Download Week 4 - Familiarization with Drosophila Fruit Flies v v+ Cy Cy+

Bio 122
Spring 2005
Week 4 - Familiarization with Drosophila Fruit Flies
Introduction
In this laboratory, we will become acquainted with one of the most well-known organisms in
genetics: the fruit fly Drosophila melanogaster. These flies possess several characteristics that
make them particularly suitable for genetic analysis: 1) it is relatively easy to rear large numbers of
them in the laboratory, 2) they have short generation times (2 weeks), 3) they have a few, large
chromosomes. Today we will familiarize ourselves with these flies and we will study the mode of
inheritance of several mutant forms of them.
Some rules for naming alleles and genotypes in flies
In Drosophila, the most common form is called the 'wild type'. A mutant can be either dominant or
recessive to the wild type. When the mutants were discovered, they had to be named. So the locus
is usually named after the mutant phenotype. If the mutant is recessive to the wild type, its name is
given in lower-case, but if it is dominant, it is given in upper case.
Examples
wild type
phenotype
brick-red
eyes
straight
wings
mutant
phenotype
mutant/wild
type
relationship
mutant allele
name
wild type
allele name
bright red
(vermilion) eyes
recessive
v
v+
curly wings
dominant
Cy
Cy+
If it is clear that the wild type refers to the same locus as the mutant, its name is often shortened to
'+'. The two alleles at a locus can be indicated by doubling the letters and separating them by a slash
'/'.
Genotype names for the vermilion eye locus
mutant homozygote
heterozygote
wild type homozygote
v/v
v/v+ or v/+
v+/v+ or +/+
Finally, if you are dealing with a sex linked trait, it is easier to visualize the transmission pattern if
you remember that the locus is segregating along with the 'X' chromosome. So for example, with the
sex-linked white eye locus in Drosophila, you can depict the alleles like this:
Genotype names for the white eye locus
female
male
white eye phenotype
XwXw
XwY
wild type
Xw+Xw or Xw+Xw+
Xw+Y
Bio 122
Familiarization with Fruit Flies
Spring 2007
Activity 1- Familiarization with the flies
The names of mutants often make it seem as if their differences from 'normal' types are
obvious. However, the differences are often much more subtle than one might guess. In order for
you to understand the efforts required to recognize mutants, you will receive a bottle of mutant flies
that are marked with a letter code. You will not be told what the official names of the mutant flies
are. You will have to make up your own names as you complete the exercise.
SPECIFICS:
1. Each group will be given two bottles of flies, one of wild type and one mutant type. Record the
letter designation for your group's mutant flies. Each group will also have at least two fly
anaesthetizers. Write the letter designation of the mutant on a separate piece of white label tape and
put one on each anaesthetizer.
2. Transferring flies from the bottle to the anaesthetizer will be demonstrated. Make sure that the
anaesthetizing chamber (opened from the top) is clean and contains no flies, fly food or debris. If it
does tell your instructor first and then clean it out yourself.
3. Use the unlabeled anaesthetizer for the wild type flies and check to make sure that you use the
correct anaesthetizer for each type of fly. Now you can transfer as many adults as possible from the
bottle to an anaesthetizer.
4. While your flies are being anaesthetized your instructor will show you how to distinguish the
sexes. After they are "asleep" dump each type out onto a separate card marked with its letter.
Identify males and females under the dissecting scope. Then separate flies into two separate groups
according to sex using a small paintbrush (or pencil) to move them. Ask your instructor to verify
that you identified the sexes correctly.
5. Each member of your group will get a new card and mark two distinct areas on it, one area for
your wild type and one for your mutant type. Share the flies on the old cards from step 4, within
your group, distributing several of each type of fly to its respective area on each group member's
new card. Using your dissecting scope, each member of your group will compare the three groups
of flies, trying to distinguish differences in eyes, body color, or appendages. Discuss what you find.
When you reach consensus as to the differences between each of the mutant types and wild for any
characteristic, then call your instructor over and explain what you have found. Use this as a basis to
complete tasks 1 and 2 of Exercise 2.
6. On the basis of your description select a one or two word descriptive name for each mutant type.
Based on that descriptive name, propose a symbol set to represent the alleles at the locus. The
groups will share their discoveries and make a list on the board of all the mutant types to include the
names and symbols for each. When that is done begin to work on task 3 of Exercise 2.
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Bio 122
Familiarization with Fruit Flies
Spring 2007
Activity 2- Description of mutant forms
1. Write a short description of how mutant #1 differs from the wild type.
2. Now examine the mutant type that students from a neighboring table examined. Describe how
their mutant (mutant #2) differs from the wild type.
3. Fill out the following table for the flies you have examined, following the standard rules for
naming your symbol. Remember that these flies come from stocks that breed true with respect to
their particular mutant form. Assume that the mutants are recessive.
fly type
trait #1 only
trait #2 only
both traits simultaneously
wild
mutant #1
mutant #2
4. What would be the phenotype of the offspring of an individual who is a pure-breeding mutant at
both genetic loci crossed with an individual who is pure-breeding wild type at both loci?
5. What would be the genotype of those offspring?
6. Assuming that both loci are autosomal, if you self-crossed those offspring, what possible
phenotypes would you expect to see in the F2 generation?
7. In what ratio would you expect those offspring to occur?
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