A single nucleotide polymorphism, or SNP (pronounced “snip”), is the most common type of genetic variation among people. The name describes the concept: a “single” change in a “nucleotide,” a building block of DNA, that is a common “polymorphism” or variation in a population. For example, an SNP occurs when a specific stretch of DNA with the nucleotide cytosine (C) is replaced by the nucleotide thymine (T). SNPs appear about once every 1,000 nucleotides, meaning a person’s genome contains 4 to 5 million of them. To be classified as an SNP, this variation must be present in at least 1 percent of the population.
The Building Blocks of Genetic Variation
Our DNA is composed of a sequence of four chemical bases: A, T, C, and G. These bases form the instructions for building and maintaining an organism. An SNP is like a single-letter spelling difference in this genetic instruction book. For instance, where one person’s genetic code reads AAGCCTA, another person’s might read AAGCTTA. These variations are inherited from parents and contribute to the genetic diversity across the human population.
SNPs are a normal part of our DNA and are distinct from genetic mutations, which are often rare and can lead to disease. They are found throughout a person’s DNA, both within genes and in the DNA between them. The vast majority of these variations have no negative effect on a person’s health or development.
The patterns of SNPs vary between individuals and different populations around the world. This variation contributes to what makes each person unique. Scientists have identified more than 600 million SNPs in human populations, providing a vast catalog of information for understanding human biology, health, and ancestry.
Influence on Human Traits and Health
The effect of an SNP depends on its location within the genome. Most SNPs are found in the DNA between genes and have no observable impact on health. However, when they occur inside a gene or a nearby regulatory region, they can influence the gene’s function. This can lead to variations in physical traits, disease susceptibility, and how a person responds to medication.
SNPs in the coding regions of genes can directly alter the structure of a protein, which may change its function. For example, specific SNPs are linked to physical characteristics like eye color and whether a person can digest lactose in adulthood. These SNPs change the proteins involved in pigment production or enzyme function, leading to different physical outcomes.
Other SNPs do not cause diseases directly but can influence an individual’s risk of developing complex conditions. An example involves the APOE gene, where certain SNPs are associated with an increased risk of developing Alzheimer’s disease. These variations don’t guarantee the disease will occur, but they do change the statistical likelihood. This pattern is seen in conditions like heart disease, diabetes, and cancer, where SNPs contribute to overall susceptibility.
An application of SNP knowledge is in pharmacogenomics, which studies how genes affect a person’s response to drugs. SNPs can alter how the body metabolizes certain medications, making a standard dose ineffective or increasing the risk of side effects. Understanding a person’s SNP profile helps doctors predict their response to certain drugs, allowing for more personalized treatment plans.
Identifying and Applying SNP Knowledge
Scientists identify SNPs using technologies like genotyping, which is often performed with a microarray that can scan for millions of known SNPs at once. This is the technology that powers many direct-to-consumer genetic testing services. More comprehensive methods, like whole-genome sequencing, read a person’s entire DNA sequence, identifying both common SNPs and rarer variations.
The information gathered from identifying SNPs has numerous practical applications. In personalized medicine, a patient’s SNP data helps guide treatment decisions. This information can determine the most effective drug and dosage for conditions like heart disease or depression, minimizing trial and error.
Ancestry testing companies use SNP patterns as markers to trace a person’s lineage. Different populations around the world have distinct frequencies of certain SNPs. By comparing a customer’s SNPs to a reference database, these companies can estimate their ancestral origins from various global regions.
In scientific research, SNPs are used in genome-wide association studies (GWAS). In a GWAS, researchers scan the genomes of thousands of people, comparing those with a specific disease to those without. By looking for SNPs that are more common in the group with the disease, scientists can pinpoint regions of the genome that may contain genes associated with that condition. This approach has helped discover genetic links to numerous complex diseases.