Drosophila melanogaster is a common fruit fly that has been useful for most experiments in the study of Genetics. The male and the female fruit fly are similar and different in regards to how they look, structurally. They are similar because both genders have a head, thorax, proboscis, antennae, eyes, and mouth parts. However, males are smaller than females and have about five abdominal segments as opposed to the seven that the female has. The life cycle of these fruit flies consist of egg, larvae, metamorphosis, and then adult stages.
During the egg stage of their life cycle, the eggs are sunk into the food until they become larvae that spend all of their time eating. After, the larvae go through two molting phases called instars, they shed their outer coat and grow a bigger size. When they molt the second time, the larvae make cocoons where the stage of metamorphosis takes place. After this stage, an adult emerges from the cocoon and only after a certain amount of hours will the wings grow to a full size. The males also have sex combs at the first pair of appendages. Their sexual structures are also different: In males, the last abdominal is larger than the female. These sex combs helped us identify whether the fruit fly was a male or female.
In figure 1, you can see wild type male and female fruit flies and how they differ: Figure 1: Male fruit flies on the left in comparison to female fruit flies on the right. Wild type fruit flies are normal fruit flies where they are usually brown or yellow with red eyes and black rings across the abdomen. These flies were observed in the lab by anaesthetizing them with triethylamine so they won’t fly away. They were then inserted in a vial with a foam stopper so then it was easier to observe them. A dissecting microscope was used to identify the color of their bodies, wing shape, sex characteristics, and any other body characteristics. We used female fruit flies who were virgins six hours after they came out of their cocoons and mated them with male fruit flies. This is how crosses were made while there were a few Genetic applications.
During this experiment, autosomal chromosomes were observed in the fruit flies which are non-sex chromosomes. This was observed in the vestigial wings in the fruit flies used. When we observed white eves in these fruit flies, we were introduced to sex linkage. Wild type flies were seen to have red eyes while mutant flies had white eyes. Sex linkage is a phenotypic expression of an allele related to the sex chromosome of an individual. If this allele was dominant, then the fly would only need one copy of this allele to show the phenotype. Dominance means that the allele of one gene in effect on the phenotype masks the second allele in the same locus. If this allele was recessive, the fly would need to have two copies of the allele to show the phenotype. Recessive would mean that even though the individual fruit fly is receiving the allele, it may be masked by the more dominant one. Incomplete dominance in these fruit flies show a blend of phenotypes between two homozygotes but the alleles do not blend. Therefore, this was observed by eye color.
In order to determine patterns of inheritance in Genetics, there are three basic principles: each trait is determined by genes passed down generations, each individual inherits one gene from each parent, and a trait might not show up in an individual but it can still be passed onto the next generation. In order to determine patterns of inheritance for the mutant white strain in fruit flies, the ratios of phenotypes in the offspring is observed. In a sex linked cross of Drosophila Melanogaster, a phenotypic ratio of 1:1 will be obtained. Hypothesis: The F1 generation will have 50% of the total progeny will have males that have the dominant gene and 50% will consist of females with the recessive gene.
The F2 generation will consist of 25% male dominant, 25% male recessive, 25% female dominant, and 25% female recessive. Experimental Design P1 generation: Genotype – Male: XeXe – Female: XEY Phenotype – Male: White eyes – Female: Red eyes F1 generation: Genotype – Male: XEXe – Female: XeY Phenotype – Male: Red eyes – Female: White eyes Constant conditions of this experiment were that in order to collect virgin female fruit flies, it had to be within the time frame of six hours after removing the adult flies from the vial. Another condition was that we had to make sure that the females were female and the males were male by identifying their sexual structures such as sex combs in the males. In order for this experiment to go further, the fruit flies had to be anesthetized and they had to be put in a vial where there was soft food available to them. Procedure Throughout these weeks, this experiment was conducted by using culturing and transferring methods from vials to more vials.
The flies were anesthetized in order for us to observe them. This experiment was done so that every week we were able to observe the fruit flies. Their optimal temperature was constant throughout every lab period: 25-30 degrees Celsius. Before we started to record our results, we had to make sure that virgin female fruit flies were used so as to not have an outside influence in the new generation of flies. Each week we repeated the steps of anesthetizing, transferring, and culturing the fruit flies.
The crosses were made up by observing the traits that show from the fruit fly’s phenotype. This is seen in traits passed from parent to offspring or adult to progeny. Results Phenotype females males Red eyes 50% 0% White eyes 0%50% P1 Generation Phenotype females males Red eyes 0%50% White eyes 50% 0% F2 Generation Phenotype Male Female Red eyes 25% 25% White eyes 25% 25% F1 Generation Note: White eyes: mutant; Red eyes: wild type Conclusion These crosses were sex-linked because there was a one to one ratio of the phenotypes involved.
The F1 generation had a one to one ratio of red eyed female fruit flies to white eyed male fruit flies and the same goes for the F2 generation but reversed. Therefore, there was a one to one ratio of red eyed males to white eyed males as well as red eyed females and white eyed females. This is where you observe that the allele for red eyes are dominant over the alleles for white eyes. There is no incomplete dominance because the eye color was not in between dominant and recessive phenotypes. The trait is sexlinked because the trait for eye color in our experiment with these fruit flies is on the X gene in males. Therefore, the sex linked chromosomes are unrelated to sex.
According to Carter, “Because a male typically only has one X chromosome, he would normally be either XY and have normal, red eyes, or X’Y and have white eyes. The only way a female with two X chromosomes could have white eyes is if she would get an X’ allele from both parents making her X’X’ genotype” (2014). Below are the P1 and the F1 punnet squares for their crosses. Parental cross Y Xe Xe YXe Xe Xe Xe YXe Xe Xe F1 cross YXe XE XE YXE XE Xe YXE XEXE XE XE XE Xe YXe XE Xe Xe YXe XeXe YXe X EXe Xe YXe XeXe YXe XEXe The next chart compares the actual ratio from the crosses to the ratios derived from these punnet squares. Expected Genotypic Ratio Expected Phenotypic ratio P1 1:11:1 F1 1:11:1 Recommendations for future study, in my opinion, include studying as many flies as possible in one setting. I come to this conclusion because I would like to see if a female could get white eyes from both the mother and father.
In other words, to see if something like this is possible would be helpful in this field of Genetics. Questions: 1. Why was it necessary for the females of the parental generation to be virgins? The females store sperm in their receptacles and if they were not virgins we would not be able to tell who the fathers were. 2. Why was it not necessary to isolate virgin females for the F1 cross? The females store sperm in their receptacles and if they were not virgins we would not be able to tell who the fathers were 3. Why the adult flies were removed from the vials at weeks 2 and 4? So, they are not being able to mate with the next generation. So from the results of the experiment I can conclude that the results were close to a 1:1 ratio.
