Answer: The founder effect
Step-by-step explanation:
The founder effect is a phenomenon which occurs when a small group of individuals is isolated from a larger group. Regardless of the original characteristics of the large group, the community that results from the small group will have the genetical traits of the members that were isolated. This results in a loss of genetic variation.
In this case, if Vikings had survived, they would have been the founders of the small group that later would have resulted in the whole population of America. Therefore many Americans would have Nordic traits because those are the traits the original isolated group had.