The association between genetic ancestry and skin color has been well demonstrated (see here). The heritability of skin color is quite high suggesting that contribution of genetic components to skin color is high as well. Many genetic research projects support this showing that variants in pigmentation candidate genes are associated with skin color or other pigmentation traits (reviewed here). However, it is often difficult to demonstrate the association between genetic ancestry and common diseases because social-cultural factors (socio-economic status, education, income, employment status, access to health care, etc.) confound the relationship.
The association between skin color and blood pressure among African Americans has been investigated by many researchers (see here), but more recently researchers started using genetic ancestry estimates to test the association between African ancestry and blood pressure. In genetic studies, information on only clinically relevant variables are collected, so association between genetic ancestry and disease phenotype (here blood pressure) is often tested without adjusting for socio-cultural and environmental factors. Only a few studies conducted so far examined 1) effects of genetic ancestry and socio-cultural factors simultaneously and 2) interactions of these factors.
Non et al. (2012) explored the association between genetic ancestry and blood pressure and between education and blood pressure using publically available Family Blood Pressure Program (FBPP) study data. They found that education, not genetic ancestry, was significantly associated with blood pressure in African Americans and interaction between education and race also plays a role in the racial difference in blood pressure.
First, they examined if African ancestry estimated using 294 autosomal STR was a predictor of blood pressure in African Americans. They did not observe significant difference in African ancestry between hypertensive and non-hypertensive African Americans. In the linear regression model, education was significantly associated with blood pressure but African ancestry was not. They did not observe interaction between African ancestry and education either.
Second, they examined if education explains difference in blood pressure between African Americans and European Americans. African Americans with high-school degree or who did not complete high-school degree have significantly higher blood pressure than people with higher educations. They did not observe the same pattern in European American, however. In the regression analysis, they observed the interaction between education and race. In addition, they showed that decline in SBP is sharper with increasing educational levels in African Americans than in European Americans.
This is an interesting study to show that education (and probably poverty), not genetic ancestry, is a very important factor affecting blood pressure in African American, suggesting that obtaining higher education will benefit African Americans more than European Americans. As noted by the authors, one of the limitations of this study is that education was only social-environmental factor available in the dataset and they used it as a proxy for socio-economic status and other social environmental factors. While I agree that education can be very useful for capturing the many social-environmental factors and their analyses clearly show that socio-environmental factors measured with education are very important predictors or risk factors of blood pressure in African Americans, African American societies are too diverse and heterogeneous, and there should be many socio-environmental factors that affect blood pressure but not captured with education. They also could explore more what socio-economic factors were captured with education.
It is interesting to note that anthropologically trained scientists are showing that socio-cultural factors are as important as genetic factors assessed with genetic ancestry estimates and skin color. In other papers, Gravlee at al. (2005 and 2009) have shown that skin color or genetic ancestry is associated with blood pressure in Puerto Ricans, but social classification based on skin color is associated with blood pressure in Puerto Ricans. They also observed statistically significant interaction between skin color and socio-economic status.