The grapefruit is one of the Groups of cultivated Citrus included under the hybrid name, C. x aurantium (C. maxima [pumelo] x C. reticulata [mandarin]). C. x paradisi (also called a grapefruit) is regarded as a synonym of the grapefruit Group, being a backcross (C. aurantium x C. maxima). Bitter orange/ Sour Orange Group is also included in C. x aurantium (syn C. sinensis). Thanks to John David at the RHS for this information.

Mabberley, D.J. (2008) The Plant Book, ed.3, Cambridge University Press