Summary
A genetic marker is a gene or DNA sequence with 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 in the genomic loci) that can be observed. A genetic marker may be a short DNA sequence, such as a sequence surrounding a single base-pair change (single nucleotide polymorphism, SNP), or a long one, like minisatellites. For many years, gene mapping was limited to identifying organisms by traditional phenotypes markers. This included genes that encoded easily observable characteristics such as blood types or seed shapes. The insufficient number of these types of characteristics in several organisms limited the mapping efforts that could be done. This prompted the development of gene markers which could identify genetic characteristics that are not readily observable in organisms (such as protein variation). Some commonly used types of genetic markers are: RFLP (or Restriction fragment length polymorphism) SSLP (or Simple sequence length polymorphism) AFLP (or Amplified fragment length polymorphism) RAPD (or Random amplification of polymorphic DNA) VNTR (or Variable number tandem repeat) Microsatellite polymorphism, (or Simple sequence repeat) SNP (or Single nucleotide polymorphism) STR (or Short tandem repeat) SFP (or Single feature polymorphism) DArT (or Diversity Arrays Technology) RAD markers (or Restriction site associated DNA markers) (using Sequence-tagged sites) Molecular genetic markers can be divided into two classes: a) biochemical markers which detect variation at the gene product level such as changes in proteins and amino acids and b) molecular markers which detect variation at the DNA level such as nucleotide changes: deletion, duplication, inversion and/or insertion. Markers can exhibit two modes of inheritance, i.e. dominant/recessive or co-dominant. If the genetic pattern of homo-zygotes can be distinguished from that of hetero-zygotes, then a marker is said to be co-dominant.
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