define single gene mutation
A gene mutation is a permanent alteration in the DNA sequence that makes up a gene, such that the sequence differs from what is found in most people.Mutations range in size; they can affect anywhere from a single DNA building block (base pair) to a large segment of a chromosome that includes multiple genes.n some cases, the mutation occurs in a personâs egg or sperm cell but is not present in any of the personâs other cells. In other cases, the mutation occurs in the fertilized egg shortly after the egg and sperm cells unite.