Gregor Mendel is considered the father of genetics. His work can be summarized with his two laws of inheritance. The first is the law of segregation. This law states that for every gene there are two alleles that segregate during meiosis (Griffiths et al. 2000). During fertilization, the alleles come together at random, one form each parent. The second law of Mendel’s is the Law of Independent Assortment. This law states that during meiosis, different alleles segregate independently of each other (Palu 2020). In this experiment, we will be seeing an example of both of Mendel’s laws by working with Drosophila.
Drosophila, also known as fruit flies, are an important species when it comes to genetics. They are extensively used as a model organism for genetic investigations and have been for over a century (ModENCODE 2020). Drosophilae are also an ideal organism for genetics because they share many similarities with humans on a genetic level including pathways. Many genetic human diseases that involve mutation, amplification, or deletion also have a counterpart in Drosophila (ModENCODE 2020). This provides a more ethical way for scientist to learn about genetics that may apply to humans. It is also beneficial that the flies have a short lifespan. This allows the scientist to see many generations much quicker than one would see with humans or many other organisms. They are also easy to maintain and culture, which is greatly beneficial for a scientist working with them (Palu 2020). For this experiment specifically, Drosophila were a great organism to use, as we wanted to look at Mendelian Inheritance, so phenotypes of parents and their offspring, and we only had a brief time span to observe this.
In this experiment, we observed the phenotypic ratios in the F1 and F2 generations from a dihybrid cross to draw some conclusions from the cross. We wanted to determine whether the mutant alleles were dominant or recessive, whether they were X-linked or autosomal, and whether the two genes are linked. To assist us with making these determinations, we used a chi- squared analysis to look at how the observed data compared to the expected data. For this dihybrid cross, we expected to see a ratio of 9 wild-type phenotypes, 3 with white eyes and wild- type wings, 3 with wild-type eyes and short wings, and one with both white eyes and short wings. We expect this ratio because the parents did not display the mutant phenotypes, so this would be a heterozygous dihybrid cross, and it is expected that the two traits will both be autosomal and unlinked. After crossing the F1 generation, observations were made, and a chi- squared analysis was performed. It was found that the hypothesis was wrong as we failed to reject our null hypothesis, so that means that these flies did not follow basic Mendelian Inheritance, but rather we saw a case involving an X-linked trait.
Materials and Methods
For this experiment, I was given a container of Drosophila marked with yellow tape and the number three. In order to sort my flies, I used a microscope, paintbrush, index card, and some fly nap. First, I used the fly nap to get the flies to stay still while I sorted them. After they were asleep, I put them on an index card. Next, I put approximately 20 on another index card and would sort and record the flies. I tried to sort first by gender, and then go through the groups and record for each gender the number of each phenotype I observed. After I was done observing the 20 on my notecard, I would move them to the next area they needed to be. For the parental cross, I would move the flies into a clean test tube with their food and a foam cork. For the F generation, I would put the flies into the fly morgue provided by my instructor. I would then use
be less than 0. This means that we fail to reject the null hypothesis. The null hypothesis would be that the offspring would not show a 9:3:3:1 phenotypic ratio.
Phenotype WT eyes/WT Table 3: Chi-Square Analysis for Individual Data Table wings WT eyes/smallwings White eyes/WT wings White Eyes/Small Wing Total ExpectedObserved 6657331921906101101 O-E=dd^2 981 14196 -17 289 -6 36 d^2/E 1 10 15 6 32. In Table 4, the class data for the yellow #3 cross is displayed. Here we can see that 792 flies were observed in total. Of these, 363 were females, while 429 were males. Similar to my individual data, no females were seen with white eyes. Most of the offspring observed displayed wild-type features for both their eye color and their wing size. Of the flies with white eyes, most of them had wild-type wings.
Table 4: F2 Class Data for Yellow #3 Cross TablePhenotypes # Males # Females Wild-type / Wild-typeWild-type / small wings 2285328182 white eyes / Wild-type 108 0 white eyes / small wingsTotal 404290363 Table 5 shows the chi-squared analysis done on the class data for the yellow #3 cross. Here we see that the x^2 value is about 23. We know that degrees of freedoms for this is 3. Based on those values, we see that the p-value is less than 0. This means that we fail to reject our null hypothesis. The null hypothesis in this case is that the offspring would not show a 9:3:3:1 phenotypic ratio.
Phenotype WT eyes/WT Table 5: Chi-Squared Analysis for Class Data Table wings WT eyes/smallwings White eyes/WT wings White Eyes/Small Wing Total ExpectedObserved 5094461351481081484050792792
O-E=dd^2 633969 -13 169 -40 1600 -10 100 d^2/E 8 1 10 2 22. Discussion/Conclusion My raw numbers of the F2 generation are a little different from the classes. I saw only 2 flies with white eyes, both of which were male and had wild-type wings. The class also only saw male flies with white eyes, but they saw these flies with both wild-type wings and the short wings. Despite differences between my data and the compiled data, both chi-squared analyses both failed to reject the null hypothesis. The observations I gathered from the first day with the F1 generation did differ from the observations I gathered with the F2 generation. In the F generation all of the flies looked to be the wild-type, while in the F2 generation I was able to see some mutant phenotypes on some of the flies. This is what indicated to me that the two phenotypes I observed were recessive. Based on these observations, it appears that the mutant allele for small wings is autosomal and recessive, while the mutant allele for white eyes is X- linked and recessive. The chi-square analysis does not support my hypothesis because it failed to reject the null hypothesis. This means that the progeny did not just show a 9:3:3:1 relationship. Tables 2 and 4 help show that the allele for white eyes is x-linked by showing that no females displayed this phenotype. In x-linked genes, males display the recessive phenotype more often than females because they typically only have one X chromosome, so it does not matter whether the allele is recessive or dominant, because there is no other allele.
For experiments like this, it is vital to collect a large dataset. This allows the observations to be more precise and the conclusions drawn from these observations to be more accurate. The more offspring we can observe, the more accurately the data represents the species, and the more accurate the conclusions we draw are. My data seems to indicate that the two genes for these mutant phenotypes could be linked since I only saw male flies with wild type wings, but when
Griffiths A.J., Miller, J., Suzuki D., et al. (2000) “An Introduction to Genetic
ModENCODE (2020). “Drosophila as a Model Organism”
Palu, R. (2020). “Introduction to Drosophila” Lab Manual. p. 14-21.