A genetic variant is any difference in the DNA sequence compared with a reference sequence: it may involve a change in a single nucleotide or the loss or addition of a segment of DNA.
Its effect may be neutral, which is the most common outcome, or it may increase or decrease susceptibility to a condition or, in some cases, directly cause an inherited condition. Most variants are inherited from parents; those that arise for the first time in an individual are called "de novo" variants.