What are single nucleotide polymorphisms (SNPs)?

• A. Variations at a single nucleotide position in the DNA sequence among individuals.
• B. Variations at multiple nucleotides in a gene.
• C. Large chromosomal rearrangements.
• D. Methylation patterns across the genome.

Answer: (A)

Single nucleotide polymorphisms are the most common genetic differences between individuals, occurring when a single base in the DNA sequence varies at a particular position. These substitutions are inherited and usually present in a sizable portion of the population (minor allele frequency typically over 1%). SNPs can reside in coding regions, where they might change an amino acid or have no effect on the protein, or in noncoding regions, where they can influence gene regulation. Clinically, they’re useful as markers in studies of disease risk and drug response. They differ from other types of variation: changes involving multiple nucleotides in a gene aren’t SNPs; large chromosomal rearrangements affect big chunks of DNA; and methylation patterns are epigenetic modifications that do not alter the DNA sequence itself.