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Home >>Molecular Maps of Animal Genome >>Molecular Genetic Maps in Humans - Homosapiens
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Molecular Genetic Maps in Humans - Homosapiens - The concept of RFLPs as genetic markers in humans was proposed as early as 1980 (or even earlier), with an objective to facilitate early diagnosis of diseases. Both genomic DNA (gDNA) and cDNA clones have been used, although it was suggested that cDNA sequences being conserved would not reveal much polymorphism.

Random genomic clones art also sometimes unusable due to the occurrence of repetitive DNA sequences in human genome. In view of this, DNA clones suitable for RFLP mapping are first collected using the criteria outlined above.

As discussed earlier, RFLPs are inherited as simple Mendelian codominant markers. Linkage relationships among RFLPs in humans can be established using pedigree analysis involving several generations.

Restriction fragment patterns in individuals can be obtained easily using small volumes of peripheral blood, which should give sufficient lymphocyte DNA for analysis.

The data for restriction patterns from each probe/enzyme combination are recorded for each individual and analysed. for linkage using standard Lod score analysis (explained below).

Although in mouse, recombinant inbred lines (RILs) are produced by inbreeding (sib mating) and have been used successfully for mapping a large number of RFLPs; in humans inbreeding can not be practised and time required for raising another generation is long.

Therefore, alternative approaches have been used. PNA markers are first ordered and their spacings (recombination frequencies) determined by analysis of segregation patterns of the markers within a panel of reference families, which represent finite and limited sample sizes.

Once ordering and spacing are determined between different RFLPs, models are fitted by maximizing the likelihood of the parameters. The likelihoods of different models are compared by calculating the likelihood ratios.

When the likelihood ratio is 1000 : 1(or atleast 100 : 1) then the odds of one model are 1000 (or atleast 100) fold greater than the odds of another, and therefore, the first model is accepted. The 10gIO likelihood ratio is called LOD score (e.g. for 1000 : 1 likelihood ratio, 10g10 = 3, LOD score = 3; for 100 : 1 likelihood ratio, LOD score = 2), and is used for linkage analysis.

A LOD score of 1 to 2 is interesting, 2 to 3 is suggestive and> 3 is proof of linkage. It has been shown that no adjacent pair of markers is separated by more than 32% recombination.

Primary genetic linkage maps with markers spaced approximately 15 cM apart are now (1992) available for most human chromosomes. These primary genetic maps are being used for the construction of high resolution linkage maps with markers spaced only 1-2 cM apart.

Such high resolution maps have already been prepared for some specific regions containing specific disease genes, but eventually they will be obtained for the entire chromosomes. They will also be used for the preparation of a physical map of the human genome.

For the construction of the above panel of 59 families (27 from Utah, primary genetic linkage maps, a USA; 29 from France; 3 from Venezuela) containing 1212 informative meioses was used. To provide a permanent source of DNA, cell lines have been established from these 9 families.

Centre for Study of Human Polymorphism or CEPH (Centre d'Etude du Polymorphism Humain) in Pairs has undertaken to distribute )NA from these cell lines to laboratories collaborating to map the human genome. (CEPH was created by Jean Dausset using money for his Nobel Prize and a large legacy.).

The decision to use the same material allowed markers studied in different laboratories to be combined in a single linkage map, and by the end of the year 1993, resolution of 1 cM may prove feasible. A complete 1 cM resolution map of human genome may contain as may as 7000-8000 loci, of which about 3000 loci were already mapped in 1991.

The primary linkage map can be utilized for locating genes for specific disease on a specific region of a chromosome. This is done through the study of cosegregation of disease with a molecular marker in a reference family.

Once linkage of disease with a marker is known through a study of cosegregation, if the marker is already mapped, the chromosome region carrying the disease gene is also automatically known and can be subjected to the development of high resolution map for this region.

From a closely linked marker, one could walk or jump to the disease gene using the technique of chromosome walking jumping, to isolate the gene eventually. This technique has been used for isolation of gene for cystic fibrosis (CF), a recessive disorder of some importance in humans.

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