European Americans have a larger proportion of potentially harmful variants than African Americans --- probably an artefact of their original migration out of Africa.
The human genome has been busy over the past 5,000 years. Human populations have grown exponentially, and new genetic mutations arise with each generation. Humans now have a vast abundance of rare genetic variants in the protein-encoding sections of the genome.
A study published in Nature now helps to clarify when many of those rare variants arose. Researchers used deep sequencing to locate and date more than one million single-nucleotide variants — locations where a single letter of the DNA sequence is different from other individuals — in the genomes of 6,500 African and European Americans.
The findings confirm their earlier work suggesting that the majority of variants, including potentially harmful ones, were picked up during the past 5,000–-10,000 years.