What is a Single Nucleotide Polymorphism (SNP)?

• A. SNPs are variations at a single nucleotide position on a chromosome, the most common type of polymorphism.
• B. SNPs are variations at multiple positions.
• C. SNPs are sequence repeats.
• D. SNPs are large deletions.

Answer: (A)

A SNP is a single base change at a specific position in the genome that is common in the population, making it the most frequent type of genetic variation. In one person, that position might have one nucleotide, while in another person it has a different nucleotide, and this difference is present in a sufficient fraction of people (typically at least 1%). SNPs can occur in coding regions, potentially altering amino acids or, more often, not changing the protein, or in regulatory and noncoding regions where they can influence gene expression. They are inherited and widely used as genetic markers in studies of disease associations and pharmacogenetics. The other options describe different forms of variation: changes across several bases (multi-nucleotide variants), repeating sequence variations (variable number tandem repeats), or large deletions (structural variants), none of which are SNPs.