The movement or spread of alleles of one species into the gene pool of another through repeated backcrossing of interspecific hybrids with members of one of the parental species.
[From Latin intrōgressus, past participle of intrōgredī, to step in : intrō-, intro- + gradī, to step; see ghredh- in the Appendix of Indo-European roots.]
in′tro·gressive (-grĕsĭv) adj.
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Indo-European & Semitic Roots Appendices
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