Segmental aneuploidy

Segmental aneuploidy is the duplication or deletion of a chromosomal segment rather than an entire chromosome, producing copy number alterations that range from single genes to multi-megabase regions. Unlike whole-chromosome aneuploidy, which disrupts the stoichiometry of hundreds or thousands of genes simultaneously, segmental aneuploidy can be locally tolerated and even advantageous when the altered region contains genes whose increased dosage confers a selective benefit.

Segmental duplications are a major engine of genome evolution. The human genome contains hundreds of segmentally duplicated regions, many of which harbor gene families that expanded through successive rounds of duplication and divergence — olfactory receptors, immunoglobulins, and Hox genes among them. In cancer, somatic copy number alterations (SCNAs) are the most common type of structural variation and frequently target oncogenes or tumor suppressor loci. The boundary between evolutionary raw material and pathological disruption is not a property of the alteration itself but of the selective environment in which it arises.

Segmental aneuploidy is evolution's preferred tool: precise enough to be useful, messy enough to be creative.