Another thing that counter your point of view is the Bottleneck effect. White north americans are LESS MIXED than white Europeans; the Bottleneck Effect, or Founder effect being "the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population". The White american population comes from a very small part of the European genepool who spreaded inside north America, there is actually less genetic variation within the American population, as there is typically less genetic variation within a newly settled area.