Genetic variation · BEGINNER LESSON
SNP
A single-nucleotide polymorphism, or SNP, is a single-letter DNA difference that is common in humans, meaning it is present in at least 1% of the population. It is a subtype of the broader category single-nucleotide variants, or SNVs.
What a SNP is
A SNP is a difference at one DNA position, where one person may have one nucleotide and another person may have a different nucleotide at the same spot. Nucleotides are the DNA letters A, C, G, and T. The term SNV is broader and includes all single-letter differences, while SNP is reserved for those found in at least 1% of the human population. Because the cutoff is a frequency rule, not a different kind of DNA change, some sources use SNP and SNV differently.
[1] Human Genomic Variation [2] SNPs in cancer research and treatment - British Journal of Cancer [3] 1.3
What is measured
In SNP studies, researchers measure how often a variant appears in a population, often through minor allele frequency, which is the frequency of the less common version of the DNA letter. They also study whether a SNP is associated with a change in risk, gene expression, or protein function. Many SNPs are simply genetic markers, meaning they help track ancestry or regions of DNA without causing the trait themselves.
[2] SNPs in cancer research and treatment - British Journal of Cancer [3] 1.3
How SNPs arise and what they can do
SNPs arise from point mutations that are maintained in populations over time. Their frequencies are shaped by time since the mutation, natural selection on useful or harmful variants, random genetic drift, and bottlenecks, which are sharp reductions in population size. Most SNPs do not change visible traits, but some can alter protein-coding sequence or gene regulation by changing transcription factor binding, which can increase or decrease gene expression.
[2] SNPs in cancer research and treatment - British Journal of Cancer [3] 1.3
Limitations and interpretation
A SNP finding is not automatically a cause of a disease or trait. In complex disease studies, a SNP result may be neither necessary nor sufficient, and nonreproducible results can happen when studies are too small or when population mixing confounds the analysis. This is why replication matters before a claim is treated as reliable.
[2] SNPs in cancer research and treatment - British Journal of Cancer
SEE THE IDEA
How to think about a SNP
One site in the genome where people can differ in a single letter.
The difference is common enough to appear in at least 1% of the population.
The SNP may be a marker, or it may affect gene expression or protein sequence.
Researchers check whether the SNP is linked to risk, function, or no effect.
A SNP is a common single-letter DNA difference; researchers then test whether that difference has a biological or statistical association.
From a single DNA-letter change to possible biological effect, with many SNPs having no direct effect.
TRY IT YOURSELF
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.