Single Nucleotide Polymorphism
A Single Nucleotide Polymorphism is also known as a SNP or snp (pronounced 'snip'). [These and other terms are briefly defined in our Glossary page.]
The importance of SNPs comes from their ability to influence disease risk, drug efficacy and side-effects, tell you about your ancestry, and predict aspects of how you look and even act. SNPs are probably the most important category of genetic changes influencing common diseases. And in terms of common diseases, 9 of the top 10 leading causes of death have a genetic component and thus most likely one or more SNPs influence your risk.
These youtube video clips explain
All humans have almost the same sequence of 3 billion DNA bases (A,C,G, or T) distributed between their 23 pairs of chromosomes. But at certain locations there are differences - these variations are called polymorphisms. Polymorphisms are what make individuals different from one another. Current estimates indicate that up to .1% of our DNA may vary a bit, meaning any two unrelated individuals may differ at less than 3 million DNA positions. While many variations (SNPs) are known, most have no known effect and may be of little or no importance.
SNPedia is a collection of the subset of SNPs that have been reported to be meaningful, either medically or for other reasons (such as for genealogy). The emphasis in SNPedia is on SNPs that have significant medical consequences, are common, are reproducible (or found in meta-analyses or studies of at least 500 patients), and/or have other historic or medical significance.
This example SNP rs1234 will introduce you to the report format used within SNPedia.
The most obvious DNA-based differences are external, such as rs1805009 which affects red hair color. Most polymorphisms have far less obvious effects though, and many of these may have medical consequences. We are just beginning to learn which of the 30 million or so possible polymorphisms influence health, either individually or in sets. Many polymorphisms are likely to have either no effect at all, or to have such subtle effects that it will be many years before their consequences are understood.
Thomas Mailund explains how scientists and statisticians determine which SNPs are related to which diseases.
These sites provide helpful introductions:
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