Thirteen sets of monozygotic (MZ) twins from an extended multi-generation family are reported. Zygosity was determined by interviewing families for overall physical similarity and by assessment of facial photographs. A hypothetical gene was traced back five generations to a common grandfather. Familial monozygotic twinning in this pedigree is compatible with autosomal dominant inheritance with reduced penetrance. Other plausible mechanisms of inheritance are discussed.

The purpose of this study was to examine whether dichorionic twins conceived by assisted reproductive technology (ART; intracytoplasmic sperm injection [ICSI], in vitro fertilization [IVF], gameteintrafallopian tube transfer [GIFT]) have a higher risk of birth defects compared to dichorionic twins conceived naturally. We reviewed the medical records of 406 mothers with dichorionic twin pregnancies, who received continuous antenatal care from ≤ 20 weeks of gestation and gave birth to infants after ≥ 24 weeks of gestation in our institute. Birth defects were diagnosed at the time of hospital discharge according to the International Classification of Diseases, 10th Revision. Occurrence of birth defects was compared between twins conceived by ART and those conceived naturally using logistic regression analysis. Overall, 51 of 812 infants (51/812 = 6.2%) had birth defects. The incidence of birth defects in ART-conceived twins was significantly higher than that of naturally conceived twins with an odds ratio of 6.9 (95% confidence interval [CI] 2.1, 22.5), 3.7 (95% CI 1.2, 12.0), and 4.3 (95% CI 1.4, 14.3) for ICSI, IVF, and GIFT, respectively. The higher frequency of birth defects in ART-conceived twins was still significant after adjusting for higher maternal age in the ART group, with an adjusted odds ratio of 6.7 (95% CI 2.1, 21.9), 3.6 (95% CI 1.1, 11.5), and 3.7 (95% CI 1.2-11.8) for ICSI, IVF, and GIFT, respectively. Dichorionic twins conceived by ART, compared to dichorionic twins conceived naturally, had a much higher risk for birth defects diagnosed at hospital discharge.

Though twinning rates have been rapidly increasing in Japan, the problem of zygosity misclassification at birth has been paid little attention. By analyzing four independent samples, the authors found that at a constant rate about 25–30% of monozygotic twins were misclassified as dizygotic twins at birth. This percentage is in very good accordance with that of monozygotic twins having dizygous placenta. Generally the obstetricians informed twins' parents about their children's zygosity. The number of placentas, as informed by obstetricians, was very strongly associated with zygosity. Concluding, even now many monozygotic twins in Japan may be misclassified as dizygotic at birth by obstetricians based solely on the number of placenta.

Stillbirth rates of triplet births in the whole of Japan were analyzed using vital statistics from 1975 to 1998. Stillbirths were registered at 12 weeks gestation or later. The stillbirth rate was significantly higher in like- than in unlike-sex triplets for 1975–1998. During the 23-year period the stillbirth rate decreased from 342 to 49 per 1000 total births for like-sex and from 195 to 54 for unlike-sex triplets. The decrease in the stillbirth rate in the 23-year period was greater in both like- and unlike-sex triplets than in singleton and twin births. Risk factors for stillbirth in triplets were like-sex, youngest or oldest maternal age groups, shorter gestational age and lower birthweight. It is recommended that the optimum period to give birth for triplet pregnancies is 34–35 weeks of gestation for Japanese women.

Although genetic factors are recognised as major contributors to otitis media, the presence of sex differences in heritability needs clarification. The aim of this study was to estimate the relative contribution of genetic and environmental effects in otitis media liability with particular focus on sex differences. Data from a cohort of Norwegian twins born between 1967 and 1979 with repeated measures on recurrent childhood otitis media were analysed. Altogether the sample included 4247 twin pairs. The tetrachoric correlations for monozygotic twins were .71 and .65 for males and females respectively. In dizygotic twins the correlations were .35 and .25 for males and females, respectively, and was .34 in opposite sexed pairs. The contribution of genetic and environmental effects was analyzed using structural equation modeling. The best fitting model showed that additive genetic effects explained 72% and 61% of the variance in males and females, respectively. The remaining variance was attributed to individual environmental effects. A model specifying equal heritability estimates for males and females yielded an almost equivalent fit. We found substantial genetic effects for liability to otitis media. There is no evidence that different sets of genes influence liability in males and females, but there may be sex differences in the relative importance of genetic effects.

