Report On Genetics Experiment On Dihybrid Crosses Between The Drosophila Melanogaster Flies

The main aim of this experiment was to observe the results from the dihybrid crosses between the drosophila melanogaster flies. Two traits were studied in this analysis, eye colour and the type of wings. In addition, being able to understand the key genetic crosses discovered by Gregor Mendel and the types of traits that are being passed on from one generation to another was equally essential. The experimental data was compared to the theoretical data which allowed us to attain the phenotypic ratio between the crosses. Furthermore, the results were in accordance to the laws of independent assortment. From the F2 generation a series of conclusions were made and the autosomal and sex linked traits were inspected. Using the chi-square tests the confidence level of the analysis was tested.

Introduction

The study of genetics is very broad and it is mostly studied to understand the traits inherited and passed on from one generation to another. It is also known as the study of heredity in living organisms. The law of independent assortment unfolds how numerous genes are separated from one another when gametes are produced which in turn gives rise to genetic variation. This takes place during meiosis, a cell division process producing 4 daughter cells that are haploid. Drosophila melanogaster flies are commonly used to analyse how particular traits are inherited from one generation to another and they are highly favourable by geneticists as these flies breed rapidly given its short life cycle and are also cost-efficient. These are some of the reasons as to why chose to study the traits inherited by these flies. In this analysis, a dihybrid cross was carried out to observe the phenotypic traits between the flies. A dihybrid cross can be defined as a cross observing two different characters. There is a complete dominance recessive link between the alleles of the flies. This experiment aimed to study both the autosomal trait and sex-linked trait. An autosomal recessive trait is present when there are two copies of an abnormal gene that causes a specific trait to develop. On the other hand, a sex-linked trait is present when the character of a gene is dependent on the location on the X chromosome. White eye coloured Drosophila melanogaster flies are X-linked recessive allele and is usually inherited by males as they are hemizygous. Males are identified as hemizygous given that there is only one X chromosome in them. Moving forward, a wild type-mutant cross was carried out to determine the phenotypic results between the Drosophila melanogaster flies. Two traits were observed in this experiment, wing type and eye colour. It has been hypothesised that the expected results from an F2 dihybrid cross will act in accordance with a ratio of 3:3:1:1 for males and 6:0:2:0 for females. Any variation from these expected ratios is due to random chance alone. Chi-square tests are used to conclude any differences between expected and observed values. Therefore, the hypothesis can be accepted or rejected using the chi-square test.

Methods and Materials

Using the Drosophila Genetics Lab Software, to simulate the breeding of the fruit flies, a sample size of the Drosophila flies was bred modifying the genotypes of the F2 generation. For the parental flies, a wild-type male and a wild-type female were selected to create the F1 generation. From the F1 cross, one male fly and one female fly were selected to create the F2 generation flies. Lastly, each fly was count manually to analyse the traits inherited and the gender as well.

Results

The results of the analysis are tabulated below. The ratios obtained were as predicted. For males the ratios observed were 3:3:1:1 and 6:0:2:0 for females. parental genotypes: XwY +vg+vg X X+X+ vgvg parental phenotypes: white males X vestigial females

F1 genotypes: X+Y/+vgvg X XWX+/vgvg

F1 phenotypes: Red eyes, Normal/vestigial wings X White/Red eye/vestigial wings

F2 Phenotypes

F2 Genotypes Wild type (Normal wings, red eyes) X+ +vg/Y +vg White-eyed (Normal wings, white eyed) Xw +vg Vestigial-winged (vestigial wings, red eyes) X +vg /Y +vg White-eye-vestigial winged (Vestigial wings, white eyes)

The X2 values for males was calculated to be 2. 6667 and as for the females 3. 0303. The p value for the male was between 0. 5 and 0. 3. Likewise, for the females the p value was between 0. 1 and 0. 05.

Discussion and Conclusion

Comparing the results at the end of the study, white eye colour was only observed in males but not in females. This indicated that the white eye colour is a sex link recessive trait and it was only found in the males which have the X chromosome. Furthermore, an X- linked recessive trait is passed on to females when they have 2 copies of the X chromosome. The female progeny obtains one of their X chromosomes from the father, hemizygous dominant. As for the male progeny, the X chromosomes were obtained from the mother, heterozygous dominant resulting in having half a red-eye allele and white eye allele. In addition, as for the vestigial wing type, it is an autosomal recessive trait that is passed on from one generation to another. Adding on, it is evident that there is a complete dominant-recessive linkage between the flies as the results containing the wild types (Normal wing and eye) are of a greater ratio. Besides, complete dominance is when one allele completely takes over the effect of the recessive allele in a heterozygous cross.

Applying the chi-square test to conclude between the observed and expected results, the null hypothesis was accepted as the p values for both the male and female was more than 0. 05 which is higher than the usual figure. The figures from the chi-square test appeared to show that independent assortment whereby various genes separate from each another, could have occurred and the p values In conclusion, the results of the analysis have supported the hypothesis which states that the expected results from an F2 dihybrid cross will act in accordance with a ratio of 3:3:1:1 for males and 6:0:2:0 for females. Any variation from these expected ratios is due to random chance alone. There were some limitations in this experiment one of it being the sample size. Instead of a small sample size, a larger sample size would have boosted the accuracy of the results.

Another limitation that might have affected the accuracy of the results was incorrectly identifying the traits and gender of the flies. Furthermore, one can easily make a mistake during the counting of the flies as it was done manually. One possible way to revise the study can be to include a function in the software that enables it to count and differentiate the different gender and traits observed.