As twin zygosity status had been based on ratings of twin resemblance during the Wave I interviews, zygosity status of the twins was again determined at Wave III using a panel of 11 highly polymorphic, unlinked short tandem repeat (STR) markers. These markers included D1S1679, D2S1384, D3S1766, D4S1627, D6S1277, D7S1808, D8S1119, D9S301, D13S796, D15S652 and D20S481 and the sex-determining locus, amelogenin (IBG-Hvar1, see http://ibgwww.colorado.edu/genotyping_lab/). The criterion used to assign monozygosity to a twin pair was 100% concordance of all genotypes at all 12 loci. A total of 34 pairs (9%) were found to have been incorrectly assigned based on questionnaire information, including 18 pairs for whom zygosity status could not be assigned from questionnaire information.
