- Males are usually smaller than females.
- Males have dark, blunt abdomens, and females have lighter, pointed abdomens.
- Only males have sex combs, which are groups of black bristles on the upper most joint of the forelegs.
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Obtain a vial containing a mutant white-eyed male fly and a wild type red-eyed female. Record the cross number of the vial. This number will serve as a record as to which cross you obtained. These flies are the parental generation (P1) and have already mated. The female should have already laid eggs on the surface of the culture medium. The eggs represent the first filial, F1 generation and will be emerging from their pupal cases in about a week.
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First week: Immobilize and remove the adult flies. Observe them carefully under the dissecting microscope. Separate the males from the females and look for the mutation(s) using a sorting brush. Note whether the mutation(s) is/are associated with the males or females. Identify the mutation(s) and give them names: (W) for white-eyed, (+) for wild type red-eyed. Record the phenotype and symbol in the Data Table.
- Set aside the beaker containing the parents for a later cross. Label the vial containing the offspring (eggs or larvae) with the symbols for the mating. Also label the vial with your name and date. Place the vial in the incubator.
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Second week: Begin by observing the F1 flies. Immobilize (with ether) and examine all the flies. Record their sex and characteristics under a microscope. Consider the conclusions that can be drawn from these data. Place some remaining F1 flies in a vial for a later cross. Label the vial with the symbols , name, and date.
- Cross two flies from the F1 generation with each other following the previous steps, examine under a microscope, and record their sex and the presence or absence of mutation(s).
- Cross flies from the F1 generation with the original male fly with white eyes following the previous steps, examine under a microscope, and record their sex and the presence or absence of mutation(s).
- Calculate the proportions of each phenotype.
Control of Variables:
I will control the independent Variable, phenotype and genotype for the eye color of Drosophila parents (wild type red vs. white) in order to see its effect on the dependent variable, phenotype and genotype for eye color of Drosophila offspring, by maintaining several constants. I will ensure that eye color is the only trait being manipulated through careful analysis of each parent fly under a microscope prior to mating. I will consequently discard any fly with a mutation other than eye color such as, body type, bristles, antennae, eye shape, wing size, wing shape, wing vein, and wing angle. I will keep all growing larvae/eggs in the incubator for two weeks in order to ensure equal exposure to heat upon each mating. Additionally, I will maintain the same heat for all incubations in order to prevent fluctuation in the rate or quality of growth.
Method for Sufficient and Relevant Data Collection:
In order to collect at least ten sufficient forms of raw data, I will perform 4 crosses: Cross #1 yielding 2 values, Cross #2 yielding 3 values, Cross #3 yielding 4 values, and the Control Cross yielding 2 values for a grand total of 11 entries of raw data. The data for each cross will be divided by males and females in order to discern proportions indicative of a sex-linked trait (gene located on the X chromosome). Qualitatively, I will draw a diagram of each offspring in order to display the phenotypes of each cross, either wild type red-eyed or white-eyed. Qualitatively, I will count the number of flies in each cross as a total in addition to the total of each phenotype from that cross. I will then use the raw data to calculate proportions/ratios of phenotypes which will be converted into percentages for easier interpretation.
Protocol Diagram:
Layout of the Lab
Data Collection and Processing:
Quantitative:
Uncertainty: In regards to the lab guideline, “for 1 measurement (ex. A thermometer), the uncertainty would be +/- 0.5 of the smallest gradation. (ex. +/- 0.5 degrees Celsius),” the uncertainty of this lab would be +/- 0.5 flies since the data being collected is the number of flies. However, this number is irrelevant and does not accurately reflect the data as there are no “half-flies” implicated in this lab and such mutations are seen rarely in the natural/biological world. Additionally, the means of measurement being utilized to measure the number of flies is unquantifiable in this case, due to the fact that it is based on perception and the fundamentals of counting, and thus, error would arise from personal misinterpretation of what is in front of you.
Number of Flies (Drosophila)
Number of Flies (Drosophila)
Number of Flies (Drosophila)
Control:
Number of Flies (Drosophila)
Qualitative:
Control:
Data Processing (Calculations):
Overview:
Sample Calculation:
Cross #1:
Phenotype: Male Wild Type Red Eye
Phenotype: Female Wild Type Red Eye
Presentation of Processed Data:
I will arrange the processed data into corresponding pie graphs in order to display the phenotypic ratios and to enable a clear view of each phenotype divided into each gender so that the evidence of sex-linked (X chromosome) inheritance is presented.
Conclusion and Evaluation
Based on my raw data and calculations, my hypothesis was correct in that the phenotype of eye color in Drosophila is in fact a sex linked trait located on the X chromosome. First I crossed the mutant male fly with a normal female with red eyes, to observe whether the white or red eyes were dominant. The F1 generation all had red eyes, which indicates that red eyes were dominant over white. I continued crossing two flies from the F1 generation with each other. Out of 1182 flies in his F2 generation, 285 had white eyes but surprisingly all the flies with white eyes were also male. I then crossed flies from the F1 generation with the original male fly with white eyes. This cross resulted in white-eyed and red-eyed males and females, making a 1:1:1:1 ratio. This ratio essentially illustrates that, in Drosophila, the number of copies of the X chromosome determines the sex. In essence, an individual that has two X chromosomes is female and an individual with one X chromosome, which then joins with the Y chromosome, is male. During fertilization, if the egg joins with an X sperm, the zygote is XX, which becomes female. If the Y sperm is involved in fertilization, there is a XY zygote, which develops into a male. Thus, the reasoning for my results is due to the fact that the gene for while eyes in Drosophila is located on the X chromosome and not the Y chromosome. Genes on the X chromosome that determine a trait are called sex linked. After I understood the white-eye trait is recessive to the red-eye trait, I easily noticed that my results follow Mendel's assortment of chromosomes.
The procedure was near flawless in terms of collecting sufficient data. The progression of each cross lead up to the final cross between the first generation female and the parental generation male which yielded the 1:1:1:1 phenotypic ratio explicative of a sex-linked trait in comparison to the previous cross. However, in terms of relevant data, the control group served no purpose and did not support nor negate my hypothesis. Overall, the limitation of the lab in a real-life setting rather than a computer would be the inconvenience of breeding all of the flies in such a short amount of time.
Possible improvements in procedure would be to remove the control group completely because all variables are sufficiently controlled and the data derived from the control group itself is irrelevant and extraneous. Furthermore, the counting of the flies could be executed in a more quantifiable manner in which exact measurements by device are implicated, perhaps by weighing total mass of the flies and dividing by the total number to attain an average weight.