Fruit Fly Lab Report

Drosophila melanogaster

(commonly known as “Fruit Fly”)

Christopher Moody

Lab TA: Xeniya Rudolf

General Genetics Lab BIOL 2321L- Spring 2017

Section 03

Introduction:

Drosophila melanogaster are great model organisms for the study of genetics. This is because there are approximately 16,000 genes observed in fruit flies and we observe much homology in the genomes of fruit flies and humans. For example, “75% of know human disease genes have a recognizable match in the genome of fruit flies” (Xeniya Rudolf, Lecture 8, slide 3).

In Drosophila melanogaster, there are two phenotypes for eye color: red and white. A fruit fly exhibiting a red phenotype for eye color possess the normal, wild-type allele for eye color. A fruit fly that exhibits

a white phenotype for eye color possesses the mutant allele. Fruit flies that exhibit the mutant white-eyed allele do not contain any pigment in their eyes, like normal fruit flies with red eyes do. Fruit flies with red eyes are denoted “w+,” while flies with the mutant white gene are denoted “w.”
In our experiments with the fruit flies, we were studying the X-linked (sex-linked) inheritance of an allele that causes a white mutant phenotype in the eyes of the fruit flies. We learned that X-linked inheritance in an extension of Gregor Mendel’s pattern for inheritance, which was autosomal inheritance (General Genetics Lab…). We were observing how this X-linked inheritance affected the occurrence of the mutant phenotypes in males and females. In fruit flies, females will carry two copies of the gene for eye color and males will only carry one copy of the gene for eye color. This is because females have two X chromosomes, while males only have one X chromosome. Because of this female fruit flies can be heterozygous, “w+w,” or they can be homozygous, “w+w+,” “ww.” On the other hand, since males only possess one X chromosome they can only be hemizygous, “w” or “w+,” for a particular gene.
This leads me to the purpose for performing our experiment. We had a few reasons for performing our experiment including: I) testing to see whether the inheritance was X-linked or autosomal (even though we knew beforehand that it was X-linked) II) observing the X-linked inheritance of the mutant white gene and III) to establish this X-linked inheritance of the white gene (Xeniya Rudolf, Lecture 7, slide 3). We were able to do this by performing several crosses of the Drosophila melanogaster.
For our experiment, our null hypothesis was that we would observe no significant difference between the expected number of red and white-eyed flies verses the observed number of red and white-eyed flies. Our alternative hypothesis was that we would observe some difference between the two groups mentioned above. We used chi square analysis was used to determine whether we could reject or fail to reject the null hypothesis. We received a p-value from the chi square test, which then allowed us to determine whether or not to reject the null hypothesis. Typically, when the p-value is less than or equal to 0.05 we are able to reject the null hypothesis. However, when the p-value is greater than 0.05 we fail to reject the null hypothesis (General Genetics Lab…).

Procedures:
Week 1:
Our purpose during week one of our experiment was to set up two separate crosses (Cross A and Cross B), which were to be our parental or P crosses.

For Cross A, we crossed 5 red-eyed females (w+w+) with 5 white-eyed males (w). For Cross B, we crossed 5 white-eyed females (ww) with 4 red-eyed males (w+; only four males because one of our red-eyed males died).

We began this experiment by obtaining four separate vials that contained the four fly genotypes that I mentioned above. We then took the vials of the respective crosses, gently tapped them on the desk to knock the flies to the bottom of the vials, took the stoppers out of the vials and put the open ends together. We had to tap the vials on the desk again to get all of the flies into one vial. We had to perform these procedures for both Cross A and B. After combining the flies into the proper vials, we labeled and turned in to our TA and allowed them to mate for a week.

Week 2:

During week two, we had to remove our parental generation from our vials so as to prevent parents and offspring mating. Mating between the parental generation and offspring would result in gene mixing and skew the results of our crosses. To discard of the parental generation, we had to tap the vials on the desk, remove the cotton stopper, then empty the parental generation into a container of alcohol, otherwise known as the “fly

