The apterous and wild type cross examined the wing type of the Drosophila studied. This specific cross of genes had an autosomal mode of inheritance. This mode of inheritance was determined as in both the F1 and F2 generation the ratio of female to male flies was balanced as opposed to one gender of fly having a significantly larger population thus signifying the sex-linked mode of inheritance. Additionally, as aforementioned the fact that no change would occur to the outcome depending on which gender fly contained which trait signifies an autosomal mode of inheritance. The gene for wild type wings is a dominant trait while the gene for recessive is recessive (The Museum of Science, Art and Human Perception at the Pier, 2017). Due to the fact that the gene in examination simply inspected solely the wing type of the flies, the cross was a monohybrid cross.
Meiosis, or the reproduction of sex cells, is essential to understand when examining how the F2 generation ended up with specific traits such as the apterous and wild type wing traits examined in this lab. Meiosis as a process includes two separate cell divisions resulting in four genetically different cells for sperm and one egg with three polar bodies. The most important part of Meiosis to consider when determining how the F2 generation ended up with specific traits is Meiosis I and specifically Anaphase I. During Prophase I homologous chromosomes line up during synapsis. These tetrads then cross over thus increasing genetic diversity. Metaphase I lines up these tetrads along the cell equator with pole determined yet otherwise random orientation. The next step and the most significant in explaining how the F2 generation acquired specific traits is Anaphase I and the process of Independent Assortment. In Meiosis I Independent Assortment is defined as the random separation of homologous chromosomes (Reece and Campbell, 2011). This process of Independent Assortment is also known as the Law of Independent Assortment a law created by Gregor Mendel in his Pea Plant experiment and can be defined as “the alleles of two (or more) different genes get sorted into gametes independently of one another.” This law signifies that the two daughter cells produced at the end of Meiosis I are equally likely to receive either allele present (“The Law of Independent Assortment,” n.d.). There is one more law created by Mendel that must be mentioned to further explain the acquisition of specific traits by the F2 generation: The Law of Segregation. The Law of Segregation is defined as “only one of the two gene copies present in an organism is distributed to each gamete (egg or sperm cell) that it makes, and the allocation of the gene copies is random.” This law is essentially stating that each parent gives donates one gene copy of their two to each of the two daughter cells produced through Meiosis I and how these are donated is random (“The Law of Segregation,” n.d.). Consequently, the F2 generation ended up with the specific traits of apterous and wild type wings with expected genotype ratio of 1:2:1 and phenotype ratio of 3:1 because both Mendel’s Laws of Segregation and Independent Assortment ensure that the resulting species have equal possibility of receiving the different alleles on a gene coding for a certain trait.
The actual results presented in this experiment minimally deviated from the expected outcome. The expected outcome as presented in the hypothesis (see hypothesis) stated that approximately 25% of the resulting F2 generation would be apterous while 75% would be wild type. With a total fly count of 231 flies the expected number of apterous flies was 57.75 and the total expected number of wild type flies was 173.25. The actual observed number of these flies were extremely close with regard to the expected ratios: 171 wild type observed and 60 apterous. Ultimately, the observed percentage of wild type flies was 74% and the observed percentage of apterous flies was 26%. Both percentages only differentiated from the expected by 1% thus consequently signifying that the results presented in this data did not deviate from the expected values by a significant amount. Although these results do not deviate from the expected values by an extreme amount the phenotype ratio of the offspring (3:1) would be altered to ≈ 2.85:1, a ratio ultimately different but still very close to the expected ratio.
Females of the parental generation of Drosophila must be virgins to ensure that the mating process is controlled. The controlled mating requires virginity of female parental generation flies as a result of the Drosophila mating process itself. For example, once a female fly mates with a male she stores his sperm inside herself in areas known as “receptacles.” As the female fly then lays her eggs she fertilizes them with the sperm previously stored inside of her receptacles. If a female fly is not a virgin before this experiment begins it can not be ensured that the flies produced are of the correct phenotypes and genotypes necessary for the success of the specific cross (The College Board, 2010).
It was unnecessary to isolate virgin females for the F1 cross as a result of the same aforementioned Drosophila mating process. For example, the primary concern for the parental generation with regard to ensuring the virginity of the female flies is for controlled mating. If the females were exposed to males of another genotype or phenotype than the one intending to be examined and held that sperm in their receptacles it can not be insured that all offspring have the correct (intended) genotypes and phenotypes. This isolation of virgin females was not needed for the F1 cross because the only males that these females (previously virgins from parental generation) have mated with are the F1 males. It is not a concern that the F1 females are no longer virgins as the sperm that they are holding in their receptacles is ultimately the sperm that they are intending to be crossed with again—the sperm of the F1 males (The College Board, 2010).
