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A Study of Inheritable Traits in Fruit Flies
INTRODUCTION
The Drosophila melanogaster, more commonly known as the fruit fly, is a popular species used in genetic experiments. In fact, Thomas Hunt Morgan began using Drosophila in the early 1900’s to study genes and their relation to certain chromosomes (Biology 263). Scientists have located over 500 genes on the four chromosomes in the fly. There are many advantages in using Drosophila for these types of studies. Drosophila melanogaster can lay hundreds of eggs after just one mating, and have a generation time of two weeks at 21°C (Genetics: Drosophila Crosses 9). Another reason for using fruit flies is that they mature rather quickly and don’t require very much space. Drosophila melanogaster has a life cycle of four specific stages. The first stage is the egg, which is about .5mm long. In the 24 hours when the fly is in the egg stage, numerous cleavage nuclei form. Next, the egg hatches to reveal the larva. During this stage, growth and molting occur. Once growth is complete, the Drosophila enter the pupal stage, where it develops into an adult through metamorphosis. Upon reaching adulthood, the flies are ready to mate and produce the next generation of Drosophila melanogaster.
During this experiment, monohybrid and dihybrid crosses were conducted with Drosophila melanogaster. Our objective was to examine the inheritance from one generation to the next. We collected the data from the crosses and analyzed them in relation to the expected results.
MATERIALS AND METHODS
For the monohybrid cross in this experiment, we used an F1 generation, which resulted from the mating of a male homozygous wild-type eyed fly with a female homozygous sepia eyed fly. Males and females are distinguished by differences in body shape and size. Males have a darker and rounder abdomen in comparison to females, which are more pointed. Another difference occurs on the forelegs of the flies—males have a small bump called sex combs. At week 0, after being anaesthitized by fly-nap, three males and three females were identified under a dissecting microscope and placed in a plastic vial with a foam stopper at the end. The vial remained on it’s side until the flies regained consciousness so that they didn’t get trapped by the culture medium at the bottom. We allowed the Drosophila to incubate and reproduce for a week.
After one week, the vial contains many larva in addition to the F1 generation flies. Next, we removed the F1 generation flies to prevent breeding between the two generations. Acting as Dr. Kevorkian, we gave the F1 generation a lethal dose of the seemingly harmless anesthesia, fly-nap. A trumpet solo of "Taps" played in our minds as we said goodbye and placed them in the fly morgue. We allowed the F2 larval generation to incubate for two weeks. The experiment called for one week of incubation, but Easter fell during that week which interfered with our lab time. After the two weeks, the F2 flies were also terminally anaesthetized. Only, before saying goodbye, we separated the flies according to sex and eye color (wild-type,red or mutant, sepia), recording the results in Table 1.
The same method was used it the dihybrid cross, except, instead of one trait, two traits were observed. The traits were eye-color (wild-type, red or mutant, sepia) and wing formation (wild-type, full or mutant, vestigial). The F1 generation for the dihybrid cross came from a cross between a male homozygous wild-type for eyes and wings, and a female homozygous for sepia eyes and vestigial wings. The results of this cross were recorded and appear in Table 2.
RESULTS
The monohybrid cross of Drosophila melanogaster produced 25,893 flies for all of the sections combined. Of those flies, 75.9% had wild-type (red) eyes, and 24.1% had mutant (sepia eyes). Overall, more females were produced than males.
TABLE 1: F1 Generation Monohybrid Cross of Drosophila melanogaster (+se x +se)
PHENOTYPE CLASS RESULTS RESULTS FROM ALL CLASSES NUMBER PERCENT RATIO NUMBER PERCENT RATIO
MALES
WILD-TYPE EYES 562 74.8% 3.0 8,960 75.4% 3.1
SEPIA EYES 189 25.2% 1 2,923 24.6% 1
FEMALES
WILD-TYPE EYES 806 75.6% 3.1 10,685 76.3% 3.2
SEPIA EYES 260 24.4% 1 3,325 23.7% 1
BOTH SEXES
WILD-TYPE EYES 1368 75.3% 3.0 19,645 75.9% 3.1
SEPIA EYES 449 24.7% 1 6,248 24.1% 1
The dihybrid cross produced a total of 26, 623 flies for all of the sections combined. 54.9% of the flies had wild-type eyes(red) and wild-type wings(full), 17.7% had wild-type eyes and vestigial wings, 21.3% had sepia eyes and full wings, and 6.1% had sepia eyes and vestigial wings. Again, the number of females produced exceeded the number of males.
