A genetic marker is a known DNA sequences (e. g. a gene or part of gene) that can be identified by a simple assay, associated with a certain phenotype. part of a DNA sequence A DNA sequence (sometimes genetic sequence) is a succession of letters representing the primary structure of a real or hypothetical DNA molecule or strand, The possible letters are A, C, G, and T, representing the four nucleotide subunits of a DNA strand (adenine, cytosine, guanine... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... An assay is a procedure where the concentration of a component part of a mixture is determined. ... The phenotype of an individual organism is either its total physical appearance and constitution or a specific manifestation of a trait, such as size, eye color, or behavior that varies between individuals. ...

A genetic marker may be a short DNA sequence, such as a sequence surrounding a single base-pair change (single nucleotide polymorphism), or long one, like microsatellites. A Single Nucleotide Polymorphism or SNP (pronounced snip) is a DNA sequence variation, occurring when a single nucleotide: A, T, C or G - in the genome differs between members of the species. ... A microsatellite is a short, noncoding DNA sequence (a Tandemly Repetitive DNA sequence) that is repeated many times within the genome of an organism. ...

Uses

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 pieces of DNA which lie near each other on a chromosome tend to be inherited together. This property enables the use of a marker, which can then be used to determine the precise inheritance pattern of the gene that has not yet been exactly localized. A genetic disorder, or genetic disease is a disease caused, at least in part, by the genes of the person with the disease. ... Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... In biology, mutations are changes to the genetic material (usually DNA or RNA). ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ...

Genetic markers have to be easily identifiable, associated with a specific locus, and highly polymorphic, because homozygotes do not provide any information. Detection of the marker can be direct by DNA sequencing, or indirect using allozymes. The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... In biology, polymorphism can be defined as the occurrence in the same habitat of two or more forms of a trait in such frequencies that the rarer cannot be maintained by recurrent mutation alone. ... A homozygotes cells are diploid or polyploid and have the same alleles at a locus (position) on homologous chromosomes. ... DNA sequencing is the process of determining the nucleotide order of a given DNA fragment, called the DNA sequence. ... In biochemistry, isozymes (or isoenzymes) are isoforms (closely related variants) of enzymes. ...

To be sure that the marker and a disease locus are actually linked, one can calculate the LOD score or log of the odds. The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... Genetic linkage occurs when particular alleles are inherited together. ...

Genetic markers also play a role in genetic engineering, as they can be used to produce normal, functioning proteins to replace defective ones. The damaged or faulty section of DNA is removed and replaced with the identical, but functioning, gene sequence from another source. An iconic image of genetic engineering; this 1986 autoluminograph of a glowing transgenic tobacco plant bearing the luciferase gene of the firefly strikingly demonstrates the power and potential of genetic manipulation. ...

They remove the faulty section of DNA, replace it with the functioning gene from another patient, and place them in bacterial cells which reproduce the new DNA sequence. The scientists then need to know which bacteria have been successful in duplicating these genes, so they add another gene that changes the bacteria's resistance to antibiotics, use replica plating to grow enough bacteria to test, and then test the bacteria's resistance to antibiotics.

1. The bacterial DNA has two resistency genes: one for tetracycline and one for ampicillin. The insulin gene can be inserted in the middle of the ampicillin gene after it has been removed using restriction endonucleases.
2. The bacteria are then allowed to grow on an agar plate containing a culture medium (all the nutrients that are needed for it to reproduce), and the bacteria grow and produce colonies on the agar gel. At this stage, however, it is not yet possible to tell which cells have taken up the insulin gene.
3. A piece of filter paper can be placed onto the top of this agar plate so that the exact positions of the colonies are remembered. This produces a copy which can then be transferred onto a second agar plate containing tetracycline. All of the bacteria that are not resistant to tetracycline will die, and those containing insulin will survive and grow (showing a clouded patch on the agar again).
4. This process is repeated onto the last agar plate, this time containing ampicillin. This will kill all of those bacteria containing the insulin gene, and by comparing plates 2 and 3, scientists can determine which bacterium have taken up the insulin gene and it can be harvested accordingly.