The adult flies were removed from the vials in order to ensure that the flies being observed were only offspring of the F1 cross generation and not F0/parental or F3. If the adult flies were left in the vials, specifically after the first parental cross, the possibility of them mating with the produced F1 generation arises. If this cross were to occur it would result in the production of an F0 crossed with F1 generation. This generation would skew the results since this experiment needs the F1 generation to mate between each other to produce flies with the 100% genotype of Aa. This 100% Aa genotype is necessary for the F2 generation to be a cross of Aa and Aa genotype flies ultimately producing the intended 3:1 phenotype ratio. This experiment intends to observe the phenotypes of the F2 generation and in order to ensure that after each observation point the flies do not backcross with the previous generations the adult flies must be removed from the vials (Encyclopædia Britannica, 2011). Additionally, if the flies from F2 cross were left in the vial to mate with the upcoming F2 offspring many more phenotypes and genotypes become possible as a complete new generation— F3 is produced.
The hypothesis expressed by the punnett squares predicted that the ratio flies produced from the F2 generation would be 25% apterous and 75% wild type. With a total of 231 flies counted, according to these ratios there would be 173.25 wild type flies and 57.75 apterous. The actual observations made during this experiment produced 171 wild type flies and 60 apterous flies. After the conduction of a statistical Chi-Square analysis using the number of expected, observed, and total flies counted it was determined that the hypothesis was accepted. Despite the small variation in numbers between expected and observed the results were not different enough to be statistically different. The Chi-Square analysis resulted in a value of 0.118. With one degree of freedom, under the probability of 0.5 if the Chi-Square analysis results in a value of ≤ 3.84 the hypothesis is accepted. With a Chi-Square value of 0.118, the hypothesis is accepted.
Despite the close proximity in observed results and expected the results, the experiment conducted has three significant sources of error that can affect the ultimate results and contribute to the small, yet present, difference between the expected and observed results. Primarily, the method used to anesthetize the flies was a chemical called “FlyNap.” This chemical when exposed to Drosophila for extended periods of time can result in death. After the flies were anesthetized they were discarded into the morgue (placed into a beaker full of alcohol) so the source of error does not result from FlyNap killing those flies as a result of them being discarded after. Instead, the larva that remained in the vile may have been affected in their larval stage by the consistent exposure to FlyNap on dates of observation. If the FlyNap chemical affected the larva or pupa in the vile by ending their life cycle the total number of flies, and the resulting phenotype values would be lessened. For example, if even a number as small as three larval or pupal stage fruit flies were killed as a result of exposure to FlyNap the date of observation would be missing a total of three flies from the count and missing three phenotypes of two distinct possibilities: apterous or wild. This would be detrimental to the results of the experiment as a completely accurate count of both fly number and phenotype could not be recorded if not all flies that were intended to be born, were not. An inaccurate total number and phenotype count affects both the number of observed flies and the number of expected flies thus slightly skewing the entire data collected and Chi-Square analysis value.
The second source of error to be considered during this experiment is the possibility of an F3 generation produced. The food and fruit fly habitat itself becomes extremely sticky during the length of this experiment and flies are easily stuck to the walls and food. This makes removing every single fly from each generation of flies for observation difficult. The difficult of this process makes the probability of a fly from the previous generation stuck at the bottom unseen, or presumed dead, highly likely during the duration of this experiment across various data points. This problem can have a strong significance with regard to the outcome of this experiment. For example, if even a parental generation fly is left unseen and alive in the habitat after the count and the F1 or F2 generation flies are with it they can mate with each other. Instead of the cross being between the intended genotype Aa and Aa there would be the addition of a fly AA or aa crossing with flies of genotype Aa. This would affect the outcome of the flies produced from each following generations thus drastically changing the results by providing the flies with different genotypes and thus different phenotypes. Additionally, if flies of the F2 cross were to mate with other flies produced from an F2 cross a complete new generation of flies (F3) would be produced. With not only different numbers of phenotypes but the addition of new genotypes and phenotypes the experiment would yield incorrect results and examine more generations than intended.
The last source of error within this experiment is similar to the first in that it results in the death of flies. The surplus of food at the bottom of each fruit fly habitat poses as a necessary edition to each set up, yet has been shown to cause a significant number of deaths. When exposed to FlyNap the flies may pass out directly into the food and thus suffocate or the flies may get stuck in the food while active and suffocate. The death of flies in the food results in a loss of population. With less fruit flies in the experiment the results can be impacted in a few different ways. For example, if a significant number of female flies were to suffocate in the food the ultimate number of larva produced would be much less than if none had suffocated. Conversely, if a significant number of male flies were to suffocate in the food once again a much smaller amount of larva would be produced thus impacting the overall total observed flies. This source of error effects the results in the same way that larval exposure and death by FlyNap does, by resulting in an inaccurate total number of flies and phenotypes counted thus impacting both the number of observed flies and the number of expected flies and ultimately impacting the Chi-Square analysis value.