In this article we introduce the first twin study in Turkey with the future plan to register all twins born in Turkey for research purposes. The Turkish twin study will illuminate the etiology of health problems in the context of cultural differences, owing to the rich cultural diversity in Turkey. As a first step, we selected a sample of 618 twin individuals from 302 pairs and 14 singletons living in the Kirikkale and Ankara regions of Turkey and administered a questionnaire with items about nicotine use, psychiatric disorders, and information about the family, in addition to zygosity items. Our study provides valuable information regarding factors related to nicotine dependence, age, gender, social situation, and family structure.

Since saliva collection is noninvasive, painless and inexpensive, it may become an alternative to obtain genomic DNA, which is critical to evaluate zygosity and the role of genetic factors in twin research. This study provided a rough description of salivary DNA in Chinese twin children, and presented the DNA yield and quality extracted from saliva in a large-scale children sample, which supplied an example for saliva sample using in genetic epidemiology. Three milliliters of saliva was collected from 356 twin children aged 6 to 15, and DNA was extracted by a com mercial DNA isolation kit. The DNA yield and purity was determined by spectrophotometry at 260nm and 280nm. The zygosity determination of the same-sex twins and the assay of Polymerase Chain Reaction-Restriction Fragment Length Polymorphism indicated the quality of salivary DNA. The amount of extracted DNA from three milliliters of saliva was about 34.91μg (2.20 ~ 122.04μg), average OD260/280 values was 1.84. Saliva DNA is a reliable sample for the determination of twins' zygosity. We conclude that saliva may be a feasible and reliable source of DNA for genetic epidemiology studies, especially for twin research.

Several large studies have demonstrated that the liability to smoke cigarettes is strongly genetically influenced. However, the role of genetic and environmental risk factors in the use of other common forms of tobacco use has yet to be studied. Data on the regular use of cigarettes, cigars, pipes, dip (moist snuff), and chewing tobacco from 2634 male twins were analyzed with ACE structural equation models. Twin similarity for regular cigarette and dip use was largely genetic in origin. However, twin resemblance for chewing tobacco was just about equally the result of genes and shared environment, and twin similarity for use of pipes and cigars was entirely the result of shared environmental factors. Thus, the genetic influences on the liability for regular tobacco use appear to vary based on tobacco type. The causes for the use of different forms of tobacco are complex and worthy of further study.

The Mid-Atlantic Twin Registry (MATR) is a population-based registry of more than 60,000 twins primarily born or living in Virginia, North Carolina and South Carolina. Researchers may utilize the MATR for administration of research services, including study recruitment, data or sample (e.g., DNA) collection, archival dataset creation, as well as data collection through mailed, phone or online surveys. In addition, the MATR houses the MATR Repository, with over 1700 DNA samples primarily from whole blood available for researchers interested in DNA genotyping. For over 40 years MATR twins have participated in research studies with investigators from a range of scientific disciplines and institutions. These studies, which have resulted in numerous publications, explored diverse topics, including substance use, smoking behaviors, developmental psychopathology, bullying, children’s health, cardiovascular disease, cancer, the human microbiome, epigenetics of aging, children of twins and sleep homeostasis. Researchers interested in utilizing twins are encouraged to contact the MATR to discuss potential research opportunities.

Structural brain magnetic resonance imaging (MRI) traits share part of their genetic variance with cognitive traits. Here, we use genetic association results from large meta-analytic studies of genome-wide association (GWA) for brain infarcts (BI), white matter hyperintensities, intracranial, hippocampal, and total brain volumes to estimate polygenic scores for these traits in three Scottish samples: Generation Scotland: Scottish Family Health Study (GS:SFHS), and the Lothian Birth Cohorts of 1936 (LBC1936) and 1921 (LBC1921). These five brain MRI trait polygenic scores were then used to: (1) predict corresponding MRI traits in the LBC1936 (numbers ranged 573 to 630 across traits), and (2) predict cognitive traits in all three cohorts (in 8,115–8,250 persons). In the LBC1936, all MRI phenotypic traits were correlated with at least one cognitive measure, and polygenic prediction of MRI traits was observed for intracranial volume. Meta-analysis of the correlations between MRI polygenic scores and cognitive traits revealed a significant negative correlation (maximal r = 0.08) between the HV polygenic score and measures of global cognitive ability collected in childhood and in old age in the Lothian Birth Cohorts. The lack of association to a related general cognitive measure when including the GS:SFHS points to either type 1 error or the importance of using prediction samples that closely match the demographics of the GWA samples from which prediction is based. Ideally, these analyses should be repeated in larger samples with data on both MRI and cognition, and using MRI GWA results from even larger meta-analysis studies.

