Genetic markers are places on the DNA sequence which are recognisable and characteristic of a specific individual, population or species. They are found in all living species ranging from yeasts to mammals. The genetic differences between individuals are called polymorphisms. There are two different kinds of polymorphisms characterized by 2 different genetic markers: Microsatellites and Single Nucleotide Polymorphism (SNPs).
Microsatellites are repeats of two, three or four nucleotides or bases in the genetic sequence (see genetics). For instance, the sequence “ATT” might be repeated an indefinite number of times. This is due to mutation processes. In fact, this type of mutation occurs every 10,000 base pairs on the genetic sequence. “Microsatellites have high mutation rates and therefore may show high variation between individuals within a species for example. This makes them ideal for determining relationships between different populations of the same species (i.e. population genetics) or even between individuals. Microsatellites can be used to determine family relationships within groups and therefore to study social behaviour within natural populations. They have even been used in determining historic genetic diversity by looking at very old museum specimens. Aside from their use in population and conservation biology, microsatellites have also proven to be useful in the study of genetic disease and forensics, for example.” (credit: http://www.amonline.net.au/evolutionary_biology/tour/microsatellites.htm)
See below an example of microsatellite repeat different in two distinct individuals. (CA is the repeat unit):
individual 1 (“CA” motif repeated 13 times):
individual 2 (“CA” motif repeated 15 times):
(credit diagrams: http://www.amonline.net.au/evolutionary_biology/tour/microsatellites.htm)
Single Nucleotide Polymorphism or SNPs are point mutations; a single base of the genetic sequence is changed. This variation of the DNA sequence differs between members of a species (or between paired chromosomes in an individual). For example, two sequenced DNA fragments from different individuals, AAGCCTA to AAGCTTA, contain a difference in a single nucleotide (in red).
This type of mutation occurs every 600 base pairs and even every 50 to a 100 base pairs in mussels and oysters and is therefore much more common than microsatellites.
They can be used for the same type of analysis as microsatellites: study of relations between individuals. However, as it doesn’t mutate as often in time, this type of genetic marker is more reliable than microsatellites. In fact, if mutations appear too often, analysis carried out by the scientists won’t be as reliable as fixed data won’t be available to be used as a reference. Determining SNPs can help scientists to find differences between populations, parents and progeny but can also help to find information regarding adaptation. For example, if all individuals of a population living in a particular environment have the same SNP situated in a coding region of the genome and influencing the protein that is being coded for by this sequence, it might be related to the adaptation of the population to this specific environment.
Contributed by Stephanie Ries
