Gene map
A gene map shows the linear arrangement of genes in the genome of an organism. A distinction is made between genetic and physical gene maps. The sequence of gene locations is entered on a genetic map . The exact distances between genes measured in base pairs are entered on a physical gene map .
Although the arrangement of genes is the same on both types of cards, there is no universally valid “conversion formula” between the two types of cards. The reason for this is that recombination frequencies vary extremely widely depending on the region of the genome and depending on gender.
In order to obtain a complete physical gene map, the genome sequence of an organism must be known. This can also be used to estimate how many genes are contained in the genome.
Linkage map or genetic map
First, one can linkage maps (linking maps / linkage maps) create. The crossing-over rate is used as a measure of the distance between two gene locations : Crossing-over separates genes from one another. If two genes are close together, a separation is less likely than if they are far apart. A linkage map is created by analyzing recombination events and only gives the order of the genes on a chromosome and no precise location information.
The mapping unit here is a centimorgan . 1 cM means a recombination frequency of 1%. (If the recombination frequency of two genes is 16%, their genetic distance is 16 cM). The less frequently the genes recombine, the closer the examined gene locations are to one another. At a distance of 50 cM (recombination rate of 50%; about a third of the chromosome length) the gene locations are considered to be uncoupled, i.e. H. It is no longer possible to determine whether two genes are on one chromosome or not by means of simple linkage analyzes.
Cytological chromosome map or physical map
By contrast, other methods can be used to create cytological chromosome maps that show the physical location of genes on a chromosome . To create cytological chromosome maps, one does not use the evaluation of recombination events but, for example, the microscopic visualization ( in situ hybridization , FISH) of known genes. The distance between two genes on a cytological map is given in m (µm). Various methods are used, for example the recovery of DNA fragments by restriction enzymes with subsequent arrangement of the fragments. The arrangement of the fragments leads to known bioinformatic problems, e.g. B. the double digest problem .
Let S be a DNA sequence. A physical map consists of a set of markers M and an image that assigns a position in S to each marker in M.
Before the development of DNA sequencing , the relative position of genes was determined by three-point analysis.
comparison
When comparing linkage maps and cytological gene maps, the sequence of the genes examined is the same, but not their position on the chromosome. On the one hand, this is due to the fact that crossing-over events occur more frequently in some areas of a chromosome than in others. On the other hand, there are gender-specific differences in the recombination frequency.
See also
literature
- Khalid Meksem, Guenter Kahl (Ed.): The Handbook of Plant Genome Mapping (Genetic and Physical Mapping) . WILEY-VCH, Weinheim 2005, ISBN 978-3-527-31116-3 .
- Dan Gusfield: Algorithms on strings, trees, and sequences . Cambridge University Press, 1999, ISBN 0-521-58519-8 , pp. 398 ff . (Physical mapping).
Individual evidence
- ↑ Gerd Poeggel, Biology for Medicine, Thieme Verlag