chromosomal deletion

A chromosomal deletion is the loss of a segment of a chromosome — a torn-out section of a book that takes its pages, and the genes written on them, with it. Unlike a small point mutation that changes a single letter, a deletion removes a whole stretch of DNA at once, so it can knock out many genes in a single event.

Deletions vary enormously in size. A large one may span a visible band on a chromosome and be detected by karyotype; a tiny one may remove only part of a gene and require sequencing to find. Because the cell ends up with only one working copy of the deleted region, the effect is a kind of dosage loss for every gene in that interval.

Whether a deletion causes disease depends on which genes it removes and how sensitive the body is to having just one copy of them. Some genes tolerate a single dose; others are haploinsufficient, meaning one copy is not enough, and losing the second copy produces a recognizable disorder. Many well-known microdeletion syndromes arise exactly this way.