Drosophila Melanogaster: A Genetic Analysis

Discussion The goal of the experiment was to determine what gene or genes are responsible for the white eye phenotype of two strains of Drosophila melanogaster. According the literature, the w+ gene is located on the X chromosome at 1.5 genetic map units (1). In addition, the st+ and bw+ genes are autosomal and assort independently, meaning they are two different genes located on two different chromosomes. It is stated that the bw+ gene is located on the 2R chromosome at 104.5 map units (2) and the st+ gene is located on the 3L chromosome at 44.0 map units (3). Our hypotheses suggested that the st+ and bw+ genes were mutated in the strain A flies. Thus, the lack of scarlet and brown pigment caused the eyes of these flies to appear white
The purpose of these crosses was to determine how many genes were involved in the expression of the white eye phenotype. The F2 progeny that resulted from the P1 F1 x F1 cross, described in Table 2, consisted of four different eye color phenotypes: wild-type, white, scarlet, and brown. The appearance of two new phenotypes, scarlet and brown eyes, supports the hypothesis that more than one gene is involved and the additional genes assort independently. Since both scarlet and brown-eyed flies are observed, the st+ and bw+ genes are not linked to the same chromosome. Since four different phenotypes were observed, this supports our hypothesis that two different autosomal genes are involved, which are the st+ and bw+ genes. Thus, in total there are three different genes involved for the white eye phenotype observed in both strains of
Branch diagrams, shown in Figure 2, describe the overall phenotypic ratios for the four phenotypes. The ratio was multiplied by the total number of flies observed, which was 648, to determine the expected values. Table 2 shows a minor deviation of 4 between the observed and expected values for white eyes. It also shows a minor deviation of 5 between the observed and expected values for scarlet eyes. However, the amount of wild-type flies observed exceeded the expected amount by 48 flies, and the amount of brown eyed flies observed was extremely lower than the expected amount by 57 flies. A chi-square analysis was used to determine the goodness of fit between the observed and expected values. The hypothesis was that the observed and expected values were equal, signifying that the w+/w gene is x-linked and the st+ and bw+ are autosomal and assort independently. The alternative hypothesis was that the observed and expected values were not equal. The calculated chi-square value was greater than the critical chi-square value shown in Table 2. Thus, the p-value was less than 0.0001 meaning that there is a very low probability of the observed results matching the expected results, rejecting our hypothesis. Reasons for this result may be due to human error in classifying eye color. Perhaps the brightness of the microscope altered the