Single-nucleotide polymorphism

A single-nucleotide polymorphism (SNP, pronounced snip) is a DNA sequence variation occurring when a single nucleotide ¡ª A, T, C, or G ¡ª in the genome (or other shared sequence) differs between members of a biological species or paired chromosomes in an individual. For example, two sequenced DNA fragments from different individuals, AAGCCTA to AAGCTTA, contain a difference in a single nucleotide. In this case we say that there are two alleles: C and T. Almost all common SNPs have only two alleles.

Indel is a molecular biology term that has different definitions in different fields:

• In evolutionary studies, indel is used to mean an insertion or a deletion^[1][2] and indels simply refers to the mutation class that includes both insertions, deletions, and the combination thereof,^[3][4][5] including insertion and deletion events that may be separated by many years.^[6]
• In germline and somatic mutation studies, however, indel describes a special mutation class, defined as a mutation resulting in a colocalized insertion and deletion and a net gain or loss in nucleotides, and microindel is defined as an indel that results in a net gain or loss of 1 to 50 nucleotides.^[7]

Copy-number variation

Copy-number variations (CNVs - a form of structural variation) are alterations of the DNA of a genome that results in the cell having an abnormal number of copies of one or more sections of the DNA. CNVs correspond to relatively large regions of the genome that have been deleted (fewer than the normal number) or duplicated (more than the normal number) on certain chromosomes. For example, the chromosome that normally has sections in order as A-B-C-D might instead have sections A-B-C-C-D (a duplication of "C") or A-B-D (a deletion of "C").

This variation accounts for roughly 12% of human genomic DNA and each variation may range from about one kilobase (1,000 nucleotide bases) to several megabases in size^{[citation needed]}. CNVs contrast with single-nucleotide polymorphisms (SNPs), which affect only one single nucleotide base.