An Analysis of Mendelian Genetics Regarding Drosophila Melanogaste

An Analysis of Mendelian Genetics Regarding Drosophila melanogaster 11/12/2019 Abstract The purpose of this lab was to determine the ratio at which the traits of the F2 generation of the unknown cross will occur. This was then used to retrospectively determine the genotype of the F1 generation. It was hypothesized that the F2 generation would hold to the ratio of 9:3:3:1. This was then tested by breeding the fruit flies for 3 weeks. After this time period, data was collected and a Chi Squared test was run to test the deviance from this expected ratio. A Chi Squared value was determined to be .881 which is significant at the .05 level of significance. Due to this it was able to be determined that the genotypes of the F1 generation was AaBb x AaBb. Introduction Drosophila melanogaster, commonly known as the fruit fly, is a species of fly belonging to the Drosophilidae family. These flies breed rapidly in cycles of approximately 10 days and are extremely undemanding in terms of laboratory maintenance. Because of these characteristics, these flies have become one of the most widely used organisms studied by modern geneticists (Sandhyarani, 2017). There are also multiple common mutations in this species. Eye color of the fruit flies can vary from a red color to a white normally, however, occasionally a mutation will occur, and the fruit fly could have a black eye color instead. Additionally, a wing mutation may also be present in cases which the flies lack wings all together (Miller, 2014). Mendelian genetics are based on two governing principles or Mendel’s Laws of Inheritance: Law of Segregation and Law of Independent Assortment. His Law of Segregation states that an offspring randomly receives an allele from each parent, resulting in a pair of alleles. Additionally, his Law of Independent Assortment states that different alleles that are passed from parent to offspring are completely independent of each other. Meaning that the inheritance of certain traits in one region does not affect or alter the inheritance of genes in another location (Genetics Generation, 2015). The purpose of this experiment was to determine the expected ratio of traits of the F2 generation of fruit flies that were bred and to determine the genotypes of the F1 generation. It was hypothesized that the F2 generation would exhibit traits in a 9:3:3:1 ratio due to the expectation that the F1 generation would have the genotype of AaBb x AaBb. The alternative hypothesis was that the F1 genotype would have a differing genotype, exhibiting a ratio varying from 9:3:3:1. In order to come to this prediction, a dihybrid cross Punnett Square was used to determine the genotype of the offspring, shown below. AABB AABB AaBB AaBB AABb AABb AaBb AaBb AABB AABB AaBB AaBB AABb AABb AaBb AaBb Procedure This experiment started with the breeding of the parent generation. In order to complete this, 5 fruit flies were placed in a small vial, containing food and a cotton ball. These flies (the F1 generation) were then left for 3 weeks so that the F2 generations could be bred. This amount of time allowed for the flies to complete 2 cycles of the breeding process. Once this time period was up, data was collected from the entire generation. In order to collect data on the fruit flies, they first had to undergo a process of of being anesthetized. Once the flies were no longer moving, data was collected. Flies were sorted into groups based upon eye color and wing mutation. The flies were placed on a blank index card and carefully studied under a microscope individually. Then the data from each group was collected and compiled into a large class data pool. Once all groups’ data was collected in Microsoft Excel, a Chi Squared test was run and a p-value was calculated to determine the significance of the data’s variation. A p-value of .05 was used to test significance. Results Overall, the class collected data on a total of 307 fruit flies, finding that 172 flies did not exhibit mutations, 57 flies demonstrated the sepia eye mutation but no wing mutation, 62 demonstrated no eye color mutation but had no wings, and 16 flies had both the eye and wing mutation present. This is exhibited in Table 1 below. Unknown Cross of Drosophila Group WT/WT MUT1/WT WT/MUT2 MUT1/MUT2 1 8 3 13 2 2 3 0 2 0 3 38 6 15 2 4 36 9 10 6 5 7 6 2 0 6 19 5 3 1 7 23 19 4 1 8 38 9 62 16 Total 172 57 62 16 Table 1: The summary of the classes observations of the Unknown Cross F2 generation The results of the Chi Squared test were run also. Because a 9:3:3:1 ratio was predicted for the F2 generation of the unknown cross, it was expected that there would be 172.68 flies with no mutations, 57.56 flies would contain only the eye mutation, 57.56 would contain only the wing mutation, and that 19.18 flies would contain both genetic mutations. This is exhibited in Table 2 below. Unknown Cross Overall Data WT/WT MUT1/WT O 172 57 E 172.6875 57.5625 O-E -0.6875 -0.5625 0.47265625 0.31640625 O-E2 O- WT/MUT2 62 57.5625 4.4375 19.69140625 MUT1/MUT2 16 19.1875 -3.1875 10.16015625 Total 307 307 N/A N/A 0.003737061 0.005496743 0.342087405 0.529519544 0.880840753 E2/E Table 2: The results of the Chi Squared Test run on the class data set. Resulting in a Chi Squared value of .88. Discussion The results gathered showed to be statistically significant. This data was tested at the .05 level using 3 degrees of freedom, meaning that a Chi Squared value underneath 7.82 would be considered significant. In this case, the Chi Squared value was .881, deeming this data statistically significant. Because of this, we fail to reject the null hypothesis, and accept the notion that the F2 generation of the unknown cross did not significantly deviate from a 9:3:3:1 ratio. This supports the notion that the F1 generation followed a genotype of AaBb x AaBb, which is demonstrated in the Punnett square in the introduction section. This biologically makes sense as the F2 generation was able to demonstrate mutations while also containing small variance compared to earlier generations. Additionally, an alternative of an AABB x aabb cross could’ve occurred too and the same genotypic results would have occurred. There were some drawbacks in this study, such as the fact that some flies were killed or altered in the extraction process, meaning that their data was not able to be obtained. Additionally, the anesthesia did not work on all flies, and some were able to fly out of the tube or move under the microscope, making data collection more difficult. In order to resolve this, a stronger anesthesia could be used and a more meticulous extraction of flies from the tube could take place. However, the flies’ fast breeding process and ease to handle proved the notions mentioned earlier about their ability to be studied so easily in a lab.