An identifiable segment of DNA whose position on a chromosome is known and whose structure varies between individuals. Examples are copy number variations in repeats or SNPs (single nucleotide polymorphisms). Biochemical and blood group markers were used before DNA markers became available.
One use of markers is in research studies in families to find genes. DNA from members of families is tested with many markers to determine which (if any) of the markers used is being inherited with the condition. This is called a linkage study. Statistical analyses are performed using information from all the families in the study to determine how physically close the marker is likely to be to the gene associated with the condition. Before the human genome sequence was known, scientists then had to work along the DNA by cloning it piece by piece to move from the marker to locate and isolate the gene. This is how the gene for cystic fibrosis was found.
Since the human genome was sequenced, researchers now usually interrogate the sequence database to identify the known genes in the chromosomal area in which the marker studies suggest the gene should lie. Using DNA from people with the condition, the genes in that area would be sequenced in turn. If changes to the DNA sequence in one of the genes were found which are of a type which would affect the functioning of the gene, this is strong evidence that the gene associated with the condition has been found.
Once the gene responsible for a condition has been identified through such research studies, clinical testing becomes possible for other people with that condition. This may be carried out by directly sequencing the gene in the person who requires the clinical information, although in some circumstances a "gene tracking" study has to be performed. Gene tracking requires samples from several family members, including at least one person with the inherited condition. A gene tracking study uses the markers previously identified in the research study as being inherited with the condition, and determining the pattern of these in the family - who has inherited which allele. This allows the marker allele being inherited with the altered gene in the family to be identified. The person asking for information can then be tested to see if he or she has inherited this allele, confirming whether the person has inherited the condition (in the case of dominant and X-linked conditions) or is a carrier (in recessive and X-linked conditions).
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