The aim of the Avera Twin Register (ATR) is to establish a prospective longitudinal repository of twins, multiples, siblings and family members’ biological samples to study environmental and genetic influences on health and disease. Also, it is our intention to contribute to international genome-wide association study (GWAS) twin consortia when appropriate sample size is achieved within the ATR. The ATR is young compared with existing registers and continues to collect a longitudinal repository of biological specimens, survey data and health information. Data and biological specimens were originally collected via face-to-face appointments or the postal department and consisted of paper-informed consents and questionnaires. Enrollment of the ATR began on May 18, 2016 and is located in Sioux Falls, South Dakota, a rural and frontier area in the Central United States with a regional population of approximately 880,000. The original target area for the ATR was South Dakota and the four surrounding states: Minnesota, Iowa, North Dakota and Nebraska. The ATR has found a need to expand that area based on twin and multiple siblings who live in various areas surrounding these states. A description of the state of the ATR today and its transition to online data collection and informed consent will be presented. The ATR collects longitudinal data on lifestyle, including diet and activity levels, aging, plus complex traits and diseases. All twins and multiples participating in the ATR are genotyped on the Illumina Global Screening Array and receive zygosity results.

Previous genetic studies on hair morphology focused on the overall morphology of the hair using data collected by self-report or researcher observation. Here, we present the first genome-wide association study (GWAS) of a micro-level quantitative measure of hair curvature. We compare these results to GWAS results obtained using a macro-level classification of observable hair curvature performed in the same sample of twins and siblings of European descent. Observational data were collected by trained observers, while quantitative data were acquired using an Optical Fibre Diameter Analyser (OFDA). The GWAS for both the observational and quantitative measures of hair curvature resulted in genome-wide significant signals at chromosome 1q21.3 close to the trichohyalin (TCHH) gene, previously shown to harbor variants associated with straight hair morphology in Europeans. All genetic variants reaching genome-wide significance for both GWAS (quantitative measure lead single-nucleotide polymorphism [SNP] rs12130862, p = 9.5 × 10–09; observational measure lead SNP rs11803731, p = 2.1 × 10–17) were in moderate to very high linkage disequilibrium (LD) with each other (minimum r2 = .45), indicating they represent the same genetic locus. Conditional analyses confirmed the presence of only one signal associated with each measure at this locus. Results from the quantitative measures reconfirmed the accuracy of observational measures.

A forgotten story of monozygotic twins reared apart is described. The pair, born in 1941 in Fribourg, Switzerland, were separated due to switch-baby incident in the hospital. This caused one twin to be raised as a singleton by an unrelated family, and the other twin to be raised as a ‘dizygotic’ twin with an unrelated child. This is followed by reviews of recent twin research and case studies of X-inactivation and hemophilia, breast reconstruction, chimerism and life histories. The final section of the article includes human interest pieces on academic twins, dating twins, transsexual twins and athletic twins.

It is common opinion that the stillbirth rate is higher among monozygotic (MZ) than among dizygotic (DZ) twins. This is supported by the fact that stillbirth rates are higher among same-sexed than among opposite-sexed twins, and the relatively high stillbirth rates among twins of young mothers. In this study we present a method to estimate the stillbirth rates for MZ and DZ twins and identify the difference. We performed analyses based on the assumptions of (a) Weinberg's differential rule, including the assumption that the secondary sex ratio is 100, (b) the stillbirth rates among opposite-sexed twins hold for all DZ twins, and (c) the stillbirth rates estimated for MZ and DZ male and female twins yield for both sexes the observed total number of stillborn twins. Our methods are applied to data from Sweden, 1869–1967, the Åland Islands, 1750–1949, Saxony, 1881–1900, and England and Wales, 1996–2003. We observed that the ratio between the estimated stillbirth rates among MZ and DZ twins were on average 1.75, and the ratio among same-sexed and DZ (opposite-sexed) twins were on average 1.31. For Sweden and Saxony similar values were obtained, but for England and Wales the values were higher and for Åland lower. With exception of Åland, the estimated stillbirth rates were in all populations the lowest for DZ, medium for same-sexed and highest for MZ twins.

This study investigates how consistent genetic factors are, as measured by heritability estimates (h2), in the leisure-time physical activity index (LTPAI) and sport participation index (SPI) from early (10–14 yrs) to late adolescence (15–19 yrs). The sample comprises 12,385 subjects from 3,378 Portuguese nuclear families. Height and weight were measured and body mass index (BMI) was calculated, and the LTPAI and SPI were estimated by questionnaire. Socioeconomic status (SES) was assessed by parental occupation. Analyses were done using S.A.G.E. software. Our results showed that h2 estimates for the LTPAI and SPI in the two age groups (10–14 yrs and 15–19 yrs) were stable: for the LTPAI, h2 = 0.297 and 0.322, respectively; and for the SPI, h2 = 0.413 and 0.428, respectively. Sibling correlations and environmental correlations are higher in the younger age group for both the LTPAI and the SPI. Spousal correlations are higher in the younger age group for the LTPAI and lower for the SPI than the older group. Parent–offspring correlations are similar in both age groups for the LTPAI and SPI. In conclusion, the influence of genetic factors on physical activity and sport participation remains stable across age in adolescence. However, variation in sibling correlations — in particular, environmental correlations — was observed. These findings suggest that shared/non-shared environmental factors express different degrees of importance across age. Future intervention programs aiming to promote change in behaviors need to consider these results to bring about positive changes in physical activity and sport participation behaviors within the family setting.

