Polymorphisms differ in their frequencies within human populations. Among coding region SNPs, synonymous SNPs are present, on average, at higher frequencies than nonsynonymous SNPs. For most genes, the nucleotide diversity, which reflects the number and the frequency of the SNPs, is greater for synonymous than for nonsynonymous SNPs. This reflects selective pressures (termed negative or purifying selection) that act to preserve the functional activity, and hence the amino acid sequence, of proteins. In human population studies using whole genome scanning, polymorphisms have been classified as either cosmopolitan (i.e., present in all ethnic groups) or population (or race and ethnic) specific. Cosmopolitan polymorphisms are present in all ethnic groups, although frequencies may differ among ethnic groups; they usually are found at higher allele frequencies in comparison to population-specific polymorphisms and are evolutionarily older.
African Americans have the highest number of population-specific polymorphisms in comparison to European Americans, Mexican Americans, and Asian Americans. Africans are believed to be the oldest population and therefore carry both recently derived, population-specific polymorphisms, and older cosmopolitan polymorphisms that occurred before migrations out of Africa.
Coding, nonsynonymous e.g., TPMV3A
Coding, synonymous e.g., ABCB1 C3435T
Noncoding, (promoter, intronic) e.g., CYP3A5*3
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