Lab Report: Drosophila Melanogaster Eye Color Inheritance

Introduction

The Drosophila Melanogaster, commonly known as the fruit fly, undergoes a life cycle consisting of five stages: embryo, larva, prepupa, pupa, and adult. This organism's short life span of approximately ten days makes it an ideal model organism for studying various biological phenomena. Its controlled generation time and ease of genetic manipulation further enhance its suitability for experimentation (Pierce 2012). In this experiment, we utilized both wild-type brick-red-eyed Drosophila Melanogaster and mutant white-eyed Drosophila Melanogaster to investigate the inheritance of eye color phenotypes.

The primary objective of this experiment was to analyze the mode of inheritance of the eye color phenotype, specifically whether the brick-red or white eye color phenotype is autosomal recessive, autosomal dominant, X-linked recessive, or X-linked dominant. To achieve this, we mated purebred brick-red-eyed male Drosophila Melanogaster with purebred white-eyed female Drosophila Melanogaster to produce the F1 generation. Genetic crosses were employed to assess the inheritance pattern of these eye color traits. The null hypothesis posited that brick-red eye color is X-linked dominant compared to white eye color, while the alternative hypothesis proposed that brick-red eye color is X-linked recessive compared to white eye color.

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Mutations in genetics can be either dominant or recessive, and this experiment aimed to determine the mode of inheritance.

Methods

The experiment commenced with the parent generation, or P-generation, consisting of five purebred white-eyed female Drosophila Melanogaster and five purebred brick-red-eyed male Drosophila Melanogaster. These flies were immobilized using a carbon dioxide pad and subsequently scored under a microscope to determine their sex and eye color phenotype.

Female Drosophila Melanogaster were distinguished by their round tails and vaginal plates, while males exhibited narrower tails, genital arches, and solid black tails instead of striped tails seen in females. After scoring, the P-generation flies were cleared and disposed of in a morgue (soap and water) to prevent cross-breeding with subsequent generations.

Once the F1 generation offspring reached adulthood, they were also scored based on their sex and eye color phenotype using the same criteria. To create the F2 generation, the F1 flies were separated into four different vials for mating. The first and second vials contained six males and six females each, the third vial contained seven males and seven females, and the fourth vial had six males and seven females. After mating to create the F2 generation, the F1 flies were cleared in the morgue.

Upon reaching adulthood, the F2 generation flies were scored once again to determine their sex and eye color phenotype. The F2 generation consisted of 115 male white-eyed fruit flies, 127 male brick-red-eyed fruit flies, 126 female white-eyed fruit flies, and 132 female brick-red-eyed fruit flies. This data from all three generations was collected to determine the mode of inheritance and to perform a chi-square analysis.

Results

The initial P-generation consisted of 10 Drosophila Melanogaster, with 5 females displaying the white-eyed phenotype and 5 males with brick-red eyes. The F1 generation resulted in 25 males inheriting the white-eyed phenotype and 26 females inheriting the brick-red-eyed phenotype. The F2 generation included 115 male white-eyed fruit flies, 127 male brick-red-eyed fruit flies, 126 female white-eyed fruit flies, and 132 female brick-red-eyed fruit flies.

Possible Modes of Inheritance Based on F1 Phenotypes

The table above presents possible modes of inheritance based on the F1 generation phenotypes. According to the data, the most likely mode of inheritance is X-linked recessive for the brick-red eye color. All other modes of inheritance do not align with the observed data. The null hypothesis initially proposed that brick-red eye color was X-linked dominant; however, the data suggests otherwise.

Chi-Square Test

The table above presents the chi-square test, which was calculated using the observed (O) and expected (E) values for each category based on sex and eye color in the F2 generation. The chi-square value obtained was χ² = 0.736, corresponding to a p-value of 0.90 (> p-value > 0.80) with a degree of freedom of 3. This non-significant p-value indicates that we failed to reject the null hypothesis.

Conclusions

Initially, we hypothesized that brick-red eye color in Drosophila Melanogaster was X-linked dominant. However, based on the data collected and the results of the chi-square analysis, we conclude that brick-red eye color is actually X-linked recessive. The non-significant p-value obtained from the chi-square test (p-value = 0.90) supported this conclusion, leading us to fail to reject the null hypothesis.

Possible errors that may have affected our results include human errors in scoring Drosophila Melanogaster and difficulties in counting flies that flew away or remained stuck in vials. Future experiments may involve polymerase chain reaction (PCR) analysis to further investigate the inheritance of the white-eyed phenotype in Drosophila Melanogaster.