Download www.downloadmela.com - world`s number one free essays website

www.downloadmela.com - world's number one free essays website
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
www.downloadmela.com - world's number one free essays website
ALL CLASSES
NUMBER PERCENT RATIO
MALES
WILD-TYPE EYES
75.4%
3.1
SEPIA EYES
1
FEMALES
WILD-TYPE EYES
3.2
SEPIA EYES
1
BOTH SEXES
WILD-TYPE EYES
3.1
SEPIA EYES
6,248
24.1%
NUMBER
PERCENT
RATIO
562
74.8%
3.0
189
25.2%
1
2,923
3.1
10,685
1
3,325
23.7%
3.0
19,645
75.9%
806
260
1368
75.6%
24.4%
75.3%
449
8,960
24.7%
24.6%
76.3%
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 redeyed 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
www.downloadmela.com - world's number one free essays website
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.
www.downloadmela.com - world's number one free essays website