First off the great works were done by Greek slaves who were much smarter than most of their masters. Second, the upper class was all etruscan and nothing to do with the rest, where all the genius of leadership came from. Thirdly they were forced to rise up to the occasion several times, which is what led to their discipline and cohesiveness for a long time. That combined with a few brilliant leaders of the caesar line is what propelled them to the top.
Then they had the problem of too much success, a huge proletariat on the dole, an ever decaying and more dissolute and outof touch upper class.
And of course it's been almost completely depopulated and replaced in the north. Then once again. Then a couple times in the south and struggles with the guelfs and ghibbolines that tore apart any last shred of cohesiveness. Then the venetians and genoans running everything in such a way as to turn everyone against each other and undermine christianity to boot, and taking on so many mercenaries again they practically speaking changed the character of the country again.
And of course the delightful rule of the duke of anjou, fallowed by the italian vespers and rule by aragon.
So no there's cohesion of any kind, and probably zero genetic continuity, and the great works of romans weren't due to the local italian population anyway.
The Etruscan kings once ruled Rome, but were kicked out. The Romans absorbed the Etruscans; notably, for example, we know that Claudius married an Etruscan noblewoman, but that hardly means the Romans were really just Etruscans. It's very clear that the Latin clans were still alive and well at that period, and that these peoples (the Etruscans and the Romans) spoke different languages, from totally different language families, and that the Indo-European Latin language prevailed.
As to the precise genetic differences, what I would say is that we don't have any "Roman" dna of that period, or any period for that matter, and as I've posted before, the only Etruscan dna we have is some HVRI values which could just as well have been in place since the Neolithic. So, everything is basically conjecture. The Italici, of which the Romans were one group, may have been significantly different from the Etruscans when they first arrived in the peninsula; we just don't know. What seems obvious, however, even if both groups came from elsewhere during the Bronze Age, or early Iron Age, (for the Etruscans there is absolutely no archaeological evidence of a mass migration at this time, so it would have to have been a small group that formed an elite) is that they would have mingled with the pre-existing population, and then with each other.
As to "replacement" in Italy, the actual science doesn't support any such hypothesis; quite the contrary. Rather, it paints a picture of continuity since about the middle of the first millennium B.C., a continuity that is rare in Europe. I don't know why so many people seem to be unaware of the latest research using IBD analysis.
Ralph and Coop et al:
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001555
The discussion at Discovery:
http://blogs.discovermagazine.com/gnxp/?p=20961#.Uhef6T_pxdE
This doesn't mean, of course, as Razib takes pains to point out, that there wasn't significant population substructure dating from that time, because there was, owing perhaps the most to the Celtic migrations in the north and the Greek colonization in the south.
It also doesn't mean that Italians don't all cluster together, however, as indeed they do, and which can be seem on any academic PCA plot. You don't have all these Italians clustering in Greece or Spain or France or Switzerland, the way that the lines are blurred between, say, the Low Countries and England, or the Scandinavian countries and England. You can go all the way back to Lao et al, and his finding that one of the major breaks in the European cline (another one being near Finland) can be located at the Alps. Within Italy itself, there is a lesser break in the cline just south of Rome, which may indeed be due to the Greek colonizations which I mentioned, but which could also be a result of some small influences from the Moorish kingdoms of Sicily and the southern part of the peninsula, and then to the fact that these provinces formed part of the general area of The Kingdom of the Two Sicilies for so long, and therefore what gene flow was experienced was largely confined to that area until perhaps fifty years ago. Despite the delusions of the followers of Lega Nord, there is no genetic evidence of any distinct population of "Padanians" who live north of the Po. Rather, except for the slight break south of Rome, Italian genetics is basically clinal except for some small genetic isolates.
What has to be remembered is that Italy has maintained high population densities since the Neolithic. (more Cardial in some areas, and more Danubian in others, but any rate, it does not seem that it experienced the type of population crash that took place in the LBK, or even in the Balkans) The Italici then appear all over the peninsula and into Sicily, with their new Indo-European languages. On top of those layers, you have the migrations of the first millennium B.C. of the Greeks into the south, and the "Celtici" or "Galli" into the north. (Whether they were substantially different genetically from the earlier Italici or the mysterious Liguri is a whole other discussion that I don't think can be answered at this time. What should be remembered, however, is that if the historical sources are correct, many of these late "Gallic" migrations ended in slavery for the invaders, while some of them, like the Boi who settled Bologna, left for Dacia or France. I don't mean to imply that some of them did not remain, but I think their influence can be overblown.) Following this, you have a concerted policy by Rome to settle all parts of Gallia Cisalpina, which means basically Italy from the Alps to the Rubicon with colony after colony of Roman settlers. The Romans knew what they were about in terms of pacifying and unifying the peninsula.
That is basically the ethnogenesis, so far as I currently understand it. What the Ralph and Coop study shows, if they are correct, and nobody seems to have challenged them yet, is that there were no further *major* gene flows into Italy, with the possible exception of some from the Moors in Sicily in particular, and perhaps in lesser degree in some other areas of the south. The Germanic invasions, seem to have had little influence autosomally, and the Slavic ones virtually none. (They maintain that the same is true for the Iberian peninsula) If people are looking for total population replacement, they need to look to the population history of northern Europe.
As to cultural matters, there are numerous full length books and scholarly papers on the intertwined cultures of Rome, Etruria and Greece that would clarify matters for anyone interested in the subject.