in·tro·gres·sion  (ĭn'trə-grěsh'ən)  n. Infiltration of the genes of one species into the gene pool of another through repeated backcrossing of an interspecific hybrid with one of its parents. [From Latin intrōgressus, past participle of intrōgredī, to step in : intrō-, intro- + gradī, to step; see ghredh- in Indo-European roots.] in'tro·gres'sive (-grěs'ĭv) adj. |