There is no such thing as an "Italic" (or even Italian) genotype. Nor is there a "Roman" one. There isn't a "Celtic" one either, since we're on the subject. Culturally, we don't know much about the native Italians prior to Etruscan and Greek infiltration, but we know that the Sicels were apparently originally a mainland group. That's about it. There's no such thing as an "Italic" culture, anyway, though there is evidence of a similar culture across much of the Celtic world, (the La Tene culture pretty much dominated Gaul).
This illustates, therefore, that your argument was not misunderstood, but incorrect. There can't be "interbreeding" between the same species, let alone the same genotype (though sometimes the term is used, such as if one were to say that wolves were interbreeding with coyotes, even though both are the same species, canines. I've long thought that there needs to be some intermediate term, but damned if I know what to use). The correct term is 'intermarriage,' which certainly occurred, and always has.
