Corelation Between Genetic Mapping And Physical Mapping

Mapping By “Prime Complementation” –

If one had in hand a set of primes that carried the entire bacterial chromosome, one could mate them, one at a time, into a recipient with a particular mutation and look for the correction of that mutation by complementation or recombination.
Presumably, only the primes carrying the region mutated in the recipient would be capable of correcting the mutant phenotype.
If one thinks about this a bit, it is clear that this form of mapping is a version of deletion mapping where the majority of the chromosome is deleted.
It can also be performed with smaller cloned regions on any replicating plasmid.
This system works because most mutations cause loss of function and are therefore recessive to wild-type.
This approach would fail in an attempt to map trans-dominant mutations.

As noted at the start, it is becoming possible to cut an entire bacterial chromosome into a relatively few pieces (typically with restriction enzymes with unusually large, and therefore very infrequent, target sites) and then to identify the fragment that hybridizes to any cloned piece of DNA.
Since the “marker” used is a hybridization probe, this allows mapping of regions of hybridization and rather than mutations, in contrast to genetic mapping.
The physical mapping of a transposon insertion does both, however, because the hybridizing region is the mutation.
When this approach has been performed on organisms with a preexisting body of genetic information available (e.g. E. coli), a very powerful genetic/physical composite map is generated.
On the other hand, it is unclear to this observer, at least, how such information is of particular use in understanding organisms that lack such a history of genetic characterization, since it simply locates the cloned region on a vast featureless piece of DNA.
This approach will certainly become easier as more “rarely-cutting” restriction enzymes become available and as tools are developed to introduce unique restriction sites into genomes.

The problem of “signal-to-noise ratio”, alluded to in the section on deletion mapping, is an important point.
It should be remembered that most point mutations will revert at a reasonable frequency and for many mapping systems, these revertants will confuse the results and lower the potential resolution of the mapping system.
It should be reemphasized that genetic mapping, and particularly deletion mapping, establishes genetic, and not physical, distances (though rough estimates are possible through use of certain numerical analyses).
Just because one finds more mutations in a particular region of a gene, as defined by deletion mapping, one should not assume that region is large.
It could simply be that region of the protein is critical, so that a disproportionate fraction of point mutations are detected there.
Consequently, this allows you to separate the region into more deletion intervals because the end points of more deletions are separable.