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