Deletion mapping

Deletion mapping is a technique used to ascertain the location of mutation sites within a gene.

The principle of deletion mapping involves crossing a strain who has a point mutation in a gene, with multiple strains who each carry a deletion in a different region of the same gene. Wherever recombination occurs between the two strains to produce a wild-type (+) gene (regardless of frequency), the point mutation cannot lie within the region of the deletion. If recombination cannot produce any wild-type genes, then it is reasonable to conclude that the point mutation and deletion are found within the same stretch of DNA.

This example should demonstrate how the principle works:

Suppose you have a gene (arc-946), which in wild-type (+) form can be shown linearly like so:

--------------------------------------------------------------------------------------------------------------------- arc-946, +

Suppose a strain of organisms has a point mutation in the gene (now called arc-946, - to denote that it is no longer wild-type):

------------------------------------------------------X-------------------------------------------------------------- arc-946, -

Now suppose you have two strains of organisms, each with deletions in arc-192 at different sites, called del-arc-191 - and del-arc-192 -, respectively:

---------------------(..............................................)---------------------------------------------------------- del-arc-191, -

--------------------------------------------------------------(............................................)------------------- del-arc-192, -

Because the point mutation lies within the deletion of arc-191, there will be no wild-type (+) recombinants. However, in a cross between the point mutant and arc-192, there could be a successful wild-type (+) recombinant produced.

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). In other words, there is a feasible recombination possibility between the point mutant and arc-192 in which a length of DNA could be made that contained neither the point mutation, nor the deletion, indicating to someone who hadn't seen these diagrams that the mutations in these two strains cannot be in the same region.

Note that not all crossovers between the point mutant and arc-192 will yield (+) recombinants, however - 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.