Polymorphism involves one of two or more variants of a particular DNA sequence. The most common type of polymorphism involves variation at a single base pair. Polymorphisms can also be much larger in size and involve long stretches of DNA. Called a single nucleotide polymorphism, or SNP (pronounced snip), scientists are studying how SNPs in the human genome correlate with disease, drug response, and other phenotypes.
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Polymorphism, by strict definitions which hardly anybody pays attention to anymore, is a place in the DNA sequence where there is variation, and the less common variant is present in at least one percent of the people of who you test. That is to distinguish, therefore, polymorphism from a rare variant that might occur in only one in 1,000 people. A polymorphism, it has to occur in at least one in 100 people. Polymorphisms could be not just single-letter changes like a C instead of T. They could also be something more elaborate, like a whole stretch of DNA, that is either present or absent. You might call that a copy number variant; those are all polymorphisms. But this is basically a general term to talk about diversity in genomes in a species.
Francis S. Collins, M.D., Ph.D.
Director, National Institutes of Health; Former Director, National Human Genome Research Institute
Dr. Francis S. Collins, director of the National Institutes of Health, is noted for his landmark discoveries of disease genes and his visionary leadership of the Human Genome Project, a complex multidisciplinary scientific enterprise directed at mapping and sequencing human DNA. Dr. Collins was the director of the National Human Genome Research Institute from 1993 to 2008. His research has led to the identification of genetic variants associated with type 2 diabetes and the genes responsible for cystic fibrosis, neurofibromatosis, Huntington's disease and Hutchinson-Gilford progeria syndrome. In 2007, Dr. Collins received the Presidential Medal of Freedom, the nation's highest civil award, for his revolutionary contributions to genetic research.