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STUDYING MECHANISMS OF INHERITANCE WITH DROSOPHILA MELANOGASTER
Use your lecture textbook to define the following terms before coming to lab:
Sex chromosomes
Autosomes
Genes
Alleles
Homozygous
Heterozygous
Hemizygous
Dominant
Recessive
Genotype
Phenotype
Wild-type
Mutant
P generation
F1 generation
F2 generation
INTRODUCTION
Gregor Mendel is known as the “father of modern genetics.” In the 1860’s, he
published the results of his hybridization studies with Pisum sativum (garden pea)
in which he proposed a mechanism for the inheritance of traits from parent to
offspring. In the early 1900’s, following the rediscovery of Mendel’s work, T.H.
Morgan began a series of studies of inheritance in Drosophila melanogaster, the
common fruit fly. This organism has now been used for over 100 years to study
the mechanisms of inheritance in animals.
WHY IS DROSOPHILA USED AS A MODEL ORGANISM FOR GENETIC STUDIES?
When choosing a model organism for biological studies, there are a certain
characteristics which make a model organism ideal: (1) Drosophila are small; you
can house 1000’s of flies in a small area; (2) they have a short life cycle (on the
order of days); (3) they are easy to maintain for multiple generations; (4) they
produce many, many offspring in a short period of time; and (5) they have a
number of different heritable characteristics that are easily distinguishable.
Week 1 – Life Cycle of Drosophila
How to anesthetize fruit flies:
Because Drosophila can fly, they must be immobilized in order to set up your crosses
and analyze progeny. If anesthetizing flies in their original culture vial, use the
following procedure:
1. Obtain an anesthetizing wand. Dip the absorbent end of the wand into the Fly
Nap and remove excess liquid by running the wand across the rim of the
bottle.
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2. Tap the vial of flies against a hard surface so that the flies are gently knocked
to the bottom of the vial.
3. Use one finger to push the vial plug slightly to the side and quickly stick the
anesthetic end of the wand into the vial. (Do this quickly or the flies could
escape!).
4. Lay the culture vessel on its side while the flies are anesthetized. (DO NOT
leave the vial upright while the flies are unconscious or you risk drowning
them in the media!)
5. Wait until the flies have stopped moving, then empty the flies out onto an
index card or piece of white paper.
6. Use a dissecting scope or hand magnifying lens to examine the flies.
How to determine the sex of fruit flies:
You need to be able to select both male and female flies in order to have success
with your crosses. Being able to determine the sex of the offspring will also allow
you to see if any of the traits you are observing are sex-linked.
Physical characteristics that are useful for distinguishing males and females are as
follows:
Body size – males tend to be slightly smaller than females
Tip of abdomen – dark in males; lighter, striped & more pointed in
females
Sex combs – males have sex combs on the uppermost joint of their
forelegs; females do not
Figure 1: Male (left) and female (right) fruit flies [image credit: NASA]
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Fruit Fly Life Cycle:
The life cycle of Drosophila can take place in plastic culture vials that contain a
source of food. The life cycle has four stages: egg, larva, pupa, and adult. You
should be able to see all four stages present in the cultures you will use today.
The eggs are small, white found on the surface of the food. The larva hatch
within 24 to 36 hours, and burrow into the food at the bottom of the vial; they
look like little wriggling grains of rice. After about 5 days, they crawl out of the
food onto the sides of the vial and transform into brown, oblong pupae. During
the pupation stage, the larvae metamorphose into adult flies. After about 4 days,
the adult flies emerge from the pupae and begin the life cycle anew. The entire
cycle takes approximately 10 to 12 days to complete.
You can watch an animation of the fruit fly life cycle at the following web site:
http://quest.nasa.gov/projects/flies/cycleAnim.html
Setting up our crosses:
The purpose of this mating experiment is to determine if the inheritance pattern of
the alleles that determine the eye color trait are autosomal dominant, autosomal
recessive, sex-linked dominant or sex-linked recessive. This can be determined
by setting up the following crosses: normal males × mutant females and normal
females × mutant males. The offspring will be either normal or mutant depending
on the pattern of inheritance.
A couple of weeks ago, the following parental cross was set up:
White-eyed (mutant) female × red-eyed (normal) males
The offspring (F1 generation) resulting from this cross are as follows:
Males (♂): all white-eyed
Females (♀): all red-eyed
Obtain a culture vial containing flies of this F1 generation. Anesthetize the flies
(as described above) and separate the males from the females using the brush
provided. Check with your instructor to see if you have done this correctly.
Make sure you can easily identify the eye color as this is the trait you will be
observing.
Transfer 3 to 5 female flies and 3 to 5 male flies to a newly prepared culture vial.
Leave the vial on its side until the flies wake up. Label your vial with your group
name, date and lab section.
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Week 2 – Removing the F1 individuals (a.k.a., the parents of the F2 generation)
Observe your culture vials for larvae and pupae. Anesthetize and remove the F1
generation from the vials; discard them in the morgue.
Week 3 – Examining the F2 generation
Anesthetize and observe the adult flies in the F2 generation. Identify the sex and
phenotype of each individual. Record your data (along with the data from the
other six lab groups) in the table below. Discard the counted flies in the morgue
after determining their phenotypes
Table 1: Data for Phenotypes by Sex of the F2 generation from your cross:
LAB
GROUP
MALES
RED-EYED
FEMALES
WHITE-EYED
1
2
3
4
5
6
7
8
Class
Total
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RED-EYED
WHITE-EYED
Analysis of the Results:
Using the data in the table above, answer the following questions:
1. Which trait is controlled by a dominant allele: red or white eyes?
2. Which trait is controlled by a recessive allele: red or white eyes?
3. Does this pattern of inheritance indicate that eye color is autosomal or sex-linked?
4. What are the genotypes of the individuals who were in the P generation?
♂________________× ♀__________________
5. What are the genotypes of the F1 generation?
♂________________× ♀__________________
6. Determining phenotype ratios:
a. In the space below, calculate the expected ratio of phenotypes in the F2
generation.
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b. What is the actual ratio of red-eyed to white-eyed flies?
c. How does the actual ratio compare to the expected ratio?
d. If there is a difference, propose an explanation for this difference?
7. In the F2 generation, what proportion of each phenotype (red-eyed or white-eyed)
was probably
a. Homozygous?
b. Heterozygous?
c. Hemizygous?
8. Male to female ratios:
a. What was the expected male to female ratio?
b. What is the actual male to female ratio?
c. Why were you asked to keep track of the number of males and females?
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