A genetic marker is a specific DNA sequence whose exact location has been identified on its corresponding chromosome and whose inheritance can be traced. The DNA sequence that forms a genetic marker can be an entire gene or just a sequence with no known function or non-coding. They are mainly used in the genetic mapping as pointers to regions of the genome of a certain organism.
One of the most outstanding characteristics of genetic markers is that they reveal the polymorphism genetics within the same species. For example, the genetic marker of the area that codes for the type of blood in humans; all humans need blood but blood can be very different from one individual to another due to this polymorphism.
The term molecular marker is very often used synonymously, although this term refers specifically to a type of genetic marker.
Types of genetic markers
Genetic markers are usually divided into two groups, biochemical markersdetected as variations in the amino acid sequence for which the marker encodes, and molecular markers DNA-based, detected at the marker nucleotide sequence level with or without observable changes in phenotype. The calls morphological markersidentified by traits in the phenotype (color, size, etc.), are nowadays much less used.
biochemical markers
Biochemical markers are based on observation of the amino acid sequence polymorphism of a protein. The most widely used biochemical markers are isozyme markersthat is, enzymes with the same function but different size, charge, or conformation.
Some of the first such markers were blood groups and immunoglobulins. These types of markers have low polymorphism and low sensitivity and therefore have limited applications compared to DNA-based molecular genetic markers.
molecular markers
These markers detect variations in the nucleotide sequence of DNA: invert, insert, duplicate or delete. They generally detect polymorphism in non-coding sequences. They can be divided into:
- Markers associated with variations in the number of repetitions of a sequence: they are divided into microsatellites and minisatellites.
- markers associated with punctual variations in the genome, detectable or not by restriction enzymes.
Some of the most used molecular markers are:
- RFLP (Restriction Fragment Length Polymorphism or Polymorphism in the length of restriction fragments)
- SSLP (Simple sequence length polymorphism or Polymorphism in the length of simple sequences)
- AFLP (Amplified fragment length polymorphism or Polymorphism in the length of amplified fragments)
- RAPD (Random amplification of polymorphic DNA or Random amplification of polymorphic DNA)
- VNTR (Variable number tandem repeat or Variable number of repetitions in tandem. Minisatellite)
- SSR (Simple sequence repeat or repetition of simple sequence. Microsatellite)
- STR (Short tandem repeat or Short tandem repeats. Microsatellite)
- SNP (Single nucleotide polymorphism or simple nucleotide polymorphism)
- SFP (Single feature polymorphism or Single Feature Polymorphisms)
- traps (Target Region Amplification Polymorphism, in Spanish Polymorphisms for the amplification of target regions)
- DART (Diversity Arrays Technology, is Spanish Technology of Vectors, or Matrices, of Diversity)
- RAD (Restriction site associated DNA markers or DNA markers associated with restriction sites)
The ideal genetic markers
DNA-based markers have a number of characteristics that make them generally more useful than protein-based markers. The characteristics of an ideal genetic marker can be summarized as:
- High ability to detect polymorphism
- hereditary
- Ability to access all regions of the genome
- Independence of the developmental stage of the individual
- Independence of the physical state of the individual
- Independence of environmental conditions
- Ease of obtaining
- Inexpensive detection methods
- Possibility of obtaining in any cell of the organism that contains a nucleus
Applications
Genetic markers have countless applications. Perhaps one of the most obvious is the localization and hereditary study of genetic traits, from hair color to genetic diseases. This location is very important, among others, in the medical field. Understanding the area of the genome involved in the inheritance of a certain characteristic can help to understand it and in some diseases it can represent an important advance in prevention, diagnosis and treatment. For example, some genetic markers associated with an increased risk of breast cancer are known (these markers indicate the location of the BRCA1 and BRCA2 genes); Analyzes of these markers are currently carried out in women with a family history of breast cancer and if they are present, these patients may decide to undergo more aggressive prevention measures.
Genetic markers are also used in paternity tests, forensic medicine and criminal investigation. The comparison of genetic markers between a known sample and an unknown sample allows to determine if both samples are related or if they are not identical. In order for the result to be considered significant, the comparison is made with numerous alleles. Phylogenetic studies are also based on this type of comparative studies.
Another of the most common applications of genetic markers is the Genome mapping and identification of genes of interest. As a tool of characterization and differentiation of the genotypethe analysis of genetic markers also makes it possible to differentiate individuals subjected to artificial selection from those that are not, which is used by geneticists in the improvement of plants and livestock.
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