The Roman legionaries and their officers of the classical Roman era were fierce, brave and resourceful warriors and soldiers. They were courageous and devoted to winning at any cost. These qualities enabled ancient, Rome and the Roman people to not only defend themselves from invaders like Hannibal, but to make successful forays into adjoining areas like cisalpine and transalpine Gaul. The present day Italians do not appear to be the same people's who created or improved so much technology and could wage total war. They do not appear to be the same people who could lose 77,000 soldiers in one battle and still win the war. *lastly, I once read something to the effect that the roman race was bred put or mostly died out of the Italian peninsula.
Yes and no. The Roman Empire was to some effect, a forced melting pot. Even when Rome was no more than a prominent city on the peninsula, they were conquering and assimilating other Italian powers under their rule.
As history progresses, it becomes an issue of when and how you want to draw the line between Romans and non-Romans. You could draw it when Rome was only a city state in Italy before it conquered and assimilated Greek colonies in the south of the peninsula, or when Romans started founding towns on the frontier in Gaul and assimilating the locals into their military. Both of the latter certainly lived under Roman rule and shared their culture, but do you consider them Roman? Later in its history, Rome was sacked several times by gothic tribes, and several more by rogue generals who would lead an army of barbarians from gaul back to Rome and appoint themselves emperor. Many of these people stayed on the peninsula and lived in Rome. Later, this mixture of Roman and barbarian peoples in Italy was conquered by Justinian of the Eastern Empire, or Byzantium. The Byzantines considered themselves Roman, but do you?
Lastly I would like to point out that sharing genetics/heritage does not equate to sharing a culture. The people of Italy today identify as many things, Italian, Catholic, Tuscan, Sicilian, ect. but none consider themselves Roman. Just as the French don't see themselves as Gauls, or Germans don't identify as people of the Holy Roman Empire. Culture evolves throughout history, and peoples can't be expected to behave as their ancestors did. The other post with with the links are also a great places to start.
You might want to step over to /r/askscience to find out how genetics works. I suspect you don't realize that your question is racist.
hi! not discouraging anyone from contributing more info, but fyi, you can get started on several related posts:
there are a few relevant posts in FAQ section Transformation of the Roman Identity in Italy
Can modern day Italians call themselves Romans (or their descendants) and be correct?
As far as genetics go...