mutation whose gene product adversely affects the normal, wild-type gene
product within the same cell, usually by dimerizing (combining) with it.
In cases of polymeric molecules, such as collagen, dominant negative mutations
are often more deleterious than mutations causing the production of no gene
product (null mutations or null alleles).
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
See also Understanding Medical Terminology.