Microdeletion
The loss of a tiny piece of a chromosome, a piece so small its absence is not apparent on ordinary examination (using a regular light microscope to look at chromosomes prepared in the usual fashion).
The detection of microdeletions requires special techniques such as high-resolution chromosome banding, molecular chromosome analysis (with FISH), or DNA analysis.
Disorders caused by microdeletions include the Angelman, DiGeorge, Prader-Willi, and Williams syndromes.
The prefix 'micro-' is derived from the Greek 'mikros' meaning small.
It diminishes whatever it precedes.