The National Project on Achievement in Twins (NatPAT) began in 2017 as part of the third funding cycle of the Florida Learning Disabilities Research Center, a program project grant funded by the Eunice Kennedy Shriver National Institute of Child Health and Development. NatPAT will have a nationally representative sample of elementary school-aged twins in the United States. The overall goal of the project is to uncover salient factors, including genetic and environmental influences, which contribute to the co-development of reading and math performance during the critical developmental period of elementary school. Here we present the specific aims, methods and materials, and future directions of the project.

Non-syndromic agenesis of permanent teeth is one of the most common anomalies in human development, a multifactorial characteristic caused by genetic and environmental factors. We describe a pair of monozygotic twins who showed second premolar and third molar agenesis, albeit with different expressions. We studied the DNA of two genes, paired domain box gene 9 (PAX9) and muscle segment homeodomain-homeobox1 (MSX1), encoding transcription factors that earlier studies found were involved in the manifestation of this condition. No specific causative mutation was found. However, we detected a C→T change in MSX1 exon 2 in both twins, suggesting that this polymorphism might be involved in the trait's expression.

Background: Monochorionic diamniotic (MCDA) twin pregnancies are associated with adverse perinatal outcome. Intertwin discordances at the time of nuchal translucency (NT) screening may have a value in the prediction of fetal loss or twin-to-twin transfusion syndrome. We aimed to determine the ability of intertwin NT and crown rump length (CRL) discordances among MCDA twins to predict adverse outcomes. Material and Methods: All MCDA twins with a documented routine ultrasound at 11 to 13+6 weeks’ gestation, and known pregnancy outcome between August 2003 and August 2012 were included. Receiver operating characteristic curves were used to determine the ideal NT and CRL discordances cut-off points that maximized the ability to predict adverse outcome, which was defined as any of: death of one or both twins, twin-to-twin transfusion syndrome, or estimated fetal weight or birth weight discordances ≥25%. Results: Of the 89 cases, 20 (22.5%) had at least one adverse outcome. NT discordance was more discriminatory of adverse outcome than was CRL discordance. The optimal values for predicting any adverse outcomes for NT were >23.7% and for CRL >3.5%. The positive predictive values for NT (52.4%) and CRL (29.8%) screening were relatively low; however, the lack of either NT or CRL discordances was more reassuring, with negative predictive values of 86.8% and 86.4%, respectively. Conclusions: NT discordance is more predictive for adverse fetal outcome in MCDA twins than CRL discordance. Neither NT nor CRL discordance are likely to modify the intensive monitoring required for these very high-risk pregnancies.

Current ultrasound techniques can accurately determine the chorionicity of twins, but not zygosity. We previously proposed that the zygosity of spontaneously conceived twins can be determined at early ultrasound, where 2 corpora lutea infers dizygosity, and 1 implies monozygosity. Here we did a case series, comparing zygosity predicted using this method with definitive DNA genotyping of twins after birth. We retrospectively identified 14 ultrasound reports of spontaneous twin pregnancies at 6(+0 days) to 13+6 weeks' gestation, where both ovaries were seen and the number of corpora lutea documented. We visited all twin pairs, obtained buccal smears, and determined zygosity by genotyping 9 independent microsatellite markers. All 8 cases where 2 corpora lutea were seen were dizygotic pregnancies. One further case where 3 corpora lutea were seen was also dizygotic. All 3 sets of monozygotic twins had 1 corpus luteum. There were 2 cases incorrectly assigned, where 1 corpus luteum was seen in dizygotic pregnancies. We conclude if 2 corpora lutea are seen at a first trimester ultrasound of spontaneously conceived dichorionic twins, they appear to be almost certainly dizygotic. However, if 1 corpus luteum is seen in dichorionic twins, zygosity cannot be determined with certainty since it is either monozygotic, or dizygotic where a second corpus luteum has been missed.

Frequencies of recurrent multiple maternities were estimated using two sets of census data in Japan in 1990 and 1995. The repeat frequency (RF) of the twinning rate is the frequency of 2 sets of twins among families or couples who have already had 1 set of twins and 2 more siblings. The overall RFs were 9.6 per 1000 couples in 1990 and 9.3 in 1995. The RFs of the monozygotic (MZ) twinning rates were 5.9 per 1000 couples in 1990 and 5.5 in 1995. The RFs of the dizygotic (DZ) twinning rates were 3.