Genetic variation refers to diversity in gene frequencies. Genetic variation can refer to differences between individuals or to differences between populations. Mutation is the ultimate source of genetic variation, but mechanisms such as sexual reproduction and genetic drift contribute to it as well.
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Variation in the human genome can take several forms. Simple single nucleotide polymorphisms, or SNPs, is one form. But in another form actually involves a larger-scale variation where you might have a stretch of DNA of hundreds, or even thousands, of base pairs that is different between people. Maybe I have three copies of that stretch and you have two. Or maybe it's a circumstance where I have the genes in the order ABC and you have them in the order of ACB because you have an inversion in that. Those don't have to be pathological. In fact, most of them won't be, but it's a different kind of variation that in some instances may be playing a role in disease risk.
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.