situation in which an individual who is heterozygous for a certain gene mutation
or hemizygous at a particular locus, often due to a deletion of the corresponding
allele, is clinically affected because a single copy of the normal gene is
incapable of providing sufficient protein production as to assure normal
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
See also Understanding Medical Terminology.