The mutation in the P-element insertion line that each group isolated as the flightless mutant is dominant flightless, recessive lethal. When the flies were originally isolated, they were found to balance over the CyO balancer. Therefore, we know that the mutation must lie somewhere on the second chromosome. In order to perform the flight testing, we crossed the flies with white-eyed, wildtype flies to remove the Curly-wing phenotype. The flies that you flight-tested are heterozygous for the P-element insertion.
In order to roughly map the mutation to a physical location on the second chromosome you could carry out genetic mapping experiments in which a mutation is mapped based on the frequency of recombination with other mutations on the same chromosome. This type of procedure is outlined in all standard genetics textbooks. You should review this on your own. Genetic mapping of P-element insertions in Drosophila is unnecessary. Now that the entire Drosophila genome has been sequenced, P-element insertions can be mapped molecularly by isolating and sequencing a plasmid rescue fragment. Since this fragment lies directly adjacent to the P-element, the sequence of the fragment can be matched to a specific area within the sequenced genome. We will do this in lab. Recombination mapping and deficiency (described below) may still be used in P-element insertion lines to demonstrate that a mutant phenotype is caused by the P-element insertion and not by a spontaneous event somewhere else in the genome.
Mutations caused by P-element deletions, on the other hand should be mapped since it is possible for a P-element to jump several times. It is possible for mutant phenotypes created by "jump-out" mutagenesis to be unrelated to the original P-element insertion. One easy experiment that can be done is to obtain and sequence a PCR product of the genomic sequence flanking the original insertion site. Another option is to deficiency map the new mutant phenotype. A set of deficiency lines, each of which has a deletion of a different region of the chromosomes, is available through the Drosophila stock center. These deficiency lines can be used for complementation testing to determine roughly where a mutation lies.
To carry out complementation testing, flies that are heterozygous for a mutation are crossed with each of the deficiency lines in the area of the chromosome of interest. The deficiency lines are also heterozygous for the deficiency. Both mutants should be balanced over the same balancer chromosome. Therefore, it should be easy to recognize flies that carry both mutations by the absence of the balancer markers.
For example, a deficiency cross for the second chromosome could have the following general genotype:
P/CyO x Def/CyO
What dominant phenotype would be absent in flies that carry both the P-element insertion and the deficiency mutation?