Mode of Inheritance
of
Bar Eyes
in
Drosophila melanogaster
Zesarae Bodie
PCB 3063- Genetics
Dr. M. A. Thornton
Fall 2004
Abstract
The question I sought to answer in this experiment with the Drosophila
melanogaster was one of inheritance. I was provided with two samples of flies one
identified for a certain mutation and the other being wild type. The mutation present was
in Strain A and was thin red eyes. In the experiment I bred the flies during a six week
period and identified whether the mutation was inherited dominantly or recessively. I also
identified any cases of sex linkage in the specified mutation. After performing reciprocal
crosses of the parent strains to produce a viable F1 progeny the flies were assessed for
sex linkage and dominant inheritance. These were then self-crossed to produce the F2.
After analyzing the phenotypes of the F2, it was determined that the mutation was
inherited autosomally and that it is recessive.
Introduction
The Drosophila melanogaster also known as the fruit fly belongs to the order
Diptera. The D. melanogaster lives in temperate regions because the offspring are
extremely dependant on temperature. The adults feed on the bacteria of rotting plants
and fruits, while the eggs are usually laid on unripened or slightly ripened fruit. The adult
fruit fly is normally a yellow brown or tan color and measures in at about 3mm in length
and 2mm in width. The shape of the fruit fly consists of a rounded head with large, red
compound eyes; three smaller eyes, and short antennae. They have a single pair of wings
that form on the middle segment of its thorax. They have black stripes on the dorsal
surface of its abdomen, which are used in the determination of gender in the fly. The
males tend to have a greater amount of black pigmentation concentrated at the posterior
end of the abdomen. The females are slightly larger than the males. D. melanogaster
reproduction is rapid with a single pair of flies producing hundreds of offspring within a
couple of weeks, and the offspring becoming sexually mature within a week.
The life cycle of D. melanogaster lasts for about fourteen days at 22oC. There are
four stages in the life cycle: egg, larva, pupa, and adult. The D. melanogaster egg is about
half a millimeter long. Approximately one day after fertilization the embryo develops and
hatches into a worm-like larva. The larvae emerge from the egg and burrow into the
medium where they feed on yeast. The larvae go through three instars (periods of growth
separated by molts). The first two instars last for about one day and the third instar for
about three days were they reach a length of 4.5mm. The larvae then crawls to the dry
surface, attach itself with ‘glue’ secreted by the salivary gland, and start to pupate.
During the pupa stage, metamorphosis (reorganization of tissue) occurs for six to seven
days. The adult emerges from the pupa case and for the next few hours they puff up their
bodies, extend their wings, harden their cuticles, and become darker. After about eight to
eleven hours the adult flies are ready to mate (Manning, 2004).
For almost a century D. melanogaster has been studied in genetic research
laboratories. D. melanogaster were first introduced into the world of scientific research in
1901 by W.E. Castle, who recognized that the ease of breeding, the short life span and
simple care were factors that made D. melonogaster the perfect candidate for inbreeding
studies in laboratories (Ashburner eds., 2). His experiments ended in 1905 and a year
later his results were published nationally. In 1910 Thomas H. Morgan used the flies to
provide the first proof that the chromosomal theory of inheritance is correct. He
discovered the first mutant allele coding for white eye color and determined it to be
inherited through X-linkage (Ashburner eds., 3). In 1913 H. Sturtevant, one of Morgan’s
students, created the first genetic maps using D. melanogaster, since that time the
genome of D. melanogaster has become very well known.
The question posed by this lab was the mode of inheritance of the alleles for eye
size and how it is transferred to future offspring. It is our hypothesis that the inheritance
of the mutant allele for eye size is sex-linked, while it is inherited recessively.
Materials and Methods
I began the experiment by making reciprocal crosses of Strain A and Strain W.
I began our reciprocal crosses by mating males and virgin females from each strain
supplied. In order to distinguish the males from the females I utilized a form of anesthesia
known as ‘Fly Nap’ to ‘knock’ the flies out. I then viewed them under the microscope
and sorted them accordingly. While the flies were still ‘napping’ I prepared four bottles
with food to put them in when they arose. To prepare the food we added one cup of dry
food mixture to one cup of cold ampicillin-treated water. Once the food solidified I
added 8-11 grains of yeast to the tubes. When the flies arose I mated Strain A males with
Strain W virgin females in two of the tubes and Strain A virgin females with Strain W
males in the other two tubes. About 7 days after performing the crosses the parents were
removed to avoid their mixing with the F1 generation. Approximately 2 weeks after first
performing the cross, the F1 started to emerge. I then put these flies to sleep and counted
the progeny to determine sex and phenotype. After a back up supply of the F1 generation
was made, the F1 flies from both crosses were allowed to self-mate and produce the F2
generation. The process was the same and in 7 days I removed these new parents from
the culture. When the F2 finally emerged we analyzed them phenotypically and sexually
and arrived at the conclusion that our hypothesis was in fact correct. The resulting
phenotypic ratios of the F2 generation were 1: 1: 1: 1 for the Strain W female/Strain A
male and 1: 1: 1: 1 for the Strain A female/Strain F male, allowing me to conclude that
our hypothesis was partially correct and that in fact bar eyes are sex-linked but it is also
dominant.
