Part 1: Observing mutant strains of flies
Be certain students have written descriptions of their mutant flies
before they begin. They have done this with wild-type flies already
so this should not be a problem now. All students should use wild type
for one cross with a mutant. Other crosses are up to the students. Some
crosses may exhibit lethality, sex-linkage or other patterns that can
be quite interesting for students to analyze. Challenge some students
with multiple phenotype flies such as vestigial/ebony (reduced wings
and darker body). Encourage multiple crosses but be sure they are recording
in their journals and on the vials. Remind them that this is good science.
to buy flies
of mutant flies
Part 2: Deciding on your crosses
You may wish to verify crosses students select so there are no duplications
within a group. The idea of replicates may need further explanation.
This is standard procedure in scientific labs. In addition, the F1 of
a monohybrid cross may generate more than two phenotypes. This is especially
true with sex-linked traits. Students need to note the sex and phenotype(s)
of each fly in the F1 and F2 generations.
Part 3: Obtaining virgin females of each strain
In "An Introduction to Drosophila
melanogaster" there are several ways to obtain
virgin females. However you choose, it is very important
that they are virgin. Students will be almost a week behind if they
see larva in these vials.
Part 4: Making your first generation (F1) cross
Multiple crosses can be made from the same parents. Remind student to
make reciprocal crosses. For example, if they cross white-eyed females
with wild type males then the reciprocal cross would be a cross between
white-eyed males with wild type females. Replicates are made of each
cross. After 4-5 days, transfer parents into a fresh vial. Parents must
be discarded before adults hatch.
Part 5: Examining the F1 generation
Ideally, students should construct their own table.
Part 6: Making the F2 cross
Students should be more comfortable making this cross. Since it has
been 12-14 days since they last worked with these flies, you may need
to have them review their notes and procedure before beginning this
Part 7: Analyzing your crosses
1. These assessments will illustrate basic Medelian concepts. It is
important that each group sees other group's data.
2. Check that journal entries are accurate and complete.
3. Making Punnett Squares of their crosses.
Students will see different outcomes depending on the crosses they do.
How will they explain this? It is assumed they have an understanding
of meiosis, independent assortment and at least a rudimentary understanding
of dominant and recessive traits. If they have an understanding of alleles,
and homozygous and heterozygous genotypes, the results should be more
apparent. If not, this is the perfect opportunity to introduce these
After students understand how meiotic division can result in different
gametic combinations, then they are ready to make Punnett Squares of
their crosses. They will need to know:
1. Which traits are dominant and which are
2. Are any traits sex-linked
3. What the parents alleles are
4. How to construct a Punnett Square
Provide students with examples of Punnett Squares, paying particular
attention to the gamete possibilities of the parents. Students should
construct Punnett Squares of their crosses on poster board and present
them to the class.