Gene mapping describes the methods used to identify the locus of a
gene and the distances between genes.
The essence of all genome mapping is to place a collection of
molecular markers onto their respective positions on the genome.
Molecular markers come in all forms. Genes can be viewed as one
special type of genetic markers in the construction of genome maps,
and mapped the same way as any other markers.
1 Genetic mapping vs physical mapping
1.1 Gene mapping
1.2 Physical mapping
3 See also
5 External links
Genetic mapping vs physical mapping
There are two distinctive types of "Maps" used in the field of genome
mapping: genetic maps and physical maps. While both maps are a
collection of genetic markers and gene loci, genetic maps' distances
are based on the genetic linkage information, while physical maps use
actual physical distances usually measured in number of base pairs.
While the physical map could be a more "accurate" representation of
the genome, genetic maps often offer insights into the nature of
different regions of the chromosome, e.g. the genetic distance to
physical distance ratio varies greatly at different genomic regions
which reflects different recombination rates, and such rate is often
indicative of euchromatic (usually gene-rich) vs heterochromatic
(usually gene poor) regions of the genome.
Researchers begin a genetic map by collecting samples of blood or
tissue from family members that carry a prominent disease or trait and
family members that don't. Scientists then isolate
DNA from the
samples and closely examine it, looking for unique patterns in the DNA
of the family members who do carry the disease that the
DNA of those
who don't carry the disease don't have. These unique molecular
patterns in the
DNA are referred to as polymorphisms, or markers.
The first steps of building a genetic map are the development of
genetic markers and a mapping population. The closer two markers are
on the chromosome, the more likely they are to be passed on to the
next generation together. Therefore, the "co-segregation" patterns of
all markers can be used to reconstruct their order. With this in mind,
the genotypes of each genetic marker are recorded for both parents and
each individual in the following generations. The quality of the
genetic maps is largely dependent upon these factors: the number of
genetic markers on the map and the size of the mapping population. The
two factors are interlinked, as a larger mapping population could
increase the "resolution" of the map and prevent the map being
In gene mapping, any sequence feature that can be faithfully
distinguished from the two parents can be used as a genetic marker.
Genes, in this regard, are represented by "traits" that can be
faithfully distinguished between two parents. Their linkage with other
genetic markers are calculated in the same way as if they are common
markers and the actual gene loci are then bracketed in a region
between the two nearest neighbouring markers. The entire process is
then repeated by looking at more markers which target that region to
map the gene neighbourhood to a higher resolution until a specific
causative locus can be identified. This process is often referred to
as "positional cloning", and it is used extensively in the study of
Since actual base-pair distances are generally hard or impossible to
directly measure, physical maps are actually constructed by first
shattering the genome into hierarchically smaller pieces. By
characterizing each single piece and assembling back together, the
overlapping path or "tiling path" of these small fragments would allow
researchers to infer physical distances between genomic features. The
fragmentation of the genome can be achieved by restriction enzyme
cutting or by physically shattering the genome by processes like
sonication. Once cut, the
DNA fragments are separated by
electrophoresis. The resulting pattern of
DNA migration (i.e. its
genetic fingerprint) is used to identify what stretch of
DNA is in the
clone. By analyzing the fingerprints, contigs are assembled by
automated (FPC) or manual means (pathfinders) into overlapping DNA
stretches. Now a good choice of clones can be made to efficiently
sequence the clones to determine the
DNA sequence of the organism
In physical mapping, there are no direct ways of marking up a specific
gene since the mapping does not include any information that concerns
traits and functions.
Genetic markers can be linked to a physical map
by processes like in situ hybridization. By this approach, physical
map contigs can be "anchored" onto a genetic map. The clones used in
the physical map contigs can then be sequenced on a local scale to
help new genetic marker design and identification of the causative
Macrorestriction is a type of physical mapping wherein the high
DNA is digested with a restriction enzyme having a
low number of restriction sites.
There are alternative ways to determine how
DNA in a group of clones
overlaps without completely sequencing the clones. Once the map is
determined, the clones can be used as a resource to efficiently
contain large stretches of the genome. This type of mapping is more
accurate than genetic maps.
Genome sequencing is sometimes mistakenly referred to as "genome
mapping" by non-biologists. The process of "shotgun sequencing"
resembles the process of physical mapping: it shatters the genome into
small fragments, characterizes each fragment, then puts them back
together (more recent sequencing technologies are drastically
different). While the scope, purpose and process are totally
different, a genome assembly can be viewed as the "ultimate" form of
physical map, in that it provides in a much better way all the
information that a traditional physical map can offer.
Identification of genes is usually the first step in understanding a
genome of a species; mapping of the gene is usually the first step of
identification of the gene.
Gene mapping is usually the starting point
of many important downstream studies.
The process to identify a genetic element that is responsible for a
disease is also referred to as "mapping". If the locus in which the
search is performed is already considerably constrained, the search is
called the fine mapping of a gene. This information is derived from
the investigation of disease manifestations in large families (genetic
linkage) or from populations-based genetic association studies.
Quantitative trait locus
^ Mader, Sylvia (2007). Biology Ninth Edition. New York: McGraw-Hill.
p. 209. ISBN 978-0-07-325839-3.
Gene mapping - Glossary Entry". Genetics Home Reference. Bethesda,
MD: Lister Hill National Center for Biomedical Communications, an
Intramural Research Division of the U.S. National Library of Medicine.
2013-09-03. Retrieved 2013-09-06. External link in work= (help)
Brown, Terry A. (2007). Genomes 3. New York, NY: Garland Science
Publishing. ISBN 9780815341383. OCLC 444522997.
"Genetic Mapping Fact Sheet". Bethesda, MD: National Human Genome
Research Institute, National Institutes of Health. Retrieved
"Canada's Michael Smith
Genome Sciences Centre". Vancouver, British