Deletions are chromosomal aberrations in which some portion of a chromosome is missing. Describe a method using Drosophila deletions to determine the actual, physical location of a gene.

Cross homozygous mutant flies to homozygous wild-type flies that have been irradiated (or those with a series of known deletions-see below). Select mutant flies in the F₁. Those of the F₁ that display the mutant phenotype may have resulted from the wild-type allele being deleted by the X-ray treatment. Establish a stock of the exceptional mutant fly; then examine polytene chromosomes in larvae. The mutation in question may be contained in the compensation loop in the homolog of the deleted chromosome. If a series of known deletions is available (and these do exist in stock centers throughout the world), one can test (by the mating described above) the mutation against each deletion. Obviously, this would be a very time-consuming task unless one had some prior knowledge as to the general location of the mutant gene. This can be accomplished using balancer chromosomes and standard linkage determination. Once the mutant gene is "exposed" by a given deletion, a series of additional deletions can be used to "fine map" the region and determine a fairly accurate location for the gene.