An Important concept of "GENE MAPPING"

Genetic map

Well lets use some imagination: Like interstate maps having cities and towns that serve as landmarks, the genetic maps have landmarks known as genetic markers, or "markers" for short. The term "marker" is used very broadly to describe any observable variation that results from an alteration, or mutation, at a single genetic locus. A marker may be used as one landmark in a map if, in most cases, that stretch of DNA is inherited from parent to child according to the standard rules of inheritance. Markers can be within genes that code for a noticeable physical characteristic such as leaf colour, or a not so noticeable trait such as a disease. The greater the distance between two linked genes, the greater the chance that two nonsister chromatids would cross over in the region between the genes and the greater the proportion of recombinants that would be produced. Thus, by determining the frequency of recombinants, we can obtain a measure of map distance between the genes. Today, several other genetic markers are used to detect linkage. There are several genetic markers:

• RFLPs/ (Restriction Fragment Length Polymorphism's): They were among the first developed DNA markers. RFLPs are defined by the presence or absence of a specific site, called a restriction site, for a bacterial restriction enzyme. This enzyme breaks apart strands of DNA wherever they contain a certain nucleotide sequence...
• VNTRs/ ( Variable Number of Tandem Repeat Polymorphisms): They mainly occur in non-coding regions of DNA. This type of marker is defined by the presence of a nucleotide sequence that is repeated several times. In each case, the number of times a sequence is repeated may vary..
• Microsatellite polymorphism's: Defined by a variable number of repeats of a very small number of base pairs. Oftentimes, these repeats consist of the nucleotides, or bases, cytosine and adenosine. The number of repeats for a given microsatellite may differ between individuals, hence the term polymorphism--the existence of different forms within a population;
• SNPs/ Single Nucleotide Polymorphism's: They are individual point mutations, or substitutions of a single nucleotide, that do not change the overall length of the DNA sequence in that region. SNPs occur throughout an individual's genome;
• AFLP/ Amplified Fragment Length Polymorphism: They mainly involve a DNA fingerprinting technique which detects DNA restriction fragments by means of PCR amplification.

Currently, the most powerful mapping technique, and one that has been used to generate many genome maps, relies on Sequence Tagged Site (STS) mapping: A STS is a short DNA sequence that is easily recognisable and occurs only once in a genome (or chromosome).