A Study Of Inheritable Traits in Fruit FliesINTRODUCTIONThe Drosophila melanogaster, more commonly known as the fruit fly, is apopular species used in genetic experiments. In fact, Thomas Hunt Morgan beganusing Drosophila in the early 1900s to study genes and their relation tocertain chromosomes(Biology 263). Scientists have located over 500 genes on thefour chromosomes in the fly.
There are many advantages in using Drosophila forthese types of studies. Drosophila melanogaster can lay hundreds of eggs afterjust one mating, and have a generation time of two weeks at 21C(Genetics:Drosophila Crosses 9). Another reason for using fruit flies is that they maturerather quickly and dont require very much space. Drosophila melanogaster has alife 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 cleavagenuclei form. Next, the egg hatches to reveal the larva. During this stage,growth and molting occur.
Once growth is complete, the Drosophila enter thepupal stage, where it develops into an adult through metamorphosis. Uponreaching adulthood, the flies are ready to mate and produce the next generationof Drosophila melanogaster. During this experiment, monohybrid and dihybrid crosses were conductedwith Drosophila melanogaster. Our objective was to examine the inheritance fromone generation to the next. We collected the data from the crosses and analyzedthem in relation to the expected results. MATERIALS AND METHODSFor the monohybrid cross in this experiment, we used an F1 generation, whichresulted from the mating of a male homozygous wild-type eyed fly with a femalehomozygous sepia eyed fly.
Males and females are distinguished by differences inbody shape and size. Males have a darker and rounder abdomen in comparison tofemales, which are more pointed. Another difference occurs on the forelegs ofthe fliesmales have a small bump called sex combs. At week 0, after beinganaesthitized by fly-nap, three males and three females were identified under adissecting microscope and placed in a plastic vial with a foam stopper at theend.
The vial remained on its side until the flies regained consciousness sothat they didnt get trapped by the culture medium at the bottom. We allowedthe Drosophila to incubate and reproduce for a week. After one week, the vialcontains many larva in addition to the F1 generation flies. Next, we removedthe F1 generation flies to prevent breeding between the two generations.
Actingas Dr. Kevorkian, we gave the F1 generation a lethal dose of the seeminglyharmless anesthesia, fly-nap. A trumpet solo of “Taps” played in our minds aswe said goodbye and placed them in the fly morgue. We allowed the F2 larvalgeneration to incubate for two weeks.
The experiment called for one week ofincubation, 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 eyecolor(wild-type,red or mutant, sepia), recording the results in Table 1. Thesame method was used it the dihybrid cross, except, instead of one trait, twotraits were observed. The traits were eye-color(wild-type, red or mutant,sepia) and wing formation(wild-type, full or mutant, vestigial).
The F1generation for the dihybrid cross came from a cross between a male homozygouswild-type for eyes and wings, and a female homozygous for sepia eyes andvestigial wings. The results of this cross were recorded and appear in Table 2. RESULTSThe monohybrid cross of Drosophila melanogaster produced 25,893 flies for all ofthe 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)PHENOTYPECLASS RESULTS RESULTSFROM ALL CLASSES NUMBERPERCENT RATIO NUMBER PERCENT RATIOMALES WILD-TYPE EYES562 74.
8% 3. 08,96075. 4% 3. 1 SEPIA EYES189 25. 2% 1 2,92324.
6%1FEMALES WILD-TYPE EYES806 75. 6%3. 1 10,68576. 3% 3. 2 SEPIA EYES260 24. 4%13,32523.
7% 1BOTH SEXES WILD-TYPE EYES1368 75. 3% 3. 019,64575. 9% 3.
1 SEPIA EYES44924. 7%16,248 24. 1%1The dihybrid cross produced a total of 26, 623 flies for all of the sectionscombined. 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 fullwings, and 6. 1% had sepia eyes and vestigial wings. Again, the number offemales produced exceeded the number of males. TABLE 2: F1 Generation Dihybrid Cross of Drosophila melanogaster(+vg+se x+vg+se)PHENOTYPE CLASS RESULTSRESULTS FROM ALL CLASSESMALESNUMBER PERCENT RATIONUMBERPERCENT RATIO WILD-TYPE EYESWILD-TYPE WINGS24447. 8% 6.