The heritability of blood pressure estimated in previous studies may be confounded by the influence of potential blood pressure risk factors. We applied the classical twin design to estimate the contribution of these covariates to blood pressure heritability. The study consisted of 173 dizygotic and 251 monozygotic twin pairs aged 18–34 years, randomly selected from the East Flanders Prospective Twin Survey. In a standardized examination, blood pressure and anthropometry was measured, a questionnaire was completed, and a fasting blood sample was taken. In univariate and bivariate modeling, diastolic and systolic heritability were estimated both unadjusted and adjusted for potential risk factors. Also, covariate interaction was modeled. Bivariate analysis gave heritability estimates of 0.63 (95%CI 0.55–0.59), 0.74 (95%CI: 0.68–0.79), and 0.78 (95%CI: 0.70–0.84) for diastolic, systolic, and cross-trait heritability, respectively. The remaining variances could be attributed to unique environmental influences. These heritability estimates did not change substantially in univariate analyses or after adjustment for risk factors. A sex-limitation model showed that the heritability estimates for women were significantly higher than for men, but the same genetic factors were operating across sexes. Sex and cigarette smoking appeared to be statistically significant interaction terms. The heritability of blood pressure is relatively high in young adults. Potential risk factors of blood pressure do not appear to confound the heritability estimates. However, gene by sex by smoking interaction is indicated.

Arange of environmental risk factors, with childbirth the most notable, have been associated with the development of pelvic organ prolapse and urinary incontinence. However, indications of genetic influence (positive family histories, ethnic differences) have prompted research into the heritability of measures of pelvic organ descent and joint mobility, which have also been associated with prolapse and incontinence. Genes appear to influence about half of the variation in these measures and, furthermore, the pelvic organ measures are associated with elbow hyperextension at a phenotypic level (r ≈ .2). We examined these measures in young, nulligravid women to determine if their association is due to a common genetic source. Data were collected from 178 Caucasian female co-twins and non-twin sisters, 50 of whom returned to be retested, which allowed reliability to be estimated and unreliable variance to be isolated in the multivariate analyses. Structural equation modeling was used to estimate genetic associations between latent elbow and bladder mobility factors for which heritabilities were estimated to be 0.80 and 0.64 respectively. The association between these factors appeared to be mediated by common genes (genetic r =.48, non-shared environmental r = −.06), with genes influencing latent elbow mobility accounting for 14% of the variation in latent bladder mobility. We speculate that genes influencing connective tissue structure may underlie this association.

This study examined genetic and environmental contributions to the covariance between aggression and hyperactivity-impulsivity as rated by twins' teachers and parents. Sex-differences in these genetic and environmental contributions and rater bias/sibling interaction effects were of interest as well. Part of an ongoing nation-wide twin-family study of behavioral development and health habits, the sample consisted of 1636 Finnish twin pairs ascertained from five consecutive and complete twin birth cohorts. Data were collected at ages 11–12, using teacher and parental rating forms of the Multidimensional Peer Nomination Inventory. Bivariate analyses were performed using structural equation modeling allowing sex-limitation effects. Results show that, in addition to significant genetic and environmental influences specific to each behavior, aggression and hyperactivity-impulsivity share common genetic and environmental etiology. Results provide evidence that both genetic and environmental factors are important in creating the observed correlation between aggression and hyper-activity-impulsivity.

The genetic and environmental relationships among measures of phoneme awareness, naming speed, Intelligence Quotient (IQ), and reading performance were investigated in 623 identical and fraternal twin pairs tested in the Colorado Learning Disabilities Research Center. A Cholesky decomposition analysis of these measures provided evidence supporting the double deficit hypothesis that difficulties in phonological processing and naming speed both contribute to reading disability. Additionally, the model revealed marginally significant genetic and significant non-shared environmental relationships between IQ and reading independent of naming speed and phoneme awareness. Thus a more complete causal model of reading disability should include IQ as well as measures of phonological processing and naming speed.

Vocabulary test scores were obtained from a total of 997 adults, all twins or a sibling of twins in this study. Some (N = 217) individuals were tested twice, around 6 years apart. Heritability varied from 50% at the first test occasion to 63% at the second test occasion. The correlation of scores across time was .74. Structural equation modelling showed that stability in vocabulary knowledge over time can largely (around 76%) be explained by genetic factors. Part of the non-shared environmental variance was stable over time also. Any influence from shared environmental factors could not be detected. Results were similar for the two sexes, except that males generally outperformed females. Results were also similar for two age cohorts, except that the older cohort generally outperformed the younger cohort.

Using univariate sum scores in genetic studies of twin data is common practice. This practice precludes an investigation of the measurement model relating the individual items to an underlying factor. Absence of measurement invariance across a grouping variable such as gender or environmental exposure refers to group differences with respect to the measurement model. It is shown that a decomposition of a sum score into genetic and environmental variance components leads to path coefficients of the additive genetic factor that are biased differentially across groups if individual items are non-invariant. The arising group differences in path coefficients are identical to what is known as “scalar sex limitation” when gender is the grouping variable, or as “gene by environment interaction” when environmental exposure is the grouping variable. In both cases the interpretation would be in terms of a group-specific effect size of the genetic factor. This interpretation may be incorrect if individual items are non-invariant.

The purpose of this article is to present alternative parameterizations of scalar and non-scalar sex-limitation models in the Mx matrix algebra program (Neale et al., 2002). These models are designed for use with extended pedigrees and take advantage of the dynamic treatment of covariates within Mx. Example scripts are provided.