outhern blotting and radioactive probes were still an obstacle to easy mapping, and VNTRs are not evenly spread across the genome, with their applicability to genome-wide maps being limited because they are mostly restricted to chromosomal regions near the telomeres. Microsatallite markers (also described as short tandem repeat polymorphisms, or STRPs) have the advantage of being abundant, dispersed throughout the genome, highly informative and easy to type (see box 11.1). The advent of PCR finally made mapping relatively quick and easy. Minisatallites are too long to amplify well, and so the standard tools for PCR linkage analysis beacame microsatellites. These are mostly (CA)n repeats. Tri- and tetranucleotide repeats are gradually replacing dinucleotide repeats as the markers of choice because they give cleaner results- dinucleotide repeat sequences are particularly prone to replication slippage during PCR application. Each allele gives a little ladder of “stutter bands” on a gel, making it hard to read (fig 6.8). Much effort has been devoted to producing compatable sets of microsatallite markers that can be amplified together in a multiplex PCR reaction and give non overlapping allele sizes, so that they can run in the same gel lane. With fluorescent labelling in several colours, it is possible to score up to ten markers on a sample in a single lane of automated gel. By focusing on this type of marker, researchers at the Genethon laboratory in france were quickly able to provide a second generation linkage map of the human genome (Weissenbach et al., 1992). Subsequently, maps have been produced with ever increasing numbers of genetic markers, especially microsatellite markers, and ever increasing resolution. Within a further two years, a genetic map with 1 cM resolution had been achieved (Murray et al., 1994). After this time the major effort switched to the construction of high-resolution physical maps.Like the genetic ...
