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Genetics Vishal Section D
Applications of Drosophila melanogaster
Introduction
Drosphila melanogaster is a species of fruit flies that has been used as a criticial tool in the
investigation of basic genetics. Drosophila are easily distingusihable and visible which allows for
simple classification of phenotypes and sex. Drosophila males have a dark and round abdomen as well
as sex combs. Female Drosophila have a colorless oval shaped abdomen with no sex combs. The small
size of Drosophila make them ideal for classroom and laboratory settings becuase they are easy to
cultivate without requiring a lot of room. Also Drosophila have a quick generation time of two days at
21 degrees celsius as well as a two week life cylcle which allows for a short-term project. Not only is
the generation time very quick but also the amount of offspring is plentiful allowing for more detailed
results. The last reason that Drosophila melanogaster are useful for laboratory experiments is that they
have mutliple chromosomes and genes which makes them to a great representation of almost all living
species in genetic studies.
This experiment is being performed to examine if there is Mendelian independent assortment
between the two traits: wing type (vg or vg+) and body color (e or e+). Also using statistical tests on the
results can determine if the traits are located on the same chromosome or on different chromosomes
and whether epitasis occured. This experiment will allow us to ascertain whether wing type and body
color assort independently in Drosophila melanogaster.
Hypothesis
The null hypothesis is that there will be independent assortment with a 9:3:3:1 ratio. The nine
would represent the wild type phenotype with normal color and normal wings. Each three would
represent a parental genotype of either ebony with normal wings or normal color with vestigial wings.
Lastly the 1 would represent the portion of flies that are ebony and vestigial. The alternate hypothesis
would be that the genes are linked and epitasis has occured which resulted in an offspring ratio other
than 9:3:3:1.
Methods
The experimenter recieved four Drosophila fruit flies from the F1 generation. After observation
under the microscope the experimenter deduced that there were 3 normal winged (vg+vg), normal body
colored (e+e) females and 1 normal winged (vg+vg), normal colored (e+e) male. They were placed into a
vial with one scoop (about tablespoon) of media, one scoop (about a tablespoon) of water, and one
pinch of yeast. The media was for yeast growth which the Drosophila larvae need for nutrition. The F1
geneneration was crossed and the larvae were allowed to grow for 14 days; however due to cold
weather the growth was slower than expected and the flies were allowed to grow for 21 days. The
experimenter recieved the vial with the live F2 flies. To put the flies asleep for easy scoring, the
experimenter used fly nap. The experimenter tapped the vial to move the flies to the bottom; he than
placed a little rope into the flynap and after making a small crevice in the foam top, placed the dipped
rope into the vial without releasing any flies. The flies were than suppose to go to sleep. However the
experimenter had a faulty fly nap kit and had to use a thrid dipped rope from another groups kit to put
the flies asleep. The napping flies were swept onto an index card using a fine point paint brush so they
did not get damaged. The index card was used as an simple background for scoring of the phenotype
and mutant characteristics of the flies. The experimenter viewed and classified all F2 offspring under a
microscope and recorded his results. Finally the flies were placed in the fly morgue.
Data/Results:
The phenotypes of the P generation were ebony colored, normal winged males (eevg+vg+) and
normal colored, vestigial winged females (e+e+vgvg). The F1 generation were all wildtype (e+evg+vg)
with normal wings and normal color. There should be four different phenotypes in the F2 generation:
wildtype (normal winged, normal colored), vestigial winged and normal color, normal winged and
ebony colored, and vestigial winged and ebony colored. The expected genotypes of the F2 generation
can be seen in Table 1 below:
Table 1 {F1XF1 Cross}
+
+
male/female
e vg
e+vg
evg+
evg
e+vg+
e+e+vg+vg+
e+e+vg+vg
e+evg+vg+
e+evg+vg
e+vg
e+e+vg+vg
e+e+vgvg
e+evg+vg
e+evgvg
evg+
e+evg+vg+
e+evg+vg
eevg+vg+
eevg+vg
evg
e+evg+vg
e+evgvg
eevg+vg
eevgvg
There were no P generation flies seen. There were 4 flies seen in the F1 generation: 3 female
and 1 male. There were 54 flies seen in the F2 generation. The phenotypes of the F2 generations are
shown in Table 2 below:
Table 2 {F2 Generation Results for Experimenter's Group}
Phenotype
Number of Offspring
Wild Type (Normal wings, normal color)
27
Ebony color, Normal wings
15
Normal color, Vestigial wings
9
Ebony color, Vestigial wings
3
The expected ratio for the F2 generation was 9:3:3:1 so by dividing 54 by 16 the experimenter
obtained the 1 in the 9:3:3:1 ratio and multiplied by 9, 3, and 3 respectively to find the expected
proportion of phenotypes. expected The ratio is: 30(wild type) :10(ebony) :10(vestigial) :3(ebony and
vestigial). The observed ratio was 27(wild type):15(ebony):9(vestigial):3(ebony and vestigial). Lab
section D had a total of 934 flies. The expected ratio according to Mendelian independent assortment
would be 525 :175 :175 :58. The observed phenotype ratio was 552: 191: 136: 55. To compare the
expected and observed results the experimenter used a chi square test. The chi square test uses the
equation:
The O represents the observed result and the E represents the expected result. The square of the
difference between the observed and the expected is divided by the expected value, and than the sum of
this value for each of the possible phenotypes gives the chi square result. This value is compared to the
probability vs. degree of freedom chart to give the probability of the results happening. The degrees of
freedom is 3 because there are four different phenotypes.
