Mendelian Genetics and the Dihybrid Cross of Drosophila Melanogaster

Genetics is a widely studied topic in science. The purpose of this experiment was to analyze and determine the dihybrid cross between drosophila and the mutations that are common. Because they have a very short reproduction period, Drosophila are commonly used to analyze mutations and dominant/recessive traits. Gregor Mendel introduced Mendelian Genetics, and Mendel’s dihybrid cross is used as reference to compare the mutation combinations for the offspring Drosophila (Holmes, 2006). Based on the class results from crossing the heterozygous parent Drosophila, there was an expected ratio of 45:15:15:5 and an observed ratio of 62:15:1:2 for the Drosophila’s possible phenotypic traits.

The calculated chi-square value equaled 21.28, which is greater than 7.82 which is derived from the degrees of freedom, therefore the null hypothesis that there will be no variation in the phenotypes of the offspring is rejected. The hypothesis stated that if there are two heterozygous Drosophila parent flies crossed, then there will be variation in the phenotypes off the Drosophila offspring.

These results can be used and studied for future research because Drosophila melanogaster are significant for studying genetics and mutations of all living species.

Introduction

Drosophila melanogaster are flies that allow scientists and researchers to study evolution and genetics (O’Grady et al., 2009). Because Drosophila reproduce in a short time period, they are easy and available for scientists to study and examine. They are great for a scientific studies because they can be tested in a laboratory (Anderson & Ehrman, 1969). There are multiple wing mutations for drosophila. The wild type has long, and full wings, the vestigial flies have short wrinkled wings.

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The apterous mutations flies have very short wings. For body color mutations, drosophila wildtype had blue ends with red eyes and the ebony mutated flies are darker with a less distinct blue ends to their body. Gregor Mendel was the first to experiment with pea plants, which was used as a way to examine dominant and recessive traits (Holmes, 2006). Mendel experimented in a garden with pea plants to analyze the generations of the plants and their traits (Sandler, 1986). The monohybrid cross is a cross examination of two parents that codes for one trait, but a dihybrid cross is used to examine to parents and two of traits for each. Thomas Hunt Morgan, at Columbia University, began the study of flies, and it was said that the study was very affordable to conduct at the time (Shine & Wrobel, 1976). Thomas Hunt Morgan was responsible for starting the huge study of genetics and crossing over for Drosophila Melanogaster (Shine & Wrobel, 1976). A dihybrid Punnett Square was used to examine the reproduction of the drosophila offspring, as shown below.

The purpose of this experiment was to examine the mutations that occur in offspring of drosophila melanogaster. Which flies carried the recessive alleles was unknown because these mutations because it is unknown in the phenotype, because it is chromosomal. This topic on drosophila is important to understand genetics and reproduction because scientists can learn information about dihybrid crosses and heterozygotes. Genetics is an important topic in research so that researchers can use the study on drosophila mutations for further investigations. The hypothesis included for this experiment was, based on the F2 generation counts and the ratio of phenotypes of drosophila melanogaster, the F1 generation is a heterozygous. The null hypothesis is that there will be no variations in the phenotypes for the production of the offspring, based on the parent drosophila. To support the hypothesis, the Punnett Square was used to determine the possible outcomes of the offspring and determine the phenotypic ratio for the offspring. The chi-square proved that the null was rejected because there was genetic variation in the offspring of the drosophila. The dihybrid Punnett Square was used to determine possible outcomes for the offspring and the p-value from the chi-square test was used to determine whether the null hypothesis was accepted or rejected.

Materials and Methods

For the drosophila melanogaster experiment culture tube, first, culture vials were gathered, and one spoonful of Formula 4-24 Instant Drosophila Medium was added to one of the culture vials. Next, using a small 10mL beaker, tap water was added to the vial until all of the dry media was soaked. Two or three grains of yeast was added to the vial. This is the food source for the drosophila. Then, the vial was sealed using a jumbo size cotton ball and placed the vial on the lab table for around five minutes. This procedure was repeated for both culture vials. Next, a permanent marker was used to write “Sex-Linked Cross” on one of the vials with the group name for the experiment. With the flies given, they were placed on a white index card and the sex and eye color of each fly was noted while looking through a dissecting microscope. A brush was used to gently sort the flies into groups based on the eye color and sex. With the flies obtained, the jumbo cotton ball was removed, and three female flies and two male flies were placed into the vial and then the vial was closed with the cotton ball. For the unknown cross, the second vial and was used and labelled with permanent marker as “Unknown Cross,” and the group name again. With the flies given, they were placed on a white note card and sorted, based on the sex of each fly, and the eye color, the body color, and the wing shape as well were recorded using the dissecting microscope. The jumbo cotton ball was removed from the vial and three female flies and two male flies were placed in the vial, then sealed with the cotton ball. The vials were then placed on the lab table for two weeks, and then after two weeks, the two vials were collected, and fly-nap was carefully placed into both vials using a cotton swab, so the flies stop moving, then the vials were placed horizontally on the lab table. After the flies have stopped moving, the cotton ball was carefully moved from the “Sex-Linked Cross” vial and the flies were placed on a white index card. The flies were placed under the dissecting microscope and counted, while taking count of the eye color and sex of each. When finished counting, all of the drosophila was placed in the kill jar and then the vials were completely washed out and cleaned in the sink. The same steps for the “Unknown Cross” vial was repeated, and the results were taken note of. Then, the class notes were collected as well. A chi-square test was used to compare the expected results to the actual results of the phenotypes of the drosophila. With the value obtained, it was used to find if the null hypothesis was accepted or rejected.

References

1. Anderson WW & Ehrman L. 1969. The American Midland Naturalist. Mating Choice in Crosses between Geographic Populations of Drosophila pseudoobscura 81(1): 47-53.
2. O’Grady PM & Magnacca KN & Lapoint RT. 2009. Encyclopedia of Islands: Drosophila. University of California Press; 1077p.
3. Sandler I & Sandler L. 1986. On the Origin of Mendelian Genetics. American Zoologist 26(3): 753-768.
4. Shine I & Wrobel S. 1976. Thomas Hunt Morgan: Pioneer of Genetics. University Press of Kentucky; 160p.