An example of deletion mapping:
The following question shows the rationale for deletion mapping at the DNA sequence level. The wild-type segment of the arc gene is shown simply for comparison with the mutants.
Note that the DNA sequence is only shown to emphasize the rationale of deletion mapping -- deletion mapping can be done without knowing the DNA sequence of either the wild-type gene, the mutation you are trying to map, or the deletion mutations. (In fact, deletion mapping was done for many, many years before we even knew how to sequence DNA.) The position of a mutation relative to the deletion can be inferred from simple line drawings of the deletion intervals as well. For example, the following "stick figure" represents the mutations shown in the DNA sequences above.
ANSWER: If a mutant allele in the donor is within the sequence corresponding to the region deleted in the recipient, then NO (+) recombinants will be obtained (as in the cross against arc-191). To repair a deletion by recombination, the donor must have "wild-type" DNA sequence in the region corresponding to the DNA deleted in the recipient (as in the cross against arc-192). Note that not all crossovers will yield (+) recombinants -- in this case only those crossover events that occur between the arc-946 allele and the left end of the deletion would inherit the wild-type sequence.
Given these results, you would infer that the mutation lies in the within the fragment of DNA that is deleted in arc-191, but not deleted in arc-192. Using a set of additional overlapping deletions, you could further narrow down the position of the arc-946 mutation.
When deletion mapping many different mutations against multiple deletions, you simply repeat this process for each separate cross.
Deletion mapping asks the question "Do you get any (+) recombinants or not?" The frequency of (+) mutations may be very low, but if you get any recombinants then you infer that the mutation does not overlap with the deletion interval. Remember that the shorter the distance between two genetic markers, the lower the frequency of a recombination event occurring between them. Hence, if the point mutation is very close to the end of the deletion, but not within sequences deleted in the recipient, then the frequency of (+) recombinants will be low.