The findings of the study, published within the last 2 years, indicate that white people are, on average, 98.6% white.  They are less than 2/10ths of 1% African or Native American, with the numbers being 0.19% and 0.18% respectively. That is tiny, and as one researcher noted, it is in keeping with American history of genocide and slavery that endured for the entirety of America's history, and rears its ugly head in the white supremacist backlash we are seeing now.   

This sort of impenetrable racial divide doesn't occur on a conscious level. It is more indicative of, among other things, learned values, and EuroAmericans who sincerely believe their worldview and cultural values are superior to those of all other people.  It is also indicative of the kind of rejection the culture exhibits toward its members who stray from the accepted worldview and values.  Robin DiAngelo, PhD, offers excellent insight into this, and I recommend her work to anyone who truly wishes to understand racism.