GENETICS WITH THE USE OF DROSOPHILA MELANOGASTER FLIES
The fruit fly, Drosophila melangaster, was used as an instrument to study the inheritance and transmission of some characters. The characters used were eye color (red or white) and antenna mutation. Monohybrid crossings revealed phenotypic ratios of 3 1, supporting the experiments hypothesis.
Introduction
In the early twentieth century, the tiny fruitfly, Drosophila melanogaster, became the wonder geneticists tool. It was made famous by Thomas Morgan, an American geneticist in 1912, who used it extensively in his researches to verify the assertions of Gregor Mendel, and also to locate the position of genes on chromosomes (chromosome mapping).
Drosophila melanogaster, the fruit fly is a very suitable instrument for genetic studies. One of the advantages is that it is tiny, the adult is only 0.5cm long, and so can be kept in the laboratory in large numbers. Drosophila are small flies, typically pale yellow to reddish brown to black, with red eyes (Drosophila, 2009). The males can easily be distinguished from the females because the males have rounded abdomens while the females have pointed abdomens. Also, it completes its life cycle within two weeks and breeds in large numbers, enabling geneticists to follow the transmission of characters through several generations in a short period. It has only homologous pairs of fairly large chromosomes in its somatic cells and it has many easily distinguishable discontinuous characteristics.
Drosophila has been found to have four pairs of chromosomes a pair of sex chromosomes (X or Y) and three pairs of autosomes (2, 3 and 4). The size of the genome is about 165 million bases and contains an estimated 14,000 genes (by comparison, the human genome has 3,300 million bases and may have about 70,000 genes yeast has about 5800 genes in 13.5 million bases) (Introduction to Drosophila melanogaster, 2006). The analysis of the entire genome of the fruit fly has almost been completed.
The aim of this experiment is to verify the hypothesis which states that if there were 10 wild males and 10 mutated females in the F1 generation, there would be a phenotypical ratio of 3 mutated flies to 1 normal fly.
Methods
The materials used during this experiment were adsorbent wand, petri dish, several Drosophila vials and labels, FlyNap solution, fly morgue.
The whole experiment was conducted over the space of three weeks. The procedures will be divided according to the weeks.
Week 1
The first part of the experiment was to anesthetize the flies. This was done using a FlyNap. A wand was prepared and dipped inside the FlyNap solution. The wand was then introduced into the container containing the selected flies and held there for about two minutes. The anesthetized flies were shaken out on a white card. With a dissecting microscope, the adults were observed and their genders and physical characteristics were noted. The adults were separated according to their genders with the use of the properties differentiating the males from the females.
Week 2
The adults were allowed to mate the previous week and the offsprings observed the following week. The fly vials were retrieved and the eggs and larvae checked for. All the flies present were anesthetized using the FlyNap solution. The P generations of flies were removed and the eggs and larvae left in the vial. These were then allowed to incubate in the incubator, pending their observation the following week.
Week 3
A fresh vial of food was prepared for the new generation of flies. Yeast was also added to the food. The flies were labeled with the corresponding mutant letter. Again, the flies were anesthetized using FlyNap. Their characteristics were observed and recorded. The flies were sorted according to their phenotypic characteristics. Of these, 10 males and 10 females were selected and put in the new fresh vials and allowed to mate and produce the F2 generation of offsprings. After some days, the F2 offsprings were observed and their phenotypes recorded (Carolina Drosophila Manual, 2009).
Results
SHAPE MERGEFORMAT
Figure 1. A monohybrid cross between a wild type male and a mutated female.
SHAPE MERGEFORMAT
Figure 2. A second monohybrid cross between a mutated male (antenna) and a White eyed female.
Mutation A represents antenna formation, mutation B represents tan body and white eye, while mutation C represents wingless flies.
After the WB X A monohybrid cross, it was discovered that all the F1 generation of offsprings looked alike, that is all the phenotypes were of the wild type.
Observed of flies 21 males and 12 females.
During a second cross between AB and B flies, all the offsprings expressed the same mutated antennae trait. 20 offsprings had red eyes and 11 others had white eyes.
Observed of flies 8 males and 23 females.
BXbAXbaYAYaXBA Red eyes
XBXbAA
Mutated antenna Red eyes
XBXbAa
Mutated antennaB Red eyes
XBYAA
Mutated antennaB Red eyes
XBYAa
Mutated antennaXBa Red eyes
XBXbAa
Mutated antenna Red eyes
XBXbaa
Normal antennaB Red eyes
XBYAa
Mutated antennaB Red eyes
XBYaa
Normal antennaXbA White eyes
XbXbAA
Mutated antenna White eyes
XbXbAa
Mutated antennaB White eyes
XbYAA
Mutated antennaB White eyes
XbYAa
Mutated antennaXba White eyes
XbXbAa
Mutated antenna White eyes
XbXbaa
Normal antennaB White eyes
XbYAa
Mutated antennaB White eyes
XbYaa
Normal antennaFigure 3. Punnett square with a dihybrid cross showing inheritance patterns through the first and second filial generations of two characters.
Discussion
The monohybrid cross illustrated in Fig. 2 supports the hypothesis. The females which expressed the same phenotype as the parents were all mutant, that is, they had antenna mutation. During the course of the experiment, the cross between WB and A did not work. This was probably due to excess light. All the offsprings had the wild type phenotype. The correct phenotypic ratio should have been ratio 31 of red to white. This would have confirmed that the red eye trait is dominant to the white eye trait. Analysis of why this did not work is beyond the scope of this experiment.
A second cross between AB and B gave a different result. The observed phenotypes were 15 red eyed females with mutated antenna, 8 white eyed females with mutated antenna, 5 red eyed males with mutated antenna and 3 white eyed males with mutated antenna. This would give a phenotypic ratio of 20 red eyed offsprings to 11 white eyed offsprings, although, all had mutated antennae. The observed F1 phenotypes did not match with the expected F1 phenotypes. This is probably due to some errors during the experiment.
Figure 3 shows the results of the dihybrid cross to study the inheritance patterns of the two characters, that is, eye color and antenna mutation. Each member of the F1 generation undergoes meiosis to produce six kinds of gametes, XBA, XBa, XbA, Xba, YA and Ya. It can be seen that the alleles for the two characters were independently assorted. If members of the F1 generation were then allowed to cross among themselves, the F2 generation so produced shows 4 phenotypes and 12 genotypes. The four phenotypes which appear in the ratio 6 2 6 2 are as follows
six red eyed with mutated antenna
two red eyed with normal antenna
six white eyed with mutated antenna
two white eyed with normal antenna
Observing the Punnett square closely, it is noticed that the phenotypic ratio of red eyes to white eyes for each sex was 1 1, that is, the male flies had a 1 1 ratio for red eyes versus white eyes and the female flies, 1 1 ratio for red eyes versus white eyes. Again, the ratio of mutated antenna to normal antenna was 3 1 for all the red eyed males. Same thing can be noticed for all the red eyed females, white eyed males, and white eyed females.
Introduction
In the early twentieth century, the tiny fruitfly, Drosophila melanogaster, became the wonder geneticists tool. It was made famous by Thomas Morgan, an American geneticist in 1912, who used it extensively in his researches to verify the assertions of Gregor Mendel, and also to locate the position of genes on chromosomes (chromosome mapping).
Drosophila melanogaster, the fruit fly is a very suitable instrument for genetic studies. One of the advantages is that it is tiny, the adult is only 0.5cm long, and so can be kept in the laboratory in large numbers. Drosophila are small flies, typically pale yellow to reddish brown to black, with red eyes (Drosophila, 2009). The males can easily be distinguished from the females because the males have rounded abdomens while the females have pointed abdomens. Also, it completes its life cycle within two weeks and breeds in large numbers, enabling geneticists to follow the transmission of characters through several generations in a short period. It has only homologous pairs of fairly large chromosomes in its somatic cells and it has many easily distinguishable discontinuous characteristics.
Drosophila has been found to have four pairs of chromosomes a pair of sex chromosomes (X or Y) and three pairs of autosomes (2, 3 and 4). The size of the genome is about 165 million bases and contains an estimated 14,000 genes (by comparison, the human genome has 3,300 million bases and may have about 70,000 genes yeast has about 5800 genes in 13.5 million bases) (Introduction to Drosophila melanogaster, 2006). The analysis of the entire genome of the fruit fly has almost been completed.
The aim of this experiment is to verify the hypothesis which states that if there were 10 wild males and 10 mutated females in the F1 generation, there would be a phenotypical ratio of 3 mutated flies to 1 normal fly.
Methods
The materials used during this experiment were adsorbent wand, petri dish, several Drosophila vials and labels, FlyNap solution, fly morgue.
The whole experiment was conducted over the space of three weeks. The procedures will be divided according to the weeks.
Week 1
The first part of the experiment was to anesthetize the flies. This was done using a FlyNap. A wand was prepared and dipped inside the FlyNap solution. The wand was then introduced into the container containing the selected flies and held there for about two minutes. The anesthetized flies were shaken out on a white card. With a dissecting microscope, the adults were observed and their genders and physical characteristics were noted. The adults were separated according to their genders with the use of the properties differentiating the males from the females.
Week 2
The adults were allowed to mate the previous week and the offsprings observed the following week. The fly vials were retrieved and the eggs and larvae checked for. All the flies present were anesthetized using the FlyNap solution. The P generations of flies were removed and the eggs and larvae left in the vial. These were then allowed to incubate in the incubator, pending their observation the following week.
Week 3
A fresh vial of food was prepared for the new generation of flies. Yeast was also added to the food. The flies were labeled with the corresponding mutant letter. Again, the flies were anesthetized using FlyNap. Their characteristics were observed and recorded. The flies were sorted according to their phenotypic characteristics. Of these, 10 males and 10 females were selected and put in the new fresh vials and allowed to mate and produce the F2 generation of offsprings. After some days, the F2 offsprings were observed and their phenotypes recorded (Carolina Drosophila Manual, 2009).
Results
SHAPE MERGEFORMAT
Figure 1. A monohybrid cross between a wild type male and a mutated female.
SHAPE MERGEFORMAT
Figure 2. A second monohybrid cross between a mutated male (antenna) and a White eyed female.
Mutation A represents antenna formation, mutation B represents tan body and white eye, while mutation C represents wingless flies.
After the WB X A monohybrid cross, it was discovered that all the F1 generation of offsprings looked alike, that is all the phenotypes were of the wild type.
Observed of flies 21 males and 12 females.
During a second cross between AB and B flies, all the offsprings expressed the same mutated antennae trait. 20 offsprings had red eyes and 11 others had white eyes.
Observed of flies 8 males and 23 females.
BXbAXbaYAYaXBA Red eyes
XBXbAA
Mutated antenna Red eyes
XBXbAa
Mutated antennaB Red eyes
XBYAA
Mutated antennaB Red eyes
XBYAa
Mutated antennaXBa Red eyes
XBXbAa
Mutated antenna Red eyes
XBXbaa
Normal antennaB Red eyes
XBYAa
Mutated antennaB Red eyes
XBYaa
Normal antennaXbA White eyes
XbXbAA
Mutated antenna White eyes
XbXbAa
Mutated antennaB White eyes
XbYAA
Mutated antennaB White eyes
XbYAa
Mutated antennaXba White eyes
XbXbAa
Mutated antenna White eyes
XbXbaa
Normal antennaB White eyes
XbYAa
Mutated antennaB White eyes
XbYaa
Normal antennaFigure 3. Punnett square with a dihybrid cross showing inheritance patterns through the first and second filial generations of two characters.
Discussion
The monohybrid cross illustrated in Fig. 2 supports the hypothesis. The females which expressed the same phenotype as the parents were all mutant, that is, they had antenna mutation. During the course of the experiment, the cross between WB and A did not work. This was probably due to excess light. All the offsprings had the wild type phenotype. The correct phenotypic ratio should have been ratio 31 of red to white. This would have confirmed that the red eye trait is dominant to the white eye trait. Analysis of why this did not work is beyond the scope of this experiment.
A second cross between AB and B gave a different result. The observed phenotypes were 15 red eyed females with mutated antenna, 8 white eyed females with mutated antenna, 5 red eyed males with mutated antenna and 3 white eyed males with mutated antenna. This would give a phenotypic ratio of 20 red eyed offsprings to 11 white eyed offsprings, although, all had mutated antennae. The observed F1 phenotypes did not match with the expected F1 phenotypes. This is probably due to some errors during the experiment.
Figure 3 shows the results of the dihybrid cross to study the inheritance patterns of the two characters, that is, eye color and antenna mutation. Each member of the F1 generation undergoes meiosis to produce six kinds of gametes, XBA, XBa, XbA, Xba, YA and Ya. It can be seen that the alleles for the two characters were independently assorted. If members of the F1 generation were then allowed to cross among themselves, the F2 generation so produced shows 4 phenotypes and 12 genotypes. The four phenotypes which appear in the ratio 6 2 6 2 are as follows
six red eyed with mutated antenna
two red eyed with normal antenna
six white eyed with mutated antenna
two white eyed with normal antenna
Observing the Punnett square closely, it is noticed that the phenotypic ratio of red eyes to white eyes for each sex was 1 1, that is, the male flies had a 1 1 ratio for red eyes versus white eyes and the female flies, 1 1 ratio for red eyes versus white eyes. Again, the ratio of mutated antenna to normal antenna was 3 1 for all the red eyed males. Same thing can be noticed for all the red eyed females, white eyed males, and white eyed females.
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prof premraj pushpakaran writes -- 2018 marks the 100th birth year of Edward Butts Lewis!!!
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