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A foundational question in the social sciences concerns the interplay of underlying causes in the formation of people’s political beliefs and prejudices. What role, if any, do genes, environmental influences, or personality dispositions play? Social dominance orientation (SDO), an influential index of people’s general attitudes toward intergroup hierarchy, correlates robustly with political beliefs. SDO consists of the subdimensions SDO-dominance (SDO-D), which is the desire people have for some groups to be actively oppressed by others, and SDO-egalitarianism (SDO-E), a preference for intergroup inequality. Using a twin design (n = 1,987), we investigate whether the desire for intergroup dominance and inequality makes up a genetically grounded behavioral syndrome. Specifically, we investigate the heritability of SDO, in addition to whether it genetically correlates with support for political policies concerning the distribution of power and resources to different social groups. In addition to moderate heritability estimates for SDO-D and SDO-E (37% and 24%, respectively), we find that the genetic correlation between these subdimensions and political attitudes was overall high (mean genetic correlation 0.51), while the environmental correlation was very low (mean environmental correlation 0.08). This suggests that the relationship between political attitudes and SDO-D and SDO-E is grounded in common genetics, such that the desire for (versus opposition to) intergroup inequality and support for political attitudes that serve to enhance (versus attenuate) societal disparities form convergent strategies for navigating group-based dominance hierarchies.

We investigated the heritability of educational attainment and how it differed between birth cohorts and cultural–geographic regions. A classical twin design was applied to pooled data from 28 cohorts representing 16 countries and including 193,518 twins with information on educational attainment at 25 years of age or older. Genetic factors explained the major part of individual differences in educational attainment (heritability: a 2  = 0.43; 0.41–0.44), but also environmental variation shared by co-twins was substantial (c 2  = 0.31; 0.30–0.33). The proportions of educational variation explained by genetic and shared environmental factors did not differ between Europe, North America and Australia, and East Asia. When restricted to twins 30 years or older to confirm finalized education, the heritability was higher in the older cohorts born in 1900–1949 (a 2  = 0.44; 0.41–0.46) than in the later cohorts born in 1950–1989 (a 2  = 0.38; 0.36–0.40), with a corresponding lower influence of common environmental factors (c 2  = 0.31; 0.29–0.33 and c 2  = 0.34; 0.32–0.36, respectively). In conclusion, both genetic and environmental factors shared by co-twins have an important influence on individual differences in educational attainment. The effect of genetic factors on educational attainment has decreased from the cohorts born before to those born after the 1950s.

Monozygotic (MZ) twins are theoretically genetically identical. Although they are revealed to accumulate mutations after the zygote splits, discriminating between twin genomes remains a formidable challenge in the field of forensic genetics. Single-nucleotide variants (SNVs) are responsible for a substantial portion of genetic variation, thus potentially serving as promising biomarkers for the identification of MZ twins. In this study, we sequenced the whole genome of a pair of female MZ twins when they were 27 and 33 years old to approximately 30 × coverage using peripheral blood on an Illumina NovaSeq 6000 Sequencing System. Potentially discordant SNVs supported by whole-genome sequencing were validated extensively by amplicon-based targeted deep sequencing and Sanger sequencing. In total, we found nine bona fide post-twinning SNVs, all of which were identified in the younger genomes and found in the older genomes. None of the SNVs occurred within coding exons, three of which were observed in introns, supported by whole-exome sequencing results. A double-blind test was employed, and the reliability of MZ twin discrimination by discordant SNVs was endorsed. All SNVs were successfully detected when input DNA amounts decreased to 0.25 ng, and reliable detection was limited to seven SNVs below 0.075 ng input. This comprehensive analysis confirms that SNVs could serve as cost-effective biomarkers for MZ twin discrimination.

The present study examined the longitudinal associations between three dimensions of temperament – activity, affect-extraversion, and task orientation – and childhood aggression. Using 131 monozygotic and 173 dizygotic (86 same-sex) twin pairs from the Louisville Twin Study, we elucidated the ages, from 6 to 36 months, at which each temperament dimension began to correlate with aggression at age 7. We employed latent growth modeling to show that developmental increases (i.e., slopes) in activity were positively associated with aggression, whereas increases in affect-extraversion and task orientation were negatively associated with aggression. Genetically informed models revealed that correlations between temperament and aggression were primarily explained by common genetic variance, with nonshared environmental variance accounting for a small proportion of each correlation by 36 months. Genetic variance explained the correlations of the slopes of activity and task orientation with aggression. Nonshared environmental variance accounted for almost half of the correlation between the slopes of affect-extraversion and aggression. Exploratory analyses revealed quantitative sex differences in each temperament-aggression association. By establishing which dimensions of temperament correlate with aggression, as well as when and how they do so, our work informs the development of future child and family interventions for children at highest risk of aggression.

Childhood adverse experiences are known to induce persistent changes in the hypothalamic-pituitary-adrenal (HPA) axis reactivity to stress. However, the mechanisms by which these experiences shape the neuroendocrine response to stress remain unclear. Method We tested whether bullying victimization influenced serotonin transporter gene (SERT) DNA methylation using a discordant monozygotic (MZ) twin design. A subsample of 28 MZ twin pairs discordant for bullying victimization, with data on cortisol and DNA methylation, were identified in the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative 1994-1995 cohort of families with twins. Bullied twins had higher SERT DNA methylation at the age of 10 years compared with their non-bullied MZ co-twins. This group difference cannot be attributed to the children's genetic makeup or their shared familial environments because of the study design. Bullied twins also showed increasing methylation levels between the age of 5 years, prior to bullying victimization, and the age of 10 years whereas no such increase was detected in non-bullied twins across time. Moreover, children with higher SERT methylation levels had blunted cortisol responses to stress. Our study extends findings drawn from animal models, supports the hypothesis that early-life stress modifies DNA methylation at a specific cytosine-phosphate-guanine (CpG) site in the SERT promoter and HPA functioning and suggests that these two systems may be functionally associated.

Abstract Context Clarification of the association among phenotypes, genetic, and environmental factors with clinical laboratory traits can reveal the cause of diseases and assist in developing methods for the prediction and prevention of diseases. It is difficult to investigate the environmental effect on phenotypes using individual samples because their genetic and environmental factors differ, but we can easily investigate the influence of environmental factors using monozygotic (MZ) twins because they have the same genetic factors. Objective We aimed to examine the methylation level of CpG sites as an environmental factor affecting adiponectin levels on the basis of the same genetic background using MZ twins and to identify the epigenetic factors related to adiponectin levels and the genetic factors associated with sensitivity to acquired changes in adiponectin. Methods Using 2 groups built from each twin of 232 MZ twin pairs, we performed a replicated epigenome-wide association study to clarify the epigenetic factors affecting adiponectin levels adjusted by genetic risk score. Moreover, we divided twin pairs into concordant and discordant for adiponectin levels. We conducted a genome-wide association study to identify a genetic background specific for discordance. Results Methylation levels at 38 CpG sites were reproducibly associated with adjusted adiponectin levels, and some of these CpG sites were in genes related to adiponectin, including CDH13. Some genes related to adiponectin or insulin resistance were found to be genetic factors specific for discordance. Conclusion We clarified specific epigenetic factors affecting adiponectin levels and genetic factors associated with sensitivity to acquired changes in adiponectin.

Multiple pregnancies are associated with significant maternal, fetal, and neonatal risks, including prematurity, low birth weight, pre-eclampsia, anemia, postpartum hemorrhage, intrauterine growth restriction, neonatal morbidity, and increased neonatal and infant mortality rates. Assisted reproductive technology (ART) treatments should prioritize efforts to reduce such events, resisting patient demand for the transfer of multiple embryos at each transfer to increase success rates. Extended culture, embryo selection, and single blastocyst transfer can mitigate the risk of high-order multiple pregnancies. Intriguingly, elective single-embryo transfer (eSET) greatly reduces, but does not completely eliminate, the likelihood of multiple gestations. The occurrence of monozygotic twinning (MZT) gives rise to identical twins. It is more prevalent in women undergoing in vitro fertilization (IVF) compared with natural conception. In fact, the reported risks of monozygotic twinning in IVF and natural conception are 1.7 and 0.4%, respectively. The factors suspected to increase the risk of MZT in IVF are multiple embryo transfer, micromanipulation, and extended in vitro culture. Determining chorionicity and amnionicity is crucial in the assessment of multiple pregnancies during the first-trimester ultrasound examination. Dichorionic twins result from embryo splitting within 3 days after fertilization, while monochorionic twins occur when the splitting takes place between 4 and 8 days after fertilization. These timings are suggested by observations carried out in natural pregnancies. In ART, there is evidence of dichorionic twins derived from single embryo transfer (SET). Here, we report a case of dichorionic diamniotic triplets after a single blastocyst transfer occurred in our center. To our knowledge, this is the first case documented so far.

Monozygous twins share a common genotype. However, most monozygotic twin pairs are not identical; several types of phenotypic discordance may be observed, such as differences in susceptibilities to disease and a wide range of anthropomorphic features. There are several possible explanations for these observations, but one is the existence of epigenetic differences. To address this issue, we examined the global and locus-specific differences in DNA methylation and histone acetylation of a large cohort of monozygotic twins. We found that, although twins are epigenetically indistinguishable during the early years of life, older monozygous twins exhibited remarkable differences in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression portrait. These findings indicate how an appreciation of epigenetics is missing from our understanding of how different phenotypes can be originated from the same genotype.

PurposeTo determine whether maternal age has an impact on monozygotic twinning (MZT) rates in women undergoing single embryo transfer (SET).MethodsThis is a retrospective cohort study analyzed for the incidence of MZT of all clinical pregnancies after a single embryo transfer was carried out between 2014 and 2018. The effect of different assisted reproductive technology (ART) parameters on the incidence of MZT was evaluated.ResultsThere were a total of 8459 cycles resulting in pregnancy during the study period. Of these pregnancies, 8236 were singletons and 223 were MZT. The preterm birth rate, miscarriage rate, and cesarean section rate were higher in MZT. Birth weight and gestational age at delivery were lower and smaller. In the univariate analysis, the risk of MZT was decreased with frozen embryo transfer (ET). A nonlinear relationship was observed between maternal age and MZT. A negative relationship between maternal age and MZT was observed in the patients’ age ≥ 36 years.ConclusionAdvanced maternal age was associated with a lower rate of MZT. A threshold female age of 36 years existed for lower MZT.

Are genetic risk factors for current depressive symptoms good proxies for genetic risk factors for syndromal major depression (MD)? In over 9000 twins from the population-based Virginia Adult Twin Study of Psychiatric and Substance Use Disorders, the occurrence of all nine DSM symptomatic criteria for MD in the last year was assessed at personal interview and then grouped by their temporal co-occurrence. The DSM criteria which occurred outside (OUT) inside of (IN) MD episodes were then separated. We calculated tetrachoric correlations for OUT and IN depressive criteria in monozygotic (MZ) and dizygotic (DZ) pairs and fitted univariate and bivariate ACE twin models using OpenMx. The mean twin correlations (±95% CIs) for IN depressive criteria were substantially higher than for OUT depressive criteria in both MZ [+0.35 (0.32-0.38) 0.20 (0.17-0.24)] and DZ pairs [0.20 (0.17-0.24) 0.10 (0.04-0.16]. The mean IN-OUT cross-correlation in MZ and DZ pairs was modest [+0.15 (0.07-0.24) and +0.07 (0.03-0.12)]. The mean heritability estimates for the nine In Out depressive criteria was 0.31 (0.22-0.41) and 0.15 (0.08-0.21), in MZ and DZ pairs, respectively. The mean genetic correlation between the nine IN and OUT depressive criteria was +0.07 (-0.07 to 0.21). Depressive criteria occurring outside depressive episodes are less heritable than those occurring within. These two ways criteria can manifest are not closely genetically related. Current depressive symptoms - most of which are occurring outside of depressive episodes - are not, for genetic studies, good proxies for MD.