Deletion on a chromosome. In genetics, a deletion (also called gene deletion, deficiency, or deletion mutation) (sign: δ) is a mutation (a genetic aberration) in which a part of a chromosome or a sequence of DNA is lost during DNA replication.Any number of nucleotides can be deleted, from a single base to an entire piece of chromosome.The smallest single base deletion mutations are believed occur by a single base flipping in the template DNA, followed by template DNA strand slippage, within the DNA polymerase active site. ' Terminal Deletion' — a deletion that occurs towards the end of a chromosome. 2  Intercalary Deletion / Interstitial Deletion — a deletion that occurs from the interior of a chromosome. 3  Microdeletion — a relatively small amount of deletion (up to 5Mb that could include a dozen genes).