What Are Single Nucleotide Polymorphisms?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 14 September 2019
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Single Nucleotide Polymorphisms (SNPs) are single variations in Deoxyribonucleic Acid (DNA) base pairs between individuals. There are four nucleotides in DNA: Guanine (G), Adenine (A), Thymine (T), and Cytosine (C), all of which can combine in a variety of ways to build genetic blueprints for making proteins, regulating gene expression, and other activities. In people with single nucleotide polymorphisms, one of them is swapped out. The rest of the DNA strand should be identical, as for example in a chain like CCATCGCCTT, which might appear that way in most individuals, but transform into CTATCGCCTT in some people, replacing the second cytosine with a thymine.

The significance of a single nucleotide polymorphism can depend on the location and the specific nucleotide involved. Researchers estimate that there are around 10 million of them in the human genome, counting only polymorphisms which show up in at least 1% of the human population, which makes them statistically significant. Most appear in what is known as non-coding DNA, because it doesn’t directly code for proteins. This doesn’t mean single nucleotide polymorphisms don’t have an impact on the individual genome, however, as they may influence the expression of other DNA if they are in the wrong place.


When single nucleotide polymorphisms show up in coding DNA, they may be classified as synonymous or replacement. A synonymous single nucleotide polymorphism doesn’t change the protein produced using that strand of DNA as a guideline, while replacement polymorphisms change the nature of the proteins made. They can have a significant impact on the expression of the genome, as the protein could be important. For example, a single nucleotide polymorphism may be linked with problems with the clotting factors used in the blood, in which case a patient’s blood may fail to clot correctly after an injury.

One use for these distinctive variations is in genetic fingerprinting, where a laboratory uses a genetic sample to determine if two people are related, or if someone was present at the scene of a crime. A single nucleotide polymorphism can be a very unique identifier that allows a lab to more closely confirm the accuracy of a match, or mismatch. For example, technicians comparing blood found at a crime scene with a sample from a suspect might categorically rule the suspect out because the suspect might be missing a SNP found in the blood at the scene.

Another use is in medical research. While a single nucleotide polymorphism does not necessarily cause disease, it can increase the risk that a patient will develop a disease, and may change the way the patient responds to medication. Pharmaceutical companies could theoretically customize treatments to patients on the basis of these genetic variations. Patients could also receive testing to highlight disease risks caused by single nucleotide polymorphisms, which might help them make decisions about prevention and screenings to catch a disease early.


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