BIOL 212 Genetics Lab
Fall 2006
Lab 2: Introduction to Drosophila
Purpose:
1. To become familiar with the Drosophila life cycle and techniques for handling
Drosophila. Learn to reliably distinguish male from female flies. Observe phenotypic
differences among mutant strains of D. melanogaster.
2. To use computer simulations of genetic crosses to investigate questions about the
inheritance of traits in Drosophila, for example to determine whether traits are dominant
or recessive, sex-linked or autosomal.
References:
Carolina Drosophila Manual, esp. pp. 8-13.
Hartl and Jones, Chapter 2.
Web site: http://www.sciencecourseware.org/vcise/drosophila/Drosophila.php
Materials:
Wild-type Drosophila melanogaster.
Various mutant stocks of D. melanogaster.
Dissecting microscopes.
Fly equipment (Fly nap, brushes, index cards, vials, fly food).
Laptop computers/wireless internet
Introduction:
Fruit flies have historically been one of the best organisms for studying genetics.
In 1911, Thomas Hunt Morgan discovered that a mutant trait for eye color in flies (white
eyes) was carried on the X chromosome (one of the sex chromosomes). Alfred
Sturtevant, a student of Morgan’s, constructed the first linkage map of Drosophila genes
in 1913, showing the order of several genes on the X chromosome. Today, labs studying
Drosophila are making great progress in understanding the genetic bases for processes
such as development and cancer.
What are the properties of fruit flies that make them such useful experimental
organisms for genetics?

Controlled matings can be carried out between flies with different genotypes.
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
Flies have short generation times (about 2 weeks) and produce many hundreds of
progeny.

The chromosomes of the salivary gland of fruit flies are very large. By comparing
staining patterns of the chromosomes of certain mutants with normal chromosomes it is
possible to precisely locate gene positions on the chromosomes.

A large number of mutant strains of Drosophila are available and the chromosomal
linkage arrangements of many genes are known.

Using modern molecular techniques, it is now possible to isolate individual fruit fly
genes (by cloning), to insert genes into fruit fly embryos (by microinjection) and observe
their phenotypic effects.