3698754. 4%8. 6 WILD-TYPE EYESVESTIGIAL WINGS13225. 9% 3. 4231518% 2. 9 SEPIA EYESWILD-TYPE WINGS9518.
6%2. 4272721. 2%3. 4 SEPIA EYESVESTIGIAL WINGS397.
6%18086. 4%1 FEMALES WILD-TYPE EYESWILD-TYPE WINGS281 51. 1%7. 0761555.
2%9. 3 WILD-TYPE EYESVESTIGIAL WINGS100 18. 2%2. 5239717. 4%2. 9 SEPIA EYESWILD-TYPE WINGS129 23.
5%3. 2295321. 4%3. 6SEPIA EYESVESTIGIAL WINGS407.
3%1821 6. 0%1 BOTH SEXESWILD-TYPE EYESWILD-TYPE WINGS52549. 5%6. 614,60254.
9% 9. 0WILD-TYPE EYESVESTIGIAL WINGS23221. 9% 2. 94,71217.
7% 2. 9SEPIA EYESWILD-TYPE WINGS22421. 1%2. 85,68021. 3% 3. 5SEPIA EYESVESTIGIAL WINGS797.
5%11,6296. 1%1DISCUSSIONThe results from the monohybrid cross for both my class and for all sectionswere very close to the expected results. “Theoretically, there should bethree red-eyed flies for every one sepia-eyed fly. We call this a 3:1phenotypic ratio” (So Whats a Monohybrid Cross Anyway? 2). As indicated intable 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 dihybridcross were not quite as accurate. Mendel hypothesized and proved that adihybrid 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. Intable 2, the ratio shows 6. 6:2.
9:2. 8:1. The data obtained from all classes wereslightly more precise. All sections together (both sexes) produced a ratio of9:2.
9:3. 5:1. There are many reasons that our results did not match the expectedratios. For example, when transferring flies from one vial to another, a fewflies got away which could have a small effect on the numbers. Another factoraffecting the results also happened upon transferring flies. A number of flieswere imbedded in the cultural medium.
We were forced to leave them there sothat we didnt 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. SinceEaster vacation occurred during our lab period, our second generation flies werepermitted to stay together for two weeks instead of one. This may haveresulted in the F2 generation flies mating with their own offspring, thusthrowing off the ratio.
I feel more certain about the results in the “allclasses” column since many more trials were performed and more flies were used. In any experiment, the more trials one conducts, the more accurate the resultswill be. This makes sense when comparing the results from my class versus theresults from all classes combined. The numbers of flies used in each columnmake the difference in trials more evident: 1,060 flies were produced in myclass, whereas 26, 623 flies were produced in all classes.
In the monohybridcross, the ratio for eye color for the females were consistent with the ratiofor males. This information implies that the gene for eye color is not sexlinked. Through research, I found that in Drosophila melanogaster, chromosomeone is the sex chromosome. Eye color is not one chromosome one, but rather onchromosome three.
Therefore, eye color in Drosophila is not sexlinked(Genetics:Drosophila Crosses). In each column, the number of femalesproduced outweighed the number of males. This may imply that the X chromosomeis dominant over the Y chromosome. This would cause the X chromosome to mixwith another X chromosome, producing a female, more often than it would mix withthe 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 beincluded in the data. I would also be sure to remove the flies after just oneweek to reduce breeding between generations. This experiment caused Mendelsfindings to be more concrete and realistic in my mind.
It made the informationmore than meaningless numbers. The experiment also made me realize how easilybiological ideas can be proved. Our results agree with Mendels discoveries. The only drawback to our learning was the massacre of over 26,000 fruit flies. REFERENCESCampbell, Neil A.
, Biology: Fourth Edition. Menlo Park: Benjamin/Cummings,1996. “Genetics: Drosophila Crosses.” Lab Handouts, General Biology Lab, 1996.”So Whats a Monohybrid Cross Anyway?” Lab Handouts, General Biology Lab, 1996.a
