Biology IB HL
March 25th 2010
Fruit Fly Lab
Genes can either be sex-linked or autosomal. If a gene appears mostly in one sex chances are the gene is sex-linked and if it appears frequently in both sexes it is most likely autosomal. Using Drosophila melanogaster, also known as the fruit fly, we will determine whether the gene is sex-linked or autosomal. Drosophila melanogasters have a relatively short life span and are an excellent organism for genetic studies because it has simple food requirements, occupies little space, is hardy, completes its life cycle in about 12 days at room temperature, produces large numbers of offspring, can be immobilized readily for examination and sorting, and has many types of heredity variations that can be observed with low power magnification. Drosophila has a small number of chromosomes (four pairs). These chromosomes are easily located in the large salivary gland cells.
The idea of this experiment is to give a clearer understanding between autosomal, sex-linked, recessive and dominant genes. Furthermore, with this knowledge we can then understand how a gene is passed down through its generations and why it would appear in one individual and not in another. The experiment also helps give a better understanding of genotypes and phenotypes and what role they play in determining and predicting a genotype.
Null: The wingless mutation is autosomal recessive
Alternative: the wingless mutation is sex-linked
The Life Cycle of Drosophila
The eggs: The eggs are small, oval shaped, and have two filaments at one end. They are usually laid on the surface of the culture medium and, with practice, can be seen with the naked eye. The eggs hatch into larvae after about a day.
The larval stage: The worm like larvae eats most continuously, and its black mouth parts can be seen moving back and forth even when the larvae are less distinct. Larvae tunnel through the culture medium when eating; thus channels are a good indication of a successful growth culture. The larvae molt twice as it increases in size. In the last of the three larval stages, the cells of the salivary glands contain giant chromosomes, which may be seen readily under low-power magnification after proper staining.
The pupal stage: When a mature larvae in a laboratory culture is about to become a pupa, it usually climbs up the side of the container or on to a paper strip provided in the culture bottle. The last larval covering becomes harder and darker, forming a pupal case.
The adult stage: When metamorphosis is complete, the adult flies emerge from the pupal case. They are fragile and light in color and their wings are not full expanded. These flies darken in a few hours and take on the appearance of an adult fly. They live a month or more and then die. A female does not mate for 4-6 hours after emerging from the pupa. Once she has mated, she stores a considerable quantity of sperm in receptacles and fertilized her eggs as she lays them. To ensure a controlled mating, it is necessary to use females that have not been mated before (virgins).
Design of the Exercise
This genetics experiment will be carried on for several weeks. Drosophila with well-defined mutant traits will be assigned to you by your teacher. You are responsible for making observations and keeping records concerning what happens as mutant traits are passed from generation to the next.
You will be assigned to study a certain mode of inheritance using particular genetic crosses of flies having one or two mutations. The modes of inheritance most commonly used are:
•Monohybrid. In these experiments, the mode of inheritance is determined when a single contrasting pair of traits is involved. •Dihybrid. In these experiments, the mode of inheritance is determined when two pairs of contrasting traits are considered at the same time. •Sex-linked. In these...