morgue.”
Week 3:
For week three, we had two tasks to perform: I) count the F1 generation and II) set up our F2 crosses. We began by using FlyNap to anesthetize the flies. This was done by placing a small brush that had been submerged in FlyNap into the vials with the flies. We had to be sure to keep the vial on its side so that the flies would not fall and become stuck in the gel at the bottom of the vial. Once the flies had been anesthetized, we emptied the contents of the vials onto a dissecting microscope in order to separate and count the flies by sex and eye color. I counted the F1 generation for one cross, while my partner did the same for the other cross. After counting the flies, we recorded our results into our data table in our lab journal.
We learned how to determine the sex of the flies during our lecture. Typically, females will be larger and have a larger abdomen than males will. Females will also have an ovipositor at the end of their abdomen, while males will have an epandrium at the end of their abdomen (General Genetics Lab…). Also, the abdomens of males will typically be darker at the end than a female. Males generally have sex combs on their front limbs in order to hold on to the female during reproduction.
After counting the F1 generations of each cross, we took 6 males and 6 females from the same cross and combined them into one vial to mate. We did this for both Cross A and B. We then discarded the remaining flies from our F1 generation. After combining the flies, we labeled and turned the vials in to our TA to store and allow them to mate.
Week 4:
During week four, we had to remove flies from the F1 generation from both vials. These flies were discarded into the fly morgue.
Week 5:
During week five, we had to anesthetize the flies with FlyNap once again in order to count the F2 generation. After anesthetizing the flies, we emptied them out onto a dissecting microscope to count them. For this generation, the white phenotype will be present since it is a recessive allele (General Genetics Lab…). Therefore, we had to count the number of red-eyed and white-eyed males and females for each cross. We recorded our results into our data table in our lab journals. We had two different data tables: Individual Data and Class Data. In both tables, we recorded our data then determined the percentages of each type of fly from each cross. In order to determine our class data, we had to take the data from each group in the lab and combine them in order to evaluate the results.
Results:
Individual Data:
Cross A Cross B
F1 male, n F1 male, % F1 female, n F1 female, % F1 male, n F1 male, % F1 female, n F1 female %
Red Eyes, w+ 29 100% 35 100% 0 0% 49 100%
White Eyes, w 0 0% 0 0% 66 100% 0 0%

Total 29 100% 35 100% 66 100% 49 100%

F2 male, n F2 male, % F2 female, n F2 female, % F2 male, n F2 male, % F2 female, n F2 female, %
Red Eyes, w+ 20 57.1% 31 67.4% 24 49% 23 47%
White Eyes, w 15 42.9% 15* 32.6% 25 51% 26 53%
Total 35 100% 46 100% 49 100% 49 100%

Class Data:
Cross A Cross B
F1 male, n F1 male, % F1 female, n F1 female, % F1 male, n F1 male, % F1 female, n F1 female, %
Red Eyes, w+ 426 100% 381 100% 0 0% 402 100%
White Eyes, w
0 0% 0 0% 470 100% 0 0%
Total 426 100% 381 100% 470 100% 402 100%

F2 male, n F2 male, % F2 female, n F2 female, % F2 male, n F2 male, % F2 female, n F2 female, %
Red Eyes, w+ 129 50% 305 91.87% 120 43.48% 185 49.07%
White Eyes, w 129 50% 27* 8.133% 156 56.52% 192 50.93%
Total 258 100% 332 100% 276 100% 377 100%

(Data Tables from page 41 of Lab Manual)
F2 Chi Square Analysis for Class Data:
Cross A Observed(O) Expected(E) O-E (O-E)^2 (O-E)^2/E
Red-eyed females 305 292 13 169 0.58
Red-eyed males 129 147.5 18.5 342.25 2.32
White-eyed males 129 147.5 18.5 342.25 2.32
White-eyed females 27* 1* 27 729 729
df 3*
X^2 734.22
P-value 0<0.001
Reject Fail to Reject
(Chi Square tables from page 42 of Lab Manual)
*- mistakes to be addressed in “Discussion” section.

Punnett Squares:

Discussion:
As mentioned in the introduction, we used a chi square test to determine whether or not to reject the null hypothesis. We expected that we would observe no significant difference between our expected and observed numbers of red and white-eyed flies. However, after performing our chi square tests for both crosses (of our F2 class data), we determined that we had to reject our null hypotheses for Crosses A and B. This came as a surprise to us because based off what we have learned about X-linked inheritance and the results of our Punnett Square, we should not have had to reject the null hypothesis for Cross A. However, we had unexpected observations occur in Cross A and had to make alterations to our chi square test to coincide with these observations. For example, in both individual and class data we observed white-eyed females in our F2 generation, which should not occur. We were uncertain as to how this was a possibility. However, after some discussion, we concluded that a mistake could have occurred after we counted our F1 flies and placed flies back in their respective vials for the F2 cross. It is possible that we may have placed a fly, or a few flies, meant for Cross B in the vial that was meant to be Cross A. This could have resulted in the appearance of the mutant white gene appearing in F2, Cross A. Consequently, this mistake resulted in alterations in our chi square test for Cross A. We were forced to change our expected number of white-eyed females from zero to one, so that we could perform the test. We also had to change our degrees of freedom from two to three because instead of testing for three populations, we then had to test for four. As a result, our p-value was less than 0.05, thus rejecting the null hypothesis.

Works Cited
• Xeniya Rudolf, Lecture PowerPoints (Lecture 7, Lecture 8)
• General Genetics Lab BIOL 2321L Laboratory Manual. Spring 2017. University of Arkansas-Fayetteville. Print.