Results
The F1 and F2 progeny of the D. melanogaster were sorted and counted according
to their phenotypes. The flies could were either male or female and were either wild type
or exhibited the bar eye mutation. In our tabulations we had a total of 200 F1 progeny and
446 F2 giving a total of 646 flies. In the first cross of females from strain W and males
from strain A we got the F1 exhibited in Figure 1. Of the 100 flies counted, 58 of them
were females who exhibited the Bar-eyed phenotype. The other 42 were males and none
of them exhibited the phenotype. The second cross of females from strain A and males
from strain W yielded another 100 F1 progeny. Of this 100, 52 were females and 48 were
males. All 100 of these flies exhibited the mutant trait. When the F1 were self crossed,
the numbers began to vary. In the first cross between W females and A males, we
achieved 240 F2 progeny: 63 female, bar-eyed, 54 female, wild-type, 62 male, bar-eyed
and 61 male, wild-type (Figure 3). For the second cross between W males and A
females, we successfully counted 206 F2 progeny: 60 female, bar-eyed, 42 female, wild
type, 46 male, bar-eyed and 58 male, wild type (Figure 4). A Punnet Square was then
drawn up to determine the genotypes exhibited by the F1 and F2 progenies (Tables 1 and
2).
Figures and Tables
Figure 1
F1 - A males x W females
males
42%
females
58%
Individuals
exhibiting bar-eyed
phenotype
Figure 2
F1 - W males x A females
females
52%
males
48%
Individuals
exhibiting bar-eyed
phenotype
Figure 3
F2 - Self Cross of
Bar Eyed Females (XBXb) x Wild Type Males (XbY)
26%
25%
males, wild
males, bar-eyed
females, wild
23%
26%
females, bar-eyed
Figure 4
F2 - Self Cross of
Bar Eyed Males (XBY) x Bar Eyed Females (X BXb)
30%
22%
males, wild
males, bar-eyed
females, wild
20%
28%
females, bar-eyed
Table 1
F1-A females and W males
Xb
Y
XB
XB Xb
XB Y
XB
XB Xb
XB Y
Table 2
F1-W females and A males
XB
Y
Xb
XB Xb
Xb Y
Xb
XB Xb
Xb Y
Table 3
F2: A(female) x W(male)
Wild type female
Wild Type male
Bar-Eyed female
Bar-Eyed male
Total
chi-squared value
p-value
Observed Expected O-E
(O-E)2
(O-E)2/E
42
51.5
-9.5
90.25
1.75
58
51.5
6.5
42.25
0.82
60
51.5
8.5
72.25
1.4
46
51.5
-5.5
30.25
0.59
206
4.56
0.15
Table 4
F2: W(female) x A(male)
Wild type female
Wild Type male
Bar-Eyed female
Bar-Eyed male
Total
chi-squared value
p-value
Observed Expected O-E
54
60
61
60
63
60
62
60
240
(O-E)2
6
9
7
2
(O-E)2/E
36
81
49
4
0.6
1.35
0.816
0.667
3.43
0.28
Discussion
The proposed question was the mode of inheritance of the alleles for bar eyes and
how it is transferred to future offspring. My hypothesis was that the inheritance of the
mutant allele for bar eyes is sex linked and recessive.
I
was given the mission to find the differences between sex linked and autosomal
traits and their contribution to the modes of inheritance in all living species. I was to
determine this by crossing flies, distinguishing which traits were sex linked and which
were autosomal. After performing the first cross between the wild-type female (strain W)
and the bar-eyed male (strain A) we received a progeny of fifty-eight (58) bar-eyed
females and forty-two (42) wild-type males for the generation. This progeny did not
contain males possessing the bar-eyed allele or females possessing wild-type eyes. This
led me to believe that there was some type of sex-linkage present. After the second cross
between the F1 generation we observed 63 female, bar-eyed, 54 female, wild-type, 62
male, bar-eyed and 61 male, wild-type. A punnett square was set up for the F1 cross and
a ratio of 1:1:1:1 was ascertained. This ratio was tested using the Chi-square analysis,
receiving a 3.43 Chi square value which was further tested against the Chi-square chart
proving to be in the accepted level cutoff.
The second experimental cross was conducted between the wild type males and
the bar-eyed females resulting in a progeny of 52 females and 48 males. All 100 of these
flies exhibited the mutant trait. A self-cross between this F1 generation was performed
and the observed progeny was 60 female, bar-eyed, 42 female, wild type, 46 male, bareyed and 58 male, wild type. Another punnett square analysis was performed for the F2
of the cross. The observed ratio was again 1:1:1:1 this ratio was tested under a chi square
analysis giving us a chi square value of 4.56. This new value was placed against the chi
square chart and it too fell under the acceptable range.
The problems that arose during the experiment itself were few yet significant.
One of the reigning problems was lack of time. I did not have time to produce a larger
number of offspring in order to make sure my results were as accurate as possible, and
also if any mistakes were made we did not have time to redo anything. Another problem
was over exposure of the flies to anesthesia. As the weeks went on this problem lessened
as my experience increased.
Despite the obstacles I had to overcome, I was successful at completing the
research and coming up with a viable conclusion proving that my hypothesis was at least
partially accurate and that bar eyes are sex-linked and dominant.
Works Cited
Ashburner, M. & Novitski, E. (Eds.), 1976. The Genetics and Biology of Drosophila:
Volume 1a. San Francisco: Academic Press.
Manning, G. (2004). A quick and simple introduction to Drosophila melanogaster.
http://www.ceolas.org/fly/intro.html. Accessed Dec. 5, 2004.