Among the reports that used DNA genotyping to verify zygosity assignment, this is the first that involves comparison to questionnaire responses in adult twins. In our twins the questionnaires were completed over 30 years earlier. All of the other studies using DNA for verification involved comparison of questionnaire data completed by the parents of twin children (Spitz et al., 1996
; Charlemaine et al., 1997
; Chen et al., 1999
; Reitveld et al., 2000
.) In the NAS-NRC twins, previous comparisons with serological markers indicated that there were more MZ pairs who thought they were DZ (Jablon et al., 1967
). With over a three-fold larger sample size, Hrubec and Neel (1978) still found that 97.8% of DZ and 92.4% of MZ pairs were correctly classified via questionnaires in this cohort. Another U.S. twin adult twin cohort (King et al., 1980
) also reported better classification of DZ pairs (97% versus 83%). Much of the difference was attributed to the MZ pairs’ parents being told their twins were DZ because of two placentas at birth. Torgersen (1979)
found slightly better classification via questionnaire data based on similarity in adult DZ twin pairs, while Cederlof et al. (1961)
reported better classification in MZ pairs. In children where twins are classified based on the responses of their parents to questionnaires, generally classification is better in MZ pairs. Nichols and Bilbro (1966)
provide a good explanation for the latter observation “in that as many as 10% of MZ pairs do not appear strikingly similar and are more easily misdiagnosed as DZ while fewer DZ pairs are similar enough to erroneously be called MZ”.
The twins in the aging study had fewer errors in assignment of zygosity from MZ pairs in a cohort that overall had better classification of DZ twins. We believe it unlikely that this observation could reflect errors in handling of samples in the laboratory. The explanation could simply be a chance occurrence or the difference may reflect how the twins were ascertained for the aging study. There is some evidence in the NAS-NRC twins and the NHLBI subset for greater mortality in DZ twins (D. Carmelli, personal communication). With aging, a higher percentage of surviving pairs are more likely to be MZ. In this study we selected for pairs in which both co-twins had to survive into their 70s, both co-twins had to complete the Q8 questionnaire, at least one of the two co-twins had to meet the healthy physical aging definition, and both had to agree to go to a local health care facility or provider to have blood samples drawn. In the other two studies included in this report pairs were recruited for Alzheimer or Parkinson’s disease in at least one co-twin with likely interest for participation because of possible risk for the unaffected co-twin. In these combined groups, there was a trend towards better classification in DZ pairs more like estimates from the overall cohort. Twins of unknown zygosity via similarity questionnaires, are most likely very similar DZ pairs or dissimilar MZ pairs Allen (1976)
. Unknown zygosity pairs were spread rather evenly between MZ and DZ pairs across the three NAS-NRC sub-groups.
Overall 96.8% of the pairs who provided a blood sample for genetic markers in the three studies were correctly classified in the NAS-NRC master database based on questionnaire responses in the 1960s. Our results suggest that using a conservative estimate of 95% of pairs correctly classified based on questionnaire data for the NAS-NRC twins and other twin cohorts regardless of whether serologic markers or DNA genotyping is used (Hrubec & Neel, 1981
; Magnus et al., 1983
; Reitveld et al., 2000
) is appropriate. The error rate has little effect on heritability estimates and if anything leads to more conservative figures. We also caution that with the use of highly polymorphic DNA markers, if a pair of twins differs in genotype on just one marker, the test probably should be repeated. For example, using the 10 microsatellite markers typed for pairs at the end of the aging twin study, if a pair matches on the first nine markers the probability the pair is really DZ is 0.0001. If there is an allele difference on the tenth marker, then it may be as plausible to suspect a genotyping error as to go ahead and assume that the pair is now DZ. Repeating the test can resolve if the initial discordance is an error in genotyping; a difference in one allele might also be due to a rare post-zygotic mutation. If DZ twins are used for studies to find genes associated with complex traits (Boomsma et al. 2002
), then even small errors in zygosity assignment may affect power to detect true associations with limited sample sizes.