In meiosis, the chromosomes of a deletion heterozygote form a looped configuration. (a) In meiotic pairing, the normal homolog forms a loop. The genes in this loop have no alleles with which to synapse. (b) Because Drosophila polytene chromosomes (found in salivary glands and other specific locations) have specific banding patterns, we can infer which bands are missing from the homolog with the deletion by observing which bands appear in the loop of the normal homolog.
[(b) William M. Gelbart, Harvard University.]