Admixture is the term to describe the amount of genetics from another race different from the overall physical appearance of a selected population.The amount of non-Cacauasian admixture in Caucasian groups varies. Some populations of Ashkenazi Jews have at least 5%[].
Europe and AsiansEdit
Some White Americans have a small amount of Native American or Black , and Blacks have some White (aside from African Immigrants, who may have negligible admixture). One third of Whites have Black heritage. Skin pigmentation, biogeographical ancestry and admixture mapping
About one-third of White Americans are of between two and twenty percent recent African genetic admixture, as measured by the ancestry-informative markers in their DNA.19 This comes to about 74 million Americans. 
They look white but they aren't, 0ver 70 million Americans are white but not quite, of course none of that matters except to racists. Those racists probably don't know that one of their great grandparents did something with someone who was part black. 
No matter how much or how little race genetically exists, race is still a powerful decider of who people choose to be with (is oil inferior to water?); to most people others are foreign. Thus it's nice to know that "you" contain a little bit of "them" that look "different".
Sometimes we turn against foreigners but other times we really like them. Being attracted to someone from a different ethnicity or a different community is quite natural and humanity benefits from this as favourable genes are spread between communities and ethnicities that way. See Why humans are sometimes sexually attracted to foreigners or others from outside their community.