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Genetic markers are DNA Sequence s that can be identified by a simple assay, allowing the presence or absence of neighbouring stretches of the Genome to be inferred. If the Genome of the corresponding species is known, it is likely that the position of genetic markers in the genome will be known too. Genetic markers can be used to study the relation 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 that lie near each other on a chromosome, tend to be inherited together. This property enables the use of a marker, which in this case can be used to determine the precise inheritance pattern of the gene that has not yet been exactly localized. 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 by direct by DNA Sequencing , or indirect using Allozyme s. To be sure that the marker and a disease Locus are actually linked, one can calculate the LOD Score or '''log of the odds'''. - Some scientists use genetic engineering to cure diseases in patients who would otherwise not be able to produce normal, functioning protiens. 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. # 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 . # 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 which look like clouded patches on the agar gel. It is not yet possible to tell which cells have taken up the insulin gene, however. # 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). # 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. TYPES The marker may be short, such as a single base-pair change ( Single Nucleotide Polymorphism ) or long, like Microsatellites , which are DNA fragments generated by a Restriction Digest . |
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