Advances in molecular genetics now allow the opportunity for international studies such as ours to explore etiological hypotheses involving specific genes rather than simple comparisons between races or ethnic groups. One of the most consistent findings in AD research has been the association of the possession of the apolipoprotein E (APOE) ∊4 allele to increased susceptibility for AD.³⁸ In contrast to reports from other populations, we have found only a marginally significant association between possession of the ∊4 allele and AD in the African American population³⁹ similar to other study reports.⁴⁰ In the Yoruba, we have found no significant association between the possession of the ∊4 allele and dementia or AD in either the heterozygous or homozygous states.⁴¹ As the frequencies of the 3 major APOE alleles are almost identical in the 2 populations, this variation in the strength of the association between ∊4 and AD may account for some of the differences in incidence rates between the populations, although it is not likely to explain all of it. It also raises the possibility that some other genetic or environmental factor affects the association of the ∊4 allele to AD and reduces incidence rates for dementia and AD in Yoruba.

https://jamanetwork.com/journals/jama/fullarticle/193548