Genetics April 30, 2012 An Affirmation of Mendelian Genetics: a Dihybrid Cross of Drosophila melanogaster Lucas Anderson and Tiffany Sinclair Purpose: The purpose of this lab was to understand the nature of genetic dihybrid crosses by creating a test cross of two variations of Drosophila melanogaster to produce 9:3:3:1 genetic spread Materials & Methods A culture tube containing a cross of vestigial/wild eyes*wild winged/sepia eyed was obtained. We received the tube with the f1 generation from the cross. A pipe cleaner tip was dipped in FlyNap and placed it inside the tube. After about 10 minutes, the flies were anaesthetized. The flies were then poured onto a white piece of paper and the remaining flies were skimmed out using a paintbrush. The flies were then separated using a paintbrush. They were placed into four groups. The groups that the flies were separated into were vestigial/wild eyes, wild wings/wild eyes, vestigial/sepia, and wild wings/sepia. After separating the flies, they were counted according to their respective groups and the results were recorded. After the count of the F1 generation, the drosophila were placed back into their tube, being careful not to flip the culture tube upright. After the flies awoke from their anesthesia, we let the flies mate and lay eggs for a week. The next week the culture tube was then taken outside, and the foam plug was removed so that the entire F1 generation was released. The plug was replaced and the flies were allowed to hatch for another week. After we were certain that most of the eggs had hatched, the flies were brushed out of the tubes again after being anaesthetized. The count was repeated, keeping in mind the four previous groups. The results of the count were recorded, and the flies were returned to their tubes. Results The following data is the result of the count of the two crosses in the F1, and then the F2 generation. F1 Count Type Fly Count Wild Wings / Wild Eyes 9 Vestigial / Wild Eyes 3 Wild Wings/ Sepia 2 Vestigial / Sepia 1 F2 Count: Type Fly Count Wild Wings / Wild Eyes 104 Vestigial / Wild Eyes 23 Wild Wings/ Sepia 14 Vestigial / Sepia 5 Discussion The data from the F1 generation is impeccable for the number of flies that we had. This data agrees with Mendel’s 9:3:3:1 achieved when creating his dihybrid cross. We got a 9:3:2:1 ratio, which is as close as one could get with that amount of flies. This Punnett square shows the reason behind the ratio: W = wild wings, w = vestigial wings E = wild eyes, and e = sepia eyes. WE We wE we WE WWEE WWEe WwEE WwEe Phenotype: Phenotype: Phenotype: Phenotype: WE WE WE WE We WWEe WWee WwEE Wwee Phenotype: Phenotype: Phenotype: Phenotype: WE We WE We wE WwEE WwEE wwEE wwEe Phenotype: Phenotype: Phenotype: Phenotype: WE WE wE we WwEe Wwee wwEe wwee we Phenotype: Phenotype: Phenotype: Phenotype: WE We wE we The F1 generation showed a great distribution and was an exact representation of Mendel’s findings. The way that we looked for the differences in our counting was to look for the distinctive “shriveled” wings, or full wings, and also looked at the eye color of the flies whether they were red (wild) or black (sepia). The vestigial and the black traits were recessive. For our F2 generation this 9:3:3:1 pattern was not followed. This is because there was a larger population of wild type flies from the initial cross. This meant that there would be a higher proportion of wild type flies to non-wild flies. Yet, this does not mean that the recessive genes are receding within the population. There is a larger amount of homozygous individuals. The recessive genes continue to be passed on, as is consistent with Mendel’s law of independent assortment. We observed a ratio of 104:21:14:5 from our count, which simplified is 21:5:3:1. This number makes sense since there was an increase in wild type individuals from the F1 generation. This data continues to affirm Mendel’s laws. These predictive factors are important for the generalization of genetic principles across populations. They are important for ecology, biology, and many of the genetic cures we search for today.
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