Student Lesson Two
Making crosses!

Crossing Flies: The crossing of wild type flies and mutants, and examining the resultant outcome of F₁ and F₂ generations.

Materials:

1.  Stereomicroscope
2.  Anesthetizing system
3.  Paint brushes
4.  Vials with media,
5.  Vials of mutant and your wild type flies.

You will receive one or more different vials of flies – each a mutant type of fly. You will also need your wild type cultures from lesson one. You will obtain virgin females from each strain of fly and cross them with different strain.  

Part 1: Observing mutant strains of flies

Different types of the same species are called strains.  Wild-type flies are one strain and each mutant is considered a separate strain of Drosophila melanogaster, even though they are the same species.

In your journal, review the external anatomical features of wild-type strain from lesson one. Next, obtain a vial of mutant flies, anesthetize them and use the stereomicroscope to record you observations as you did with the wild-type, noting external characteristics of eye color, wing shape and size, antenna shape and size, body color and body striping.  In your journal, draw these different strains and write a paragraph describing how they are different from wild-type flies.

Part 2: Deciding on your crosses

You will be adding males of one strain into a vial containing virgin females of another strain.  After larvae appear in the food, you will release the parents, allow the larvae to develop into adults, and then look at their phenotypes to see if there are any differences. How long will it take for the offspring to become adults? 

You need to decide which females to mate with which males. Since each vial of flies contains purebred strains, mating a male and female of the same type will not produce phenotypic differences

Part 3: Obtaining virgin females of each strain

For each strain you will need to collect 5 virgin females.  Your instructor will explain how to do this.  It is critical to insure that the flies you collect are virgins.  In your journal, explain:

1) How you collected your virgin flies
2) How these flies look different than mature flies
3) Why you should have more than one fly in a vial

Each strain must be place in their own vial. For example, a group may have 5 wild-type flies in one vial, 5 of another mutant strain in a second vial, and so on.  Remember to label each vial and record this information in your journal.

Scientists will make multiple vials of the same strain so they have more data and to prevent the “all your eggs in one basket” situation.  Therefore, you should have at least three replicates for your crosses.  Replicates are replications of the same thing, so in this case you should have three vials, each with 5 virgin females, of each strain you are crossing. Label your replicates as you did with your first vial.  Remember to record everything in your journal.

The females need to remain in their tubes for 2-3 days to insure they are virgin.  After 2-3 days, use the stereomicroscope to look for larvae in the food. As you know, eggs are okay since they are sterile. However, if you see larvae, then one or more females were not virgins so you cannot use that female for a cross.

Make backup cultures of your mutant flies.

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