The US did undergo ethnogenesis; particularly in the southeast, there have long been large numbers of people that identify their ethnicity as "American." The process was largely disrupted/reversed in the northeast with the Ellis Islander waves and then near-totally nationwide in recent decades. (The west was too new and too churny to have undergone anything like that.)

"African Americans" certainly also separately underwent ethnogenesis, although the preferred nomenclature there has changed, and there wasn't really any disruption there. But I think it's certainly fair to count them as a distinctly and uniquely US ethnicity.