Population prevalence of the ApoE4 gene

Regarding the Alzheimer genotype, I just did a little research regarding the prevalence of the ApoE epsilon 4 allele (the allele that predisposes to Alzheimer disease). Depending on the study you read, about 20% of the population has at least one epsilon 4 allele, while about 2% have two epsilon 4 alleles. As you'd expect, having two is worse than having one in terms of Alzheimer risk. I should add that the epsilon 4 allele also predisposes to a worse outcome in recovery from traumatic brain injury and is also over-represented among those football players who suffer from Chronic Traumatic Encephalopathy.