A genetic marker is a gene or DNA sequence that has a known location on a chromosome that can be used to identify individuals or species . It can be described as a variation (which may arise due to mutation or alteration at genomic loci) that can be observed. A genetic marker can be a short DNA sequence, such as a single base-pair change ( single nucleotide polymorphism , SNP), or a long one, such as a sequence surrounding minisatellites .
For many years, gene mapping was limited to identifying organisms by conventional phenotype markers. This included genes that encode easily observable characteristics, such as blood type or seed size. The insufficient number of such features in many organisms limited the mapping efforts that could be done. This prompted the development of gene markers that can identify genetic features that are not readily observable in organisms (such as protein variation).
Some commonly used types of genetic markers are:
- RFLP (or Restriction Segment Length Polymorphism )
- SSLP (or Simple Sequence Length Polymorphism )
- AFLP (or Amplified Segment Length Polymorphism )
- RAPD (or random amplification of polymorphic DNA )
- VNTR (or Variable Number Tandem Repeat )
- SSR microsatellite polymorphism, (or simple sequence repeat )
- SNP (or single nucleotide polymorphism )
- STR (or Short Tandem Repetition )
- SFP (or Single Feature Polymorphism )
- DART (or Diversity Arrays Technology )
- RAD marker (or restriction site-associated DNA marker )
Molecular genetic markers can be divided into two classes a) biochemical markers that detect variation at the gene product level such as changes in proteins and amino acids and b) molecular markers that detect variation at the DNA level such as nucleotides. Variation: deletion, duplication, inversion and/or insertion. Markers can show two modes of inheritance, namely dominant/recessive or co-dominant. If the genetic pattern of homo-zygotes can be distinguished from that of hetero-zygotes, a marker is said to be co-dominant. Generally co-dominant markers are more informative than dominant markers.
Genetic markers can be used to study the relationship between an inherited disease and its genetic cause (for example, a particular mutation of a gene that results in a defective protein ). It is known that DNA fragments located near each other on a chromosome are inherited together. This property enables the use of markers, which can then be used to determine the exact inheritance pattern of genes that have not yet been localized exactly.
Genetic markers are employed in genetic DNA testing for genetic lineages to determine the genetic distance between individuals or populations . Uniparental markers (on mitochondrial or Y chromosomal DNA) are studied to assess maternal or paternal ancestry . Autosomal markers are used for all progenitors.
Genetic markers are easily identified, associated with a specific locus, and are highly polymorphic, as homozygotes do not provide any information. Marker detection can be direct by RNA sequencing, or indirectly using allozymes.
Some of the methods used to study genome or phylogenetics are RFLP, AFLP, RAPD, SSR. They can be used to generate genetic maps for any organism to be studied.
What is the transmissible agent of CTVT (Canine Transmissible Venereal Tumor) has been debated. Many researchers hypothesized that virus-like particles were responsible for transforming the cell, while others thought that the cell itself was capable of infecting other dogs in the form of allografts. With the help of genetic markers, the researchers were able to provide conclusive evidence that the cancerous tumor cell evolved into a transmissible parasite. In addition, molecular genetic markers were used to address the issue of natural transmission, race of origin (phylogenetics), and age of canine tumors. Genetic markers have also been used to measure the genomic response to selection in livestock. Natural and artificial selection changes the genetic structure of the cell. Presence of different alleles is indicative of differences between selected and non-selected livestock due to deleterious segregation on genetic markers.