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Human Genome News, Nov. 1994; 6(4):14
Genes and other DNA fragments located on the same chromosome are said to be linked. Genetic linkage maps (also called genetic maps or linkage maps) show the relative order of and approximate spacing between specific, identifiable DNA regions that researchers use as signposts along the chromosomes. These landmarks or markers include detectable genes or other DNA stretches. Distances between markers, measured in centimorgans (cM), are not actual physical distances but rather describe the frequency with which the markers are coinherited. Two markers are said to be 1 cM apart if they are separated (not coinherited) by recombination 1% of the time. A genetic distance of 1 cM is roughly equivalent to a physical distance of 1 million base pairs.
Human genetic mapping began in the 1930s when researchers studying family inheritance patterns noted that color blindness and hemophilia were linked to the X chromosome. Mapping proceeded slowly and was limited to genes linked to visible physical traits until the late 1970s, when a new type of marker, based on detectable DNA sequence differences, was discovered. Individual variations in base sequence cause restriction enzymes to cut DNA at specific sites, producing fragments of different lengths (restriction fragment length polymorphisms or RFLPs). Although the discovery of RFLP markers spurred genetic mapping, researchers found them to be relatively rare and unevenly dispersed in the human genome, difficult to analyze (genotype), and not variable or "informative" enough. The newer short tandem repeat polymorphism markers have several technological advantages over RFLPs, including their ability to be amplified by polymerase chain reaction.
HGMIS staff
The electronic form of the newsletter may be cited in the following style:
Human Genome Program, U.S. Department of Energy, Human Genome News (v6n4).
The Human Genome Project (HGP) was an international 13-year effort, 1990 to 2003. Primary goals were to discover the complete set of human genes and make them accessible for further biological study, and determine the complete sequence of DNA bases in the human genome. See Timeline for more HGP history.
Published from 1989 until 2002, this newsletter facilitated HGP communication, helped prevent duplication of research effort, and informed persons interested in genome research.