The experimenter's group results gave chi square values of 0.3 for the wildtype phenotype, 2.5
for ebony colored and normal winged, 0.1 for vestigial winged and normal color, and 0 for ebony and
vestigial. This totaled 2.9 and represents the range of probability from 0.5 to 0.3. This is over the 5
percent error range which means there is not independent assortment for the experimenter's sample.
The entire section D results gave chi square results of 1.39 for the wildtype phenotype, 1.46 for
the ebony colored with normal wings, 8.69 for vestigial winged with normal color, and 0.16 for ebony
and vestigial. This sums to 11.7 which corresponds to a range of probabilities between 0.01 to 0.001
which is much less than 5 percent. This means that Section D's results do show Mendelian independent
assortment.
Conclusion/Discussion:
The statistical tests disproved the null hypothesis for the entire lab section but proved the null
hypothesis correct for the experimenter's group results. This data implies that the experimenter's group
had flies that showed independent assortment and the class as a whole did not have independent
assortment. A possible explanation for these results could be that the body color and wing type are
linked. They could be close together on a chromosome and show up more frequently together than
independently assorting genes. The phenotypic ratio for the flies for the entire section was
approximately 10:4:3:1. This ratio would imply that the body color and wing type were linked and
undergoing epitasis. If the genes are linked, the explanation for the implied independent assortment for
the experimenter's group could be just genetic chance. The phenotypic ratios are only probabilities of
what is likely to happen in a cross so variance is very common. According to research, the body color
and wing type genes are not linked.
The chi square test gave numerical results which show the variance with four different
phenotypes between the observed frequency and the expected frequency. The variance in the
experimenter's group was from 50 percent to 30 percent which is a very high result. The level of
significance for this test is 5 percent so the experimenter's group results did not independent
assortment. The section D had a variance of 1 percent and 0.1 percent which is much lower than the
level of significance implying the ratio is similar to the 9:3:3:1 ratio and showing independent
assortment.
Possible sources of error could be miscounting, misclassifiying, and irregular life cycle.
Miscounting was definite a part of the error. After the flies had grown to the adult stage they were put
to sleep. The experimenter recalled seeing dead and alive flies stuck in the media in the vial. These flies
went unaccounted for. Also misclassifying by the experimenters when viewing the flies under the
microscope could have led to inaccurate results because there could have been a phenotypes incorrectly
attributed to another. Lastly the flies had an extended growth period. Usually they grow for two weeks
to become adults, however the cold weather in the laboratory caused a three week growth period. This
strange life cycle could have affected reproductive habits and natural growth tendencies.
If the experimenter could repeat this experiment to improve results he would increase the
amount of flies being tested in the F2 generation in order to get more representative data.
The two genes, body color and wing type, are clearly unlinked because they show independent
assortment. They are most likely found on separate chromosomes and assort independently. The results
showed the typical nature of unlinked genes through the 9:3:3:1 phenotypic ratio.
References:
1.
Brockett, Mirjana M. Genetics Laboratory Manual. Atlanta: Georgia Tech, 2010. Print.
2.
2. Cook, Erin. "Drosophila Lab." Drosophilia Lab. Cherry Emerson, Atlanta. Lecture.
3.
"Drosophilia Genetics." Mun.Biology. Www.Mun.ca. Web. 1 Mar. 2010.
<http://www.mun.ca/biology/dinnes/B2250/DrosophilaGenetics.PDF>.
4.
"Interactive Chi-Square Tests." Information Technology. Web. 09 Mar. 2010.
<http://people.ku.edu/~preacher/chisq/chisq.htm>.