Genetics Lab report 1 PDF

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Lab Report on Drosophila...

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Lab Report 1: Mendelian Inheritance Introduction 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 chisquared 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 wildtype 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 chisquared 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 F2 generation, I would put the flies into the fly morgue provided by my instructor. I would then use

my now fly-free index card and get about 20 more flies and repeat the process. I used the paint brush to aid me in moving the flies to where they needed to be. Besides what was described here, methods were followed from the lab manual pages 26-28 (Palu 2020). Results In Table 1, the phenotypes of the parental cross can be seen for both the males and females. All of my flies displayed the wild-type phenotypes for the three characteristics I looked at, so the mutant phenotype must be recessive. The three characteristics I looked at were their eye color, wing size, and body color.

Phenotype Eye Color Wing Size Body Color

Table 1: Parental Cross Phenotype Table F1 Male F2 Females Wild-Type (Red) Wild-Type (Long) Wild-Type (brown)

Wild-Type (Red) Wild-Type (Long) Wild-Type (brown)

Mutant Phenotype Recessive Recessive Recessive

In Table 2, the phenotype data gathered from the F2 generation is displayed. Here we can see that there were no observed females with white eyes, and only two males with white eyes, both of which had wild-type wings. Most of the offspring for both male and female were wildtype for both wing size and eye color. In total, 101 flies were observed, 39 of which were males and 62 were females.

Phenotypes WT wings/ WT Eyes Small Wing/ WT Eyes WT wings/ White Eyes Small Wings/ White Eyes Total

Table 2: F2 Phenotype Data Table # Males 21 16 2 0 39

# Females 45 17 0 0 62

Table 3 shows the chi-square analysis performed on my individual data. Here we can see that the x^2 value is about 33. The degrees of freedom for this would be 3, so the p-value would

be less than 0.005. 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

Observed Expected O-E=d d^2 d^2/E

Table 3: Chi-Square Analysis for Individual Data Table WT eyes/WT WT eyes/small White White wings wings eyes/WT Eyes/Small wings Wing 66 33 2 0 57 19 19 6 9 14 -17 -6 81 196 289 36 1.421 10.316 15.211 6

Total

101 101

32.948

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 Table Phenotypes # Males Wild-type / Wild-type 228 Wild-type / small wings 53 white eyes / Wild-type 108 white eyes / small wings 40 Total 429

# Females 281 82 0 0 363

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 0.005. 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

Observed Expected

Table 5: Chi-Squared Analysis for Class Data Table WT eyes/WT WT eyes/small White White wings wings eyes/WT Eyes/Small wings Wing 509 135 108 40 446 148 148 50

Total

792 792

O-E=d d^2 d^2/E

63 3969 8.899

-13 169 1.142

-40 1600 10.811

-10 100 2

22.852

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 F1 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 Xlinked 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

looking at the class data this does not seem to be the case. The class observed a total of 792 progeny from this particular cross, which means for this specific experiment, there should be enough to draw some conclusions, and discrepancies would be caused by something else. For example, my observations may have differed from the expected because of the lack of experience working with Drosophila. This is my first time working with drosophila, so I am sure there was some sort of learning curve, but this should not have posed too much of an issue as drosophila are a relatively easy organism to distinguish male from female and the phenotypes were distinct. There is much more a scientist could do with drosophila and genetics; as mentioned before, they are a great model organism. This experiment gave a splendid example of not only basic Mendelian genetics, but also on the special cases of Mendelian genetics as we saw with the X-linked gene associated with the eye color.

Bibliography

Griffiths A.J.F., Miller, J.H., Suzuki D.T., et al. (2000) “An Introduction to Genetic Analysis” https://www.ncbi.nlm.nih.gov/books/NBK22098/ ModENCODE (2020). “Drosophila as a Model Organism” http://modencode.sciencemag.org/drosophila/introduction. Palu, R. (2020). “Introduction to Drosophila” Lab Manual. p. 14-21....