Genetic Markers

The mapping of human disease genes requires genetic markers. Any polymorphic Mendelian character can, potentially, be used as a marker to map a disease gene. DNA polymorphisms within the human genome have provided sets of markers sufficient in number and density that have been mapped to a specific chromosomal location. Complete genetic maps allow gene localization resulting in genetic diagnosis. The first generation of DNA markers was based on restriction fragment length polymorphisms (RFLP) that were typed by Southern blotting from restriction digests of the test DNA and hybridized to radiolabelled probes. The disadvantage of this type of mapping was the presence of only two alleles, the site is either present or absent and so is not very informative. Donis-Keller et al. published the first comprehensive human genetic map in 1987 based on 403 polymorphic loci, 393 of which were RFLP markers [16]. The average resolution of the markers was about 10 centiMorgans (cM) and, as previously mentioned, they were not very informative. The development of the polymerase chain reaction (PCR) has greatly facilitated genetic mapping by allowing amplification of the marker region and analysis on automated gel analysers, and more recently capillary electrophoresis [43]. Now markers are more commonly labelled with a fluorescent dye, amplified by PCR and analysed on an automated system.

The most popular polymorphisms used presently for physical mapping are microsatellites. The discovery of the highly polymorphic microsatellite sequences within the genome has allowed the construction of genome-wide linkage maps with a high resolution [51, 63]. Microsatellites are abundant, dispersed throughout the genome, highly informative and easy to type by PCR. Analysis on automated systems then allows the genotypes to be read directly as sizes of alleles in base pairs. Human genetic mapping has focused on the construction of high-resolution linkage maps and the human genome now contains thousands of markers covering the entire genome with a density of less than 1 cM [58].

How To Reduce Acne Scarring

How To Reduce Acne Scarring

Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.

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