A deletion is a type of mutation or chromosomal aberration where genetic material is lost. Deletions can range in size from a single nucleotide, to a large piece of chromosome, to an entire chromosome in the case of monosomic disorders. Deletions are the cause of several serious genetic diseases. Types of deletion include terminal deletions, occurring towards the end of the chromosome, and intercalary or interstitial deletions, occurring on the interior of the chromosome.

Causes of deletion include losses from unbalanced translocation, unbalanced recombination during meiosis, caused by for example heterozygosity for an inversion, or a piece of chromosome breaking off and not rejoining. For the pairing of homologous chromosomes (synapsis) to occur where one chromosome comes with a large intercalary deficiency and a normal complete homolog, the unpaired region of the normal homolog must loop out of the linear structure into a deletion or compensation loop.

Small deletions are less likely to be fatal; large deletions are usually fatal. There are always variations based on which genes are lost. Some medium-sized deletions lead to recognisable human disorders. Deletion of a number of base pairs that is not evenly divisible by three will lead to a frameshift mutation, causing all of the codons occurring after the deletion to be read incorrectly during translation, producing a severely altered and potentially non-functional protein. Deletions are responsible for an array of genetic disorders, including some cases of male infertility and two thirds of cases of Duchenne muscular dystrophy. Deletion of part of the short arm of chromosome 5 results in a syndrome called Cri du chat, French for "cry of the cat" syndrome. It is found in approximately 1 in 50,000 live births. The surviving infants have a distinctive cry, severe mental retardation, and shortened life span.