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Human Genome News, September 1990; 2(3)
Mapping is the process of determining the relative position and spacing of genes or other landmarks on chromosomes. The two types of maps, genetic and physical, differ both in the methods used to construct them and in the way distance between landmarks is measured. Mapping human genes began early in the twentieth century but has been intensively pursued for only 2 decades. Recent improvements in technology have led to more extensive mapping; even so, less than 2000 of the 50,000 to 100,000 human genes have been mapped.
Genetic Map 5-Year Goal: 600-1500 Index Markers
The genetic map, or genetic linkage map, displays the relative positions of genetic markers (such as an identifiable DNA sequence or genes associated with a physical trait or disease) on a chromosome; distance is measured in centiMorgans (cM). Genetic maps have many uses, including identification of loci associated with genetic diseases, and they form an essential backbone needed to guide physical mapping efforts. Because genes that lie close together on a chromosome have a much higher chance of being inherited together than genes that are farther apart, genetic maps are often produced by studying families to determine how frequently two traits are inherited together.
Advances in genetic mapping tools have helped to make the goals of the Human Genome Project possible. The number of useful DNA index markers (such as restriction fragment length polymorphisms) has increased dramatically in the past 2 years, but about 3000 well-spaced (1 cM apart) and informative markers will be needed to achieve a completely linked framework map. The project's 5-year goal is to create a 2- to 5-cM map, which will require 600 to 1500 index markers, each identified by an STS.
Physical Map: Technology Improvements Making Construction Easier
Physical maps can be constructed in a variety of ways; distance is measured in units of physical length, such as numbers of nucleotide pairs. These maps are used as the basis for isolation and characterization of individual genes or other DNA regions of interest, as well as to provide the starting point for DNA sequencing.
Physical maps can be categorized into two general types. The cytogenetic physical map, which is based on light microscopy, indicates the location of genes or markers relative to visible chromosome bands. Another type of cytogenic physical map (i.e., the long-range restriction map) records the order of and distance between restriction sites on chromosomes. The second type of physical map consists of cloned DNA pieces that represent a complete chromosome or chromosomal segment, together with information about the order of the cloned pieces.
Technology for the construction of overlapping clone sets (contigs) is continually improving, and the new STS strategy is being developed to decrease the need for cloning but still allow the investigator access to the DNA to be sequenced. Initial stages in constructing physical maps of large genomes are becoming easier and faster because of improvements in pulsed-field gel electrophoresis, yeast artificial chromosome cloning, the polymerase chain reaction, fluorescent in situ hybridization, and radiation hybrid analysis. [For more information, see "Physical Mapping of Human DNA: An Overview of the DOE Program," Human Genome Quarterly, 1(2): 1-6 (Summer 1989).]
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 (v2n3).
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.
