wild type females (X+Xw) and wild type males (X+Y)
in a 3:1 ratio. We then crossed the F1 generation, expecting a progeny of wild type females
(X+X+), wild type females (X+Xw), wild type males (X+Y), and white eyed males (XwY). We
ended up with a progeny of 426 F2 generation flies.
Aim and Hypothesis:
The aim of our experiment was to determine whether eye color in Drosophila Melanogaster is a
sex-linked trait. In order to do this, we crossed a wild type (red eyed) female with a white eyed
male. We hypothesized that when we crossed the wild type (red eyed) female with a white eyed
male, we would observe 100% wild type (red eyed) female fruit flies and 100% wild type males.
We also hypothesized that when we crossed the F1 progeny, the F2 generation would display a
mix of all wild type females, wild type males, and white eyed males, in a 2:1:1 ratio, since we
would not be able to tell if females are carriers for white eyes or not simply based on physical
characteristics. The null hypothesis for our experiment was that there would be no deviation for
the F2 generation, displaying a 2:1:1 ratio.
Materials and Methods:
The materials used for the experiment were as follows:
-
Morgue containing 70% Ethyl Alcohol
Stocks of Drosophila Melanogaster
Dissecting microscope
Culture medium
Dropper
Teasing needle
Water
-
Marking pen
Etherizers
Clear vials with foam stoppers
Scapula
To carry out the experiment, we first set up vials for the fruit flies. In order to develop the
medium, we added one cup of instant medium to the vial, then added close to the same amount
of water, which turned the medium blue. We added a pinch of yeast which is used as food for the
vials.
We then took three virgin wild type (red eyed) flies and four white eyed male flies and placed
them in the vial. The virgin flies came from a pre-supplied vial, and the males were differentiated
with a scapula underneath a dissecting microscope after they were anesthetized. We then capped
the vial with the foam stopper and placed them horizontally into a drawer until the anesthetized
flies woke up, after which we turned the vial vertically. These formed the P1 generation, and
after about a week, the P1 generation had mated, producing the F1 generation flies.
We then anesthetized the F1 flies, put them on a note card, and viewed them under a dissecting
microscope. Our F1 generation produced 68 wild type (red eyed) and 59 wild type (red eyed)
males. We took new vials, made a new medium, and then transferred the flies to a new vial, so
the remaining vials would contain only F1 flies. We then repeated the same process after a
couple of days to produce F2 flies. We kept on transferring the flies to new vials so the progeny
would not get mixed up with the parental. Our final count of the F2 generation ended with 268
wild type (red eyed) females, 64 white eyed males, and 76 red eyed males. These flies were then
placed into the morgue after the experiment was completed.
Results:
F1 Results: Wild Type (red eyed) females*White eyed males
Classes
(Phenotypes)
Observed
(O)
Expected
(E)
Deviations
(O-E)
(O-E)2
(O-E)2/E
Red Females
68
63.5
4.5
20.25
0.3189
White Males
59
63.5
-4.5
20.25
0.3189
X2= 0.6378
F2 Results: Wild Type (red eyed) Females*Wild Type (red eyed) Males
Phenotypes
Red eyed
females
White eyed
males
Red eyed
males
Observed
(O)
268
Expected
(E)
204
Deviations
(O-E)
64
(O-E)2
(O-E)2/E
40.96
20.078
64
102
-26
676
6.6274
76
102
-38
1444
14.157
X2=40.8624
Discussion & Conclusion:
Based on our results, we accepted our hypothesis for the F1 generation and rejected the
hypothesis for the F2 generation. We had originally hypothesized that there would be 100% wild
type females and 100% wild type males. We had also hypothesized that when we crossed the F1
progeny, the F2 generation would display a mix of all wild type females, wild type males, and
white eyed males in a ratio of 2:1:1. Our results made sense because since females have a trait
for wild type eyes and males have the trait for white eyes on the X chromosome, the female
offspring will be carriers for the white eyes, and the males will display wild type characteristics.
As a result, when the F1 flies mated, their progeny would display a mix of characteristics
because the males can get either the wild type traits or white eyed traits; since they only have one
X chromosome, that trait will be displayed. For this reason, the F2 generation will display wild
type females, but some can be carriers for white eyes because white eyes is a recessive trait and
is only expressed when both chromosomes have the trait. However, our data did not match our
predicted ratio of 2:1:1, leading us to believe that there might be a mistake in our hypothesized
ratio or in our carrying out the experiment.
We did not observe any problems throughout the experiment. Any error that occurred could have
resulted from miscounting the number of progeny or the amount of flies we mated. Future
improvements that we can do include keeping better track of the flies we counted, and checking
on flies more frequently to prevent any mixing between generations. Overall, the experiment let
us have a firsthand look at the discovery and research done by Thomas Hunt Morgan.
Bibliography:

http://anthro.palomar.edu/mendel/mendel_1.htm

http://www.genomenewsnetwork.org/resources/timeline/1910_Morgan.php

http://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452