onase gene was not known.The desirability of a complete linkage map of the human genome was clear. In addition to providing a framework for studying the nature of recombination in humans, it would permit rapid gene localization, assist gene cloning, and facilitate genetic diagnosis. Almost inevitably, the realisation that a comprehensive human genetic map was now attainable sparked serious efforts to construct one. The first generation of DNA markers were restriction length polymorphisms (RFLPs). RFLPs were initially typed by preparing Southern blots from restriction digests of the test DNA, and hybridising with radiolabelled probes (fis 5.12). This technology required plenty of time, money and DNA, and made a whole genome a heroic undertaking. Nowadays this is less of a problem because RFLPs can usually be typed by PCR. A sequence including the variable restriction site is amplified, the product is incubated with the appropriate restriction enzyme and then run out on a gel to see if it has been cut (fig 6.6). A more fundamental limitation is their limited informativeness. RFLPs have only two alleles; the site is present or it is absent. In 1987, after a huge effort, the first such map was published based on the use of 403 polymorphic loci, including393 RFLP (explain)markers (Donis-Keller et al., 1987). Although this achievement was important, there remained some serious draw backs with the map: the average spacing between the markers (greater than 10 cM) was still considerable, and, more significantly, RFLP markers were not very informative and are difficult to type (see box 11.1).High-resolution human genetic maps have therefore largely been obtained through the use of microsatallite markers. Hypervariable minisatallite VNTR (variable number tandem repeat) polymorphisms as an alternative, are highly polymorphic, and were a great improvement, given that they have many alleles and high heterozygosity, however the technical problems of s...