The base sequence of the entire fruit fly genome is now known (it was a practice genome
for completion of the Human Genome Project.
Procedure:
I. Introduction to Drosophila
1. Learn to anesthetize fruit flies and be able to identify each stage of the Drosophila
melanogaster life cycle under a dissecting microscope. If available, you may want to
work with a “wild type” (or phenotypically normal) vial first. Dump the adult flies into a
clean anesthetizing bottle with Fly Nap (instructor will demonstrate), examine them and
then look at other stages of the life cycle (larvae, pupa) from the walls of the vial.
Beware of the following pitfalls: Do not overanesthetize flies or get them wet with water
or anesthetic--they will die! Also, try to keep the light from the dissecting microscope
from overheating the flies. Dead flies should be discarded in the morgues (vials of
ethanol) provided.
2. Learn to reliably distinguish the sex of adult flies. Can you explain the reasons for the
sex differences you observe? Once you can sex the flies, have your instructor observe
several flies you have sorted by sex and initial the last page of the handout. Answer the
question on the last page on sexing Drosophila. Save some of these sorted flies or some
of the mutants from below to set up a stock culture (part III).
3. Examine several mutant stocks of D. melanogaster and make side by side comparisons
to wild-type flies. What phenotypic differences do you notice? Do all flies with the same
mutant genotype have the same phenotype, that is, is the penetrance of each mutation the
same? To what degree is the effect of the mutation expressed by different individuals
(what is the expressivity?)?
4. Next, examine the demonstration microscopes. Record the mutant traits and gene
symbols in the table on the last page of the handout. Observe the flies carefully under the
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dissecting microscope, and if necessary consult the Carolina manual or the internet.
Complete the table on the last page of the worksheet.
II. Genetic crosses and stock cultures: Real fruit flies
In a few weeks, we will set up crosses of adult flies with different phenotypes
(male wild types with female mutants). Today, we will just practice setting up some
stock (pure breeding) cultures. To set up a cross, prepare a fresh vial of fly medium. Add
one small vial of water to a vial of dry powdered medium, let stand for a few minutes.
After you have been checked out for sexing the flies, take 6 anesthetized males and 6
females of the same type (all wild type or all the same mutant type) and combine them in
a new vial. Lay the vial on its side until the flies wake up from the anesthesia (several
hours or overnight). Label the vial with your initials and those of your lab partner, today's
date and the contents of the vial (wild type or particular mutant type). Progeny flies (the
F1 generation) will begin to emerge in about two weeks.
For future reference: To do controlled crosses of fruit flies with predictable genetic
outcomes, you must use females that have not mated (virgins). To collect virgin flies,
one would clear the mutant stock vial of all adult flies. Flies that emerge within 12
hours are virgins. Most flies emerge in the morning, so it is best to clear vials the night
before you want to collect virgins. Another option is to clear the vials early in the
morning and check them during the day as the flies emerge. One would then anesthetize
and sort the virgin flies by sex immediately after collecting them. The virgin female flies
would be kept for your crosses; the males will be discarded in the morgue. Males for the
crosses we will perform will come from wild type stocks and need not be virgin; they can
be anesthetized and sorted and female wild types can be discarded.
3. "Virtual" fly crosses
Simulations of fruit fly matings will be used to give you practice with simple
genetic crosses and their outcomes, with much less stress to you (and to the fruit flies).
Go to the web site http://www.sciencecourseware.org/vcise/drosophila/Drosophila.php
Select two different traits from the instructor’s basket and record each on the worksheet.
For this exercise, select “enter as guest” (double click). The “Lab Bench” window
comes up. Follow the on-screen instructions. Click on the computer image to order flies.
In the computer options screen, click on order flies. In the right hand menu, locate the
mutant phenotype from one of your two cards and click on it, to select a female with the
trait. In the left panel, click on “add to cart”. Next, in the left panel, select male wild
type and add to cart. Click on the shopping cart icon in the upper right hand corner. If
the two flies in the cart are correct, then click on checkout and confirm. Follow the
instructions on the screen to unpack and mate the flies, incubate them, anesthetize them,
and observe them under the microscope. Click on “sort flies”. You may want to use the
zoom in/zoom out features to take a closer look at the progeny flies. Click on “add to
notebook” to transfer the results to the notebook. Click on send data to the computer.
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Record your results on the worksheet or attach a printout. To repeat the process for your
other mutation, click on “return to lab”. You may want to click on the trash can (clean
up), so you don’t get your stocks or crosses mixed up.
4. Use of genetic symbols
Conventionally, the “normal” fly phenotype is referred to as wild type, which is
symbolized as +, or using a superscript on the genetic symbol for the mutant trait (for
example w+ = wild type, while w = white eye mutant). Recessive traits in Drosophila are
usually given 1-3 letter designations using lower case (dp=dumpy, v=vermillion);
dominant traits are more rare, and are given 1-3 letter designations, beginning with an
uppercase letter (B=Bar, Cy=Curly). To avoid confusion, it is best to use the already
assigned symbols for a particular trait. To learn what these are, you may need to consult
the Carolina manual, the internet, or other sources.
Assignment:
The laboratory practical worksheet should be completed today, if possible. The
practical is worth 10 points.
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Drosophila Laboratory Practical
Name______________________________
Biology 212 Lab
Section __________________
1. Sexing Drosophila. What are the major differences in phenotype (outward
appearance) between male and female fruit flies?
 Anesthetize a vial of Drosophila (can be wild type or mutant). Sort about a
dozen flies based on their sex. Once you are confident you can tell the males
from the females, have the instructor check your sorting.
OK for sexing fruit flies _______________________
2. Set up a stock culture of a particular Drosophila strain and show instructor.
stock________________________ OK for stock cross______________________
3. Drosophila mutants
 The demonstration microscopes illustrate several Drosophila single mutants.
What single characteristic is affected in the mutants? COMPLETE THE
TABLE.
Microscope Mutant
Station
Gene
symbol
Describe trait
#1
#2
#3
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Does this trait
affect wing, body
color, eye color
OR eye shape?
4. Virtual crosses
Select cards for two different Drosophila mutants from the instructor's basket. Use
the virtual fruit fly lab to carry out crosses of true breeding (homozygous) female
mutant flies with true breeding male wild type flies and complete the table below
(the results for the white eye color trait are shown as an example).
mutation
phenotype
gene
symbol
dominant or
recessive
trait
white
white eye
color
w
recessive
X-linked* F1 progeny
or
phenotypes
autosomal
trait
X-linked
Wild type
females,
White eye
males
*X-linked traits typically are passed from mothers to sons and fathers to daughters.
If the F1 progeny display different phenotypes in males vs. females, most likely the
trait is X-linked.
5. Collecting virgins for crosses. What is the maximum amount of time that can pass
after an adult female fly has emerged from its pupa case to ensure that she is a virgin?
___________________________
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