Do Sepia and Dumpy Segregate Independently

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The lifecycle of the fruit fly, Drosophila melanogaster, begins with the egg. The first instar larvae emerge from the egg after approximately one day and molt two times for a total of three larval instars. At the end of the third larval instar, the larvae pupariate and undergo metamorphosis as pupae. About 10-14 days after the eggs are laid by the females, the adult flies emerge from their pupal cases.In this lab, we performed a dihybrid cross for dumpy wing/normal eye color and normal wing/sepia eye color. In a dihybrid cross, two different mutants are crossed to each other and a sibling cross is performed with the progeny from the F1 generation. The dihybrid cross is performed to determine if two genes assort independently or if they are genetically linked. If the two genes assort independently, the expected phenotypic ratio is 9:3:3:1. If the two genes are on the same chromosome and linked, there will be fewer recombinants that have a phenotype different from either parent in the P generation. Since dumpy and sepia are both recessive traits, the F1 generation will be phenotypically wild-type.The dependent variable is the number of each class of flies in the F2 generation (wild-type, dumpy, sepia, and dumpy/sepia). The controlled variables are the genotypes of the P1 and F1 generations. The P1 generation will be either red-eyed, dumpy or sepia, normal wing. The F1 generation will all be heterozygous for both mutations. This is verified by making sure that the F1 flies used for the cross all have the wild-type phenotype for both wings and eyes.Study PurposeThe purpose of this study was to determine if dumpy and sepia assort independently or if they are genetically linked.HypothesisThe hypothesis is that dumpy and sepia assort independently and the phenotypic ratio of the F2 progeny is expected to be 9:3:3:1 (wild-type: dumpy: sepia: dumpy/sepia).ExperimentFlies were sorted by genital morphology and the presence/absence of sex combs under a dissecting microscope following anesthetization with FlyNap. Flies were cultured and allowed to develop in vials with water added to dry media and supplemental yeast. Progeny from each cross were allowed to develop in the vials and emerging adults were collected. The P and F1 generations were sorted and 10 males and 10 females were placed in new culture vials with food to set up both crosses. The F2 progeny were counted by phenotype and the data was recorded. The phenotypes were scored as either dumpy or normal wings and red or sepia eyes. Χ2 statistical analysis was performed on the data to assess statistical significance. The crossing scheme is shown below:P: dumpy wing, red-eye x normal wing, sepia eyeF1: dumpy wing/wild-type, sepia/wild-type (sibling cross)F2: normal wing, red-eyed dumpy, red-eyed normal wing, sepia dumpy, sepia Discussion and Conclusion From the Χ2 table (Table 2) for p to be less than 0.05, Χ2 must be lt. 0.352. For these data, Χ2 is greater than this value (1.15), which is not consistent with the hypothesis. Based on the data, the Χ2 statistical analysis does not support the hypothesis that sepia and dumpy segregate independently. This means that either the hypothesis is incorrect or there was not enough statistical power with the sample size to confirm the hypothesis. If it is because the hypothesis is incorrect, then these two loci may be on the same chromosome and may be genetically linked. However, it is also possible that the variation was too great to support the hypothesis without a larger sample size. Other sources of error could also be caused by recombinant flies (dumpy wings and sepia eyes) being less viable than single mutants and wild type. If this is the case, fewer double mutant adults would be scored. This seems like a reasonable possibility since most of the deviation from the expected result comes from the double mutant flies. However, this would also be the expected result if these two loci are genetically linked.