TABLE 2: F1 Generation Dihybrid Cross of Drosophila melanogaster(+vg+se x +vg+se)
PHENOTYPE CLASS RESULTS RESULTS FROM ALL CLASSES
MALES NUMBER PERCENT RATIO NUMBER PERCENT RATIO
WILD-TYPE EYES WILD-TYPE WINGS 244 47.8% 6.3 6987 54.4% 8.6
WILD-TYPE EYES VESTIGIAL WINGS 132 25.9% 3.4 2315 18% 2.9
SEPIA EYES WILD-TYPE WINGS 95 18.6% 2.4 2727 21.2% 3.4
SEPIA EYES VESTIGIAL WINGS 39 7.6% 1 808 6.4% 1
FEMALES
WILD-TYPE EYES WILD-TYPE WINGS 281 51.1% 7.0 7615 55.2% 9.3
WILD-TYPE EYES VESTIGIAL WINGS 100 18.2% 2.5 2397 17.4% 2.9
SEPIA EYES WILD-TYPE WINGS 129 23.5% 3.2 2953 21.4% 3.6
SEPIA EYES VESTIGIAL WINGS 40 7.3% 1 821 6.0% 1
BOTH SEXES
WILD-TYPE EYES WILD-TYPE WINGS 525 49.5% 6.6 14,602 54.9% 9.0
WILD-TYPE EYES VESTIGIAL WINGS 232 21.9% 2.9 4,712 17.7% 2.9
SEPIA EYES WILD-TYPE WINGS 224 21.1% 2.8 5,680 21.3% 3.5
SEPIA EYES VESTIGIAL WINGS 79 7.5% 1 1,629 6.1% 1
DISCUSSION
The results from the monohybrid cross for both my class and for all sections were very close to the expected results. "Theoretically, there should be three red-eyed flies for every one sepia-eyed fly. We call this a 3:1 phenotypic ratio" (So What’s a Monohybrid Cross Anyway? 2). As indicated in table one, the data comes within one or two tenths of the 3:1 ratio. Therefore, the monohybrid cross was very accurate. However, the results from the dihybrid cross were not quite as accurate. Mendel hypothesized and proved that a dihybrid cross should produce a 9:3:3:1 ratio(Biology 245). In our experiment, the results from my class (both sexes) were not very close to the ratio. In table 2, the ratio shows 6.6:2.9:2.8:1. The data obtained from all classes were slightly more precise. All sections together (both sexes) produced a ratio of 9:2.9:3.5:1.
There are many reasons that our results did not match the expected ratios. For example, when transferring flies from one vial to another, a few flies got away which could have a small effect on the numbers. Another factor affecting the results also happened upon transferring flies. A number of flies were imbedded in the cultural medium. We were forced to leave them there so that we didn’t loosen the medium. The largest source of error in the "my class" column came from the amount of time we allowed the flies to reproduce. Since Easter vacation occurred during our lab period, our second generation flies were permitted to stay together for two weeks instead of one. This may have resulted in the F2 generation flies mating with their own offspring, thus throwing off the ratio.
I feel more certain about the results in the "all classes" column since many more trials were performed and more flies were used. In any experiment, the more trials one conducts, the more accurate the results will be. This makes sense when comparing the results from my class versus the results from all classes combined. The numbers of flies used in each column make the difference in trials more evident: 1,060 flies were produced in my class, whereas 26, 623 flies were produced in all classes.
In the monohybrid cross, the ratio for eye color for the females were consistent with the ratio for males. This information implies that the gene for eye color is not sex linked. Through research, I found that in Drosophila melanogaster, chromosome one is the sex chromosome. Eye color is not one chromosome one, but rather on chromosome three. Therefore, eye color in Drosophila is not sex linked(Genetics:Drosophila Crosses).
In each column, the number of females produced outweighed the number of males. This may imply that the X chromosome is dominant over the Y chromosome. This would cause the X chromosome to mix with another X chromosome, producing a female, more often than it would mix with the Y chromosome, which would produce a male.
As a follow-up to the experiment, I would perform many more trials than each person did for this experiment. Also, more flies could be placed in each vial to ensure even more offspring to be included in the data. I would also be sure to remove the flies after just one week to reduce breeding between generations.
This experiment caused Mendel’s findings to be more concrete and realistic in my mind. It made the information more than meaningless numbers. The experiment also made me realize how easily biological ideas can be proved. Our results agree with Mendel’s discoveries. The only drawback to our learning was the massacre of over 26,000 fruit flies.
REFERENCES
Campbell, Neil A., Biology: Fourth Edition. Menlo Park: Benjamin/Cummings, 1996.
"Genetics: Drosophila Crosses." Lab Handouts, General Biology Lab, 1996.
"So What’s a Monohybrid Cross Anyway?" Lab Handouts, General Biology Lab, 1996.BACK TO DIRECTORY | BACK TO SUB DIRECTORY: BIOLOGY
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