COMMON GENETIC INFLUENCES ON BMI AND AGE AT MENARCHE

Genetic influences on variability of body weight and onset of menarche are well known. To investigate the genetic and environmental contribu tions to the association of body weight with onset of menarche, we stu died Finnish twins from consecutive birth cohorts (the Finn Twin 16 st udy) ascertained from the national population registry, which identifi es nearly 100% of all living twins. Baseline questionnaires were maile d to the twins within 60 days of their sixteenth birthday and later to older sibs of the twins. Pairwise response rates (similar to 85% acro ss gender and zygosity) and 30 months of data collection yielded resul ts from 1283 twin pairs, The questionnaires included a survey of healt h habits and attitudes, a symptom checklist, MMPI personality scales, and a survey of relationships with parents, peers, and the co-twin. Ag e at menarche was reported by 468 monozygotic (MZ) girls, 378 girls fr om like-sex dizygotic (FDZ) pairs, 434 girls from opposite-sex (OSDZ) pairs, and 141 older female sibs of the twins. The one-month test-rete st reliability of age at menarche in an independent sample (N = 136) o f 16-year-olds from a national survey was 0.96. Girls from OSDZ pairs had a significantly higher mean age at menarche (13.33 yr) than FDZ gi rls (13.13 yr) (difference, 0.20 yr; 95% confidence interval, 0.05-0.3 5). The MZ correlation for age at menarche was 0.75, the DZ correlatio n was 0.31, and the sib-twin correlation was 0.32. A bivariate twin an alysis of age at menarche and body mass index (wt/ht(2)) indicated tha t 37% of the variance in age at menarche can be attributed to additive genetic effects, 37% to dominance effects, and 26% to unique environm ental effects. The correlation between additive genetic effects on age at menarche and body mass index was 0.57, indicating a substantial pr oportion of genetic effects in common.