Drosophila melanogaster is a small, common fly found near unripe and rotted fruit. It has been in use for over a century to study genetics. Thomas Hunt Morgan was the best biologist studying Drosophila early in the 1900’s. Morgan was the first to discover sex-linkage and genetic recombination, which placed the small fly in the forefront of genetic research. Scientists have used Drosophila for many reasons. For one they are very easy to maintain, breed, anesthetize, and kill with little equipment. They are also very small and it is easy to distinguish males vs females and sexually mature flies and virgins. At lastly, the flies have a very short two week life span. On days 2-7 of their life after they hatch they are in the larva stage. Then on days 7-11 the flies go through the pupa stage where the go into a cocoon. Then on days 11-14 they emerge fully developed as an adult (but females won’t be sexually mature until 8-10 hours after they emerge. Form the time of egg to adult, temperate is crucial to keep the flies alive. The ideal temperate range is 21-23 degrees Celsius (or 69-75 degrees Fahrenheit). The higher temperature range the faster generation time it is, it is the reverse effect for colder temperatures. After the eggs hatch, small, white, worm-like larvae should be visible in the growth medium (blue solid in the bottom of the tube). Look for the black area at the head of the larvae. In addition, as the larvae feed they disrupt the smooth surface of the growth medium.
After approximately 7 days, the larvae will begin to migrate up the vial in order to pupate (make a cocoon). The flies will be grown in small tubes plugged with cotton to allow air into the tube but not to allow the flies to escape. A blue solid known as “growth medium” is placed at the bottom of the tube. This is the food for the organisms. This is the “home” for the flies is called a culture vial. Because the adult flies “fly” the flies must “be put to sleep” or anesthetize them to be able to observe their traits. This is a simple process using the chemical using 50% Triethylamine 25% Ethanol and 25% Fragrances which is called FlyNap®. The flies will remain asleep for 35-40 minutes. To anesthetize, dip a swab into the FlyNap® then slide the swab into the vial while keeping the cotton plug still in. Place the vial on its side with the swab on the bottom. Wait for chemical to take affect about 5 minutes or until it is seen that all flies are asleep. Finally the care for the flies, a new growth medium and transfer the adult flies each generation. In the experiments that follow, the flies will be observed to find how dominant and recessive traits are inherited from one generation to the next. The experiment will use 2 strains of flies and their life cycle. Next, males and females from different strains will be selected and “crossed” or allowed to breed.
Then, the traits will be observed in the flies that are the F1 generation. And finally, the F1 generation flies will be used to cross and create the F2 generation. As the experiment comes to an end, the chance will be given to observe the inheritance of an additional trait that will not follow the traditional modes of dominant/recessive. From the observations, it will conclude how this trait is inherited. The hypothesis for this experiment is that all traits exhibited in the Wild stain of the fly is dominant over the white eye strain, Ebony body stain, and vestigial wings strain no matter the gender combination. So in this experiment it is expected that for all crosses the outcome will be the F1generation having 100% Wild strain, and the F2 generation having 75% Wild stain and 25% of the other strain no matter the gender combination. Materials
Wild Stain of Drosophila melanogaster
White Eye Strain of Drosophila melanogaster
Ebony Strain of Drosophila melanogaster
Vestigial Strain of Drosophila melanogaster
Small Paint Brush
First take one fly of a specific gender wild strain. Then take one fly of a different strain, of the opposite gender. Put both flies into a vial with the growth medium at the bottom (every new vial should have the growth medium) and put the cotton plug on top (always anesthetize the flies when moving the flies in and out of their vial to avoid escapees). When it is seen that the flies have produce larvae that have hatch, kill of the parents. When the larvae have developed into adults, take the out of the vial and observe under the microscope in a petri dish. Their count how many flies there are according to phenotype and gender (use paint brush to move flies gently without squishing them).This is the F1 generation. Record all data. Then select one female and one male to put in new vial (kill of rest). Wait for larvae in the newest vial to appear (then kill of parents) and wait again for these flies to fully mature, this is the F2 generation. Move these flies to the petri dish and look under the microscope. Once again sort the flies by phenotype and gender. Record all Data. Kill of all remaining flies and clean the vials. Repeat all steps for all crosses.
The data shows that in wings and body color are dominant for wild type because in both trials of experimentation, the F1 generation had 100% wild type and in F2 generation had 75% wild and 25% of the recessive trait. But in eye color, the Punnet Squares above showed how the trait is sex-linked and carried on the “X’s” of the flies chromosomes for gender: “XX or XY”. This is why there where different results when the gender of the type of fly was breed (Wild ♂ x White ♀ vs. White ♂ x Wild ♀). One thing that can be learned from this experiment is how sex-linked traits worked. This process has made the idea of sex-linked traits easier to understand and simple because it can physically be seen how sex-linked traits work instead hearing and reading about it. In the class experiments, the same percentages were received for the crosses as in the computer simulation. In the Wild wings x no wings (This experiment used and Ebony fly instead of wild to represent having wings), The F1 generation showed 100% having wings. And the F2 generation showed a little over 75% having wings and less than 25% having no wings which are close to computer results. Unfortunately, our eyes color cross died during the F1 generation. And the body color notes were lost. But it if had occurred as planned, those two crosses would have also agreed with the computer results.