Across the ten SNPs analyzed, frequencies of the less-common alleles ranged from 5 percent to 44 percent. The double-helix structure of DNA means that each individual carries two copies of each nucleotide base in the DNA sequence. Each SNP genotype comprises a nucleotide from each strand. Therefore, individuals may carry 0, 1, or 2 copies of a given allele. Carriers of 0 or 2 copies are called “homozygotes.” Carriers of 1 copy are called “heterozygotes.” For several SNPs, there were very few homozygotes for the less-common allele (the expected proportion of homozygotes is the squared proportion of the allele frequency). We conducted primary analyses assuming an additive model (SNPs were coded 0, 1, or 2 according to the number of copies of the test allele). Because of the low frequencies of test-allele homozygotes for some SNPs, we repeated analyses using a dominance model. In the dominance model, a value of “1” indicated the presence of one or two test alleles and a value of “0” indicated the presence of no test alleles.
