Imperial Race Klingons were the original Klingon physical type, distinguished by a bony ridge protecting the crown of the head. Members of the Imperial Race far outnumbered other types of Klingons in the interior of the Empire, and maintained an unofficial (but recognizable) social dominance in Klingon society.
Fusions were Klingon subraces that had been genetically engineered to compete with other major spacefaring races that the Empire had encountered. Fusion races blended Klingon traits with the best and most competitive characteristics of another race to form a new branch of the Klingon species. Klingon/Human and Klingon/Romulan Fusions were known, and others were possible. The Klingon physiology was dominant in such fusions, whose body chemistry, internal structure, and so on is similar to that of Imperial Race Klingons. When Fusion-race Klingons interbred with Imperial Klingons, random genetic selection would produce offspring whose dominant characteristics were either of the Imperial Race or the appropriate Fusion race.
Though Fusions were more suited than Imperial Race Klingons to living and fighting in various alien environments, they were socially (but subtly) subordinate to members of the Imperial breed In the Empire. Fusion races outnumbered Imperial Klingons in many border areas, however, and at least one Klingon Emperor may have been a Klingon/Human Fusion.
- McLimore, Guy W. Jr., Poehlein, Greg K., and Tepool, David F., with Ippolito, Donna, and Huettel, Todd W. "The Klingons: Star Fleet Intelligence Manual." Star Trek: The Role Playing Game, Supplement 2002, Second Edition. Based on original material by Ford, John M. Illustrations by Todd F. Marsh, Dana Knutson, Jeff Laubenstein, and Mitch O'Connell. FASA Corporation. 1987.