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Dyhybrid Cross

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									                                                                                      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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