This is as clear display of the difference between biological and social race.
Populations from the three major human biological races (European Americans from Caucasoids, Yoruba from Negroids, Japanese/Chinese from Mongoloids) are clearly separable, with no overlap.
The "black race" to which African Americans are said to belong is seen as an almost perfect linear combination of Caucasoids and Negroids. It is not a biological race, but rather the result of admixture between the two races.
The same can be seen in other admixed groups such as the Uyghur, who are a combination of Caucasoids and Mongoloids. In that case, however, the admixture is more ancient, and the opportunity to further mix with representatives of the unadmixed groups is more limited. Therefore, the blend has been completed, and most individuals have similar admixture proportions from the ancestral groups. African Americans, on the other hand are much more variable in their individual ancestry components, from ~100% Negroid, to more Caucasoid than Negroid.
PLoS Genetics doi: 10.1371/journal.pgen.1000294
Effects of cis and trans Genetic Ancestry on Gene Expression in African Americans
Alkes L. Price et al.
Variation in gene expression is a fundamental aspect of human phenotypic variation. Several recent studies have analyzed gene expression levels in populations of different continental ancestry and reported population differences at a large number of genes. However, these differences could largely be due to non-genetic (e.g., environmental) effects. Here, we analyze gene expression levels in African American cell lines, which differ from previously analyzed cell lines in that individuals from this population inherit variable proportions of two continental ancestries. We first relate gene expression levels in individual African Americans to their genome-wide proportion of European ancestry. The results provide strong evidence of a genetic contribution to expression differences between European and African populations, validating previous findings. Second, we infer local ancestry (0, 1, or 2 European chromosomes) at each location in the genome and investigate the effects of ancestry proximal to the expressed gene (cis) versus ancestry elsewhere in the genome (trans). Both effects are highly significant, and we estimate that 12±3% of all heritable variation in human gene expression is due to cis variants.