Fruit Fly Lab Report

Christopher Kruzel
Bios 221
April 4, 2012
Drosophila

Introduction
The fruit fly or Drosophila melanogaster is ideal for classroom experiments. They are inexpensive, easy to nap, breed, as well as observe. It was very important to be able to tell the male and female flies apart from one another. The males are known to be usually smaller in size than the female flies and have bristles on their forelegs while the females lack this appearance. Also the males have a black or dark round end whereas the females have striped pointy ends. The Drosophila flies are small have dark red eyes and have a yellow-brown body. These flies are able to mutate within approximately ten to fourteen days at twenty-five degrees Celsius. The Drosophila has a
handful of genetic variability that can be observed within several days since they reproduce quickly laying an enormous amount of eggs.
This lab was the study of monohybrid as well as dihybrid crosses. A monohybrid cross is the study of a certain trait whereas a dihybrid cross is the study of the mating experiment between two organisms that are identically hybrid for two traits. ( Reference 1 ) This lab was done to determine the genetic mutations after each generation and to observe the ratios. As each generation of fruit flies came to existence, traits would either be different from each fly or certain traits would disappear from existence. The expected ratio for a monohybrid cross was 3:1 and 9:3:3:1 for the dihybrid cross. For the monohybrid cross, eye color was observed to be wildtype, dark red, or white eyed, which was x-linked. (Reference 2) As for the dihybrid cross, both wing shape as well as eye color was observed. The wings were either straight, wildtype, or shriveled, vestigial, depending on
the autosomal genes. The eye color was either dark red, wildtype, or sepia.
Materials
The materials used within this lab was male and female Drosophila flies, two vial tubes with a sponge plug for each, Flynap, teasing needle, light, wand, and the culture medium.
Methods
The Drosophila flies were put to sleep by using the Flynap and wand.
The wand was dipped into the Flynap and then placed into the tube for a few minutes until the flies were asleep. Once the flies were asleep, the wand was removed and the flies were distributed. From this point, the flies were counted and their sex was determined under the microscope. The monohybrid cross was sex-linked, wild x wild eye colored flies. The dihybrid consisted of vestigial winged flies x sepia eye colored. An even amount of males and females were placed in the vial tubes using a teasing needle, one labeled monohybrid and the other dihybrid. Within each tube, there was some culture media for the flies to survive and reproduce for about a week. After giving the fruit flies time to reproduce, they were put to sleep again using the Flynap and then the progeny were taken out to be counted and observed. The results were recorded and the same was done for the following generation,
F2.
Results
Monohybrid Results
Monohybrid cross of Parent Generation (White x Wild)
R was the gene for wild type, dark red, eye color r was the sex-linked recessive gene for white eye color

XR XR
XR Xr
XR Xr
XRY
XRY
Xr
Y

One hundred percent of the F1 progeny were wild type flies.

Monohybrid cross of the F1 Generation (Wild type females x Wild type males) XR Xr
XR XR
XRXr
XR Y
XrY
XR
Y

Seventy-five percent of the progeny were wild type flies and twenty-five percent are white-eyed flies resulting in a 3:1 ratio.

In order to test Mendel’s Law of Segregation, the inheritance of eye color was determined by crossing two pure breeding strains of Drosophila that was wildtype and white eyes. The phenotypes of the F1 progeny are shown below.
Phenotypes
Number of Progeny

Males
Females
Total
Wild Type
5
7
12
White Eyes
0
0
0

In order to confirm whether eye color was inherited according to Mendelian laws, the F1 progeny were crossed and the resulting phenotypes of the F2 progeny are shown below.

Phenotypes
Number of Progeny

Males
Females
Total
Wild Type
11
15
26
White Eyes
4
6
10

36

Dihybrid Results
Dihybrid cross of Parent Generation (Vestigial Wings x Sepia Eyes)
S was the dominant wild type, dark red, eye color s was the recessive sepia eye color
Vg was the dominant wild type, straight, wing shape vg was the recessive vestigial, shriveled, wing shape

Svg
SsVgvg
sVg

One hundred percent of the F1 progeny were wild type flies, dark red eyes with straight wings.

Dihybrid cross of F1 Generation (Wild Type x Wild Type)

VgS Vgs vgS vgs VgVgSS
VgVgSs
VgvgSS
VgvgSs
VgVgSs
VgVgss
VgvgSs
Vgvgss
VgvgSS
VgvgSs
vgvgSS vgvgSs VgvgSs
Vgvgss
vgvgSs vgvgss VgS

Vgs

vgS

vgs

56.25% of the of the progeny are wild type flies, dark red eyes with straight wings, 18.75% are vestigial, shriveled, wings with wild type, dark red, eye color, 18.75% are wild type, straight, wings with sepia eye color, and 6.25% of the progeny are vestigial, shriveled, wings with sepia eye color resulting in a 9:3:3:1 ratio.

In order to test Mendel’s idea of independent assortment, the inheritance of both eye color and wing shape was examined by crossing the F1 generation with itself.

Progeny
Phenotypes
Genotypes
Total
Wild Type x Wild Type
VgVgSS,VgvgSS,
VgVgSs,VgvgSs
25
Wild Type x Sepia
VgVgss,Vgvgss
4
Vestigial x Wild Type vgvgSs,vgvgSS 2
Vestigial x Sepia vgvgss 1

32

Monohybrid Chi Square

In order to check the results in comparison with the expected results of a 3:1 ratio, a Chi square test on the F2 progeny is constructed.
Class
Observed
Expected
(O-E)2
(O-E)2/Expected
Wild Type
26
27
(26-27)2 = 1
1/27 = 0.037
White Eyes
10
9
(10-9)2 = 1
1/9 = 0.111
Totals
36
36

X2 = 0.148

The degree of freedom, df = n-1 (n = total number of categories) = 2-1 = 1
X2= 0.148 is less than 3.841, so hypothesis that the experiment followed a 3:1 ratio for a monohybrid cross is accepted.

Dihybrid Chi Square
In order to check our results in comparison with the expected results of a 9:3:3:1 ratio, a Chi square test on the F2 progeny was constructed.

Class
Observed
Expected
(O-E)2
(O-E)2/Expected
Wild Type x
Wild Type
25
18
(25-18)2 = 49
1/49 = 0.0204
Wild Type x
Sepia
4
6
(4-6)2 = 4
1/4 = 0.25
Vestigial x
Wild Type
2
6
(2-6)2 = 16
1/16 = 0.0625
Vestigial x
Sepia
1
2
(1-2)2 = 1
1/1 = 1
Totals
32
32

X2 = 1.333

The degree of freedom, df = n-1 (n = total number of categories) = 4-1 = 3
X2= 1.333 is less than 7.815, so we accept the hypothesis of a 9:3:3:1 ratio being followed.

Discussion
Both the monohybrid and dihybrid crosses had some type of mutation occurring. Within the monohybrid cross, two different traits were observed, dark red eyes as well as the white eyed Drosophila flies. When the dihybrid cross was observed, there were four different possibilities that occurred after the mutations took place as seen above in the Chi Square. After all the results were observed, the expected ratios were seen throughout the experiment. By looking at the Chi Square tables, both hypotheses were accepted and a 3:1 ratio was seen for the monohybrid cross whereas a 9:3:3:1 ratio was seen for the dihybrid cross.

References

1)Nature.com. Nature Publishing Group. Web. 05 Apr. 2012. .

2) Flagg, Raymond O. "Carolina Drosophila Manual." Carolina Biological Supply Company, Print.