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Monohybrid and Dihybrid Cross of Drosophila Melanogaster Lab Report

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The process of mono- and di- hybrid crosses is a main example of how each species can express their phenotypic characteristics. For these evaluations to be accurately tested for, the model organism that is used is Drosophila melanogaster. This model organism has a quick lifecycle and can only have offspring through mating purposes between a male and female fruit fly. Drosophila melanogaster is also resourceful due to its accuracy of having dominant and recessive phenotypic traits being represented such as humans can as well. The results of this experiment will allow better understanding of how offspring can phenotypically look and how the parent’s genes were able to either stay dominant or create recessive traits through generational lifecycles.

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Within this experiment, the use of Drosophila melanogaster will be the main use of a model organism in order to identify various phenotypic relations through the stages of having a parental control group and having generational offspring after that. The use of Mendelian Law of Random Segregation, each individual organism possesses two alleles encoding a trait that separates once gametes are formed (Pierce, 2016, p. 53), and the use of Mendel’s Law of Independent Assortment, different gene pairs segregate independently during meiosis as long as they are not genetically linked (Lab Manual), will aide in the process of how each Drosophila melanogaster looks phenotypically. Mendel also created monohybrid crosses that aided in the results of having heterozygote parents creating a genotype ratio of 1:2:1 and a phenotype of 3:1. The genotype ratio was constructed as one homozygous dominant (BB), two heterozygous (Bb), and one homozygous recessive (bb). The phenotype ratio was created by three dominant traits (BB, Bb, Bb) and one recessive trait (bb). Dihybrid crosses were created to represent genes that are not linked and created phenotypic ratios of 9:3:3:1. Nine of the phenotypes had the dominant trait (A_B_), three only had a dominant trait of (A_bb), three only had a dominant trait of (aaB_), one had recessive traits (aabb).

While calculating the results, the use of the chi-square will be very helpful of determining whether the expected null hypothesis should be accepted or rejected by the result of . The Chi-Square test determines by using the formula of . O is the observed number from the original control group, E is the expected number, which is then all added together to get all of the possible values. The expected value is zero because it will hold true to the ratios of 3:1 or 9:3:3:1.

In this experiment Drosophila melanogaster will be tested for its phenotypic ratios as each generational offspring are viewed under a microscope each time that they are born. The phenotypic ratios could help aide on which trait is being expressed such as if they are heterozygote alleles or recessive alleles. Once the phenotypes have been viewed, the hypothetical conclusion should be a 3:1 ratio since there is mating happening between a controlled group and one wild type.

Instruments

1.) One shoebox

2.) Combination lock

3.) Lab notebook

4.) 4 vials

5.) 4 plugs

6.) 1 Fly-nap

7.) 4 Nets

8.) 1 Anesthetizing wand

9.) 1 plastic cup

10.) 1 paintbrush

11.) 1 index cards

12.) 4 vial labels

13.) Record sheets

14.) 2 pebbles of food for each vial

15.) Yeast

16.) 1% Propionic Acid

17.) Microscope

Methods

1.) When in possession of the vial with Drosophila melanogaster and a wild type, start the process of anesthetizing the flies.

2.) First grab the anesthetizing wand and place 2-4 drops of the fly-nap onto the piece of the cotton that is facing the interior of the anesthetizing wand. Make sure to create a small hole for the anesthetizing smell to transfer over into the vial. Then uncover the vial with the flies slowly and place the anesthetizing wand in its position.

3.) Hold the vial at about a 45˚ angle so that once the flies fall asleep it is easier to take them out. The anesthesia may take a while, just be patient.

4.) Once the flies fall asleep, use the small paint brush to take the flies out and place them on the notecard. Now use the microscope and take notes of each phenotypic characteristic such as the color of the eyes, the shape of the wings, and separate them by their sex.

5.) As you finish viewing the flies, prepare a new vial that will be labeled as F1. Place some yeast at the bottom of the vial. Then add 1% Propionic Acid until the yeast turns blue. Place to pebbles of food, make sure the food does not sink into the yeast or at the bottom of the vial. Now, place a net in a shape of a “U” into the vial so that it is easier to make future transfers when necessary.

6.) Now place an even number of males and females into the newly assembled vial. It is ok if there are more females than males because it will produce higher numbers of offspring such as a ratio of 9:16, male: female.

7.) Once the transfer has been made, cover the top of the vial with a stopper. Due to cold room conditions the lifecycle of Drosophila will be a lot slower. Check the flies about once every 3-4 days.

8.) Once the larva appears to be big and dark, take out the F1 by placing them in a zip-lock bag or release them outside of the lab.

9.) Start phenotyping each fruit fly that becomes hatched and move it/them into a newly prepared vial labeled as F2. When transferring the fruit fly make sure to use steps 2-4 to obtain the best results as possible and not lose any fruit fly.

In Table 1.1 and Table 1.2, the wild type was phenotypically labeled as red eyes and rounded wings. As well as the mutant, vestigial, was phenotypically classified as white eyes and vestigial wings. The F1 generation did not contain any visual recessive genes, but that did not exclude them from being heterozygous genes such as either dominant or recessive traits. The F2 generation contained nearly half of mutant fruit flies with a total of 5. Those 5 fruit flies had contained vestigial wings, which is a recessive trait to those that have rounded wings. The recessive trait was created during the mating of the F2 generation in which could suggest that the recessive trait could be skipping one generation because there was none in the F1 generation. As shown in Table 1.3 the chi-square test can confirm that the ratio of 3:1 is valid since is close to zero.

In conclusion, the Drosophila melanogaster lab report showd that it lasted for about 6 full continuous weeks with the F2 generation hatching at about week 4 due to the cold environment that they were in. The Drosophila melanogaster did prove my hypothesis to be correct, which was keeping a phenotypic ratio of 3:1 since the phenotypic changes would be rather small for 2 generations. The official approval mathematically had come from the Chi-square test which was = which is close to zero.

References

  1. Pierce, B. A., & Barbujani, G. (2017). Genetics: a conceptual approach. New York, NY: Freeman.
  2. Padilla, P., & Alberts, A. (2019) Genetics Laboratory Manual Fall 2019.
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