Then from the existing genetic maps of these isolated individual chromosomes (or segments), we use probes that serve as anchor points on the physical map.

The gaps between these anchor points are then filled by extensive Southern analysis. This technique has already yielded restriction maps for yeast (Saccharomyces pombe) chromosomes.

(ii) The second strategy involves preparation of a complete ordered genomic library. This is a bottom up approach, because we start with individual clones, which are fingerprinted and through detection of overlapping of DNA segments in clones, the latter arc arranged in a large number of contiguous sets of overlapping clones (called contigs), which coalesce into a map of the whole genome.

Utilizing the above two approaches involving restriction mapping and contig mapping, a hybrid map technology has also been suggested. In hybrid maps, we make use of restriction maps, linkage maps (RFLP maps), cytogenetics and somatic cell genetics, in order to orient and align a series of contigs.

In such a map the discontinuity of contig maps is largely overcome and clones will be available that will cover most of the mapped region.