Overview of the Respiratory System
A polymorphism is a DNA sequence variation that is common in the population (as opposed to a mutation, which is more rare). The term "polymorphism" also is used frequently in clinical genetics to distinguish a DNA sequence variation that is unrelated to disease (polymorphism) from a DNA sequence variation that is associated with a disease (mutation). A polymorphism occurs in the human genome roughly every 250-1,000 base pairs.
Polymorphisms provide markers, or signposts, throughout the genome that can be used to trace traits and diseases, and their associated genes, through families or large populations. This experimental approach has allowed researchers to map the genetic locations of numerous disease genes, facilitating their identification. The polymorphism itself may not necessarily be associated with the disease in question, but it may sit on the chromosome very near the genetic alteration associated with the disease, and thus, may be used as a tag for the disease gene until a more precise genetic alteration associated with the disease can be identified.
The slide shows an electropherogram generated by automated fluorescent DNA sequencing of a segment of DNA using an ABI 310 Genetic Analyzer (Applied Biosystems, Foster City, CA, USA). The arrow shows a nucleotide substitution in one of the two copies of this gene in this individual. One copy of the gene (allele) carries an A nucleotide (green peak) while the other allele carries a C nucleotide (blue peak). This nucleotide substitution is detected as a dual peak in the output of the sequence analysis software. In this case, the DNA sequence variation results in a missense mutation. Missense mutations can be associated with disease or they may be benign polymorphisms. The detection of variations in the DNA sequence of a gene that are uncertain biological consequence can complicate the interpretation of genetic test results.
The human genome also contains a number of repetitive elements or tandemly repeated sequences. The size of these repeated sequences varies among individuals. Many of these variable length repetitive sequences are benign and simply represent normal variability in the human genome, while some are associated with disease. The polymorphic nature of repetitive DNA sequences is the basis for much of the DNA-based forensic analyses performed today including paternity and personal identification testing.
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