This is most likely an example of a mutation.
Mutation is any alteration or change in DNA sequence which affects the genetic instruction carried by that gene. Mutation can occur either as a result of errors during DNA replication or due to environmental factors such as cigarette smoke and ultraviolet (UV) radiation. Mutations are heritable, especially if they have a positive effect. The disorder Down syndrome is an example of mutation which occurs due to error in cell division in which there is extra copy of a genetic material (specific chromosome); chromosome 21.