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Chapter 062. Principles of Human Genetics (Part 12)

The Genetic Map Given the size and complexity of the human genome, initial efforts aimed at developing genetic maps to provide orientation and to delimit where a gene of interest may be located. A genetic map describes the order of genes and defines the position of a gene relative to other loci on the same chromosome. It is constructed by assessing how frequently two markers are inherited together (i.e., linked) by association studies. Distances of the genetic map are expressed in recombination units, or centiMorgans (cM). One cM corresponds to a recombination frequency of 1% between two polymorphic markers; 1. | Chapter 062. Principles of Human Genetics Part 12 The Genetic Map Given the size and complexity of the human genome initial efforts aimed at developing genetic maps to provide orientation and to delimit where a gene of interest may be located. A genetic map describes the order of genes and defines the position of a gene relative to other loci on the same chromosome. It is constructed by assessing how frequently two markers are inherited together i.e. linked by association studies. Distances of the genetic map are expressed in recombination units or centiMorgans cM . One cM corresponds to a recombination frequency of 1 between two polymorphic markers 1 cM corresponds to 1 Mb of DNA Fig. 62-3 . Any polymorphic sequence variation can be useful for mapping purposes. Examples of polymorphic markers include variable number of tandem repeats VNTRs RFLPs microsatellite repeats and SNPs the latter two methods are now used predominantly because of the high density of markers and because they are amenable to automated procedures. The Physical Map Cytogenetics and chromosomal banding techniques provide a relatively low-resolution microscopic view of genetic loci. Physical maps indicate the position of a locus or gene in absolute values. Sequence-tagged sites STSs are used as a standard unit for physical mapping and serve as sequence-specific landmarks for arranging overlapping cloned fragments in the same order as they occur in the genome. These overlapping clones allow the characterization of contiguous DNA sequences commonly referred to as contigs. This approach led to high-resolution physical maps by cloning the whole genome into overlapping fragments and has been essential for the identification of disease-causing genes by positional cloning. Recent insights into the structure of the normal human genome show that certain blocks of DNA sequences often containing numerous genes can be duplicated one or several times. This copy number variation CNV which tends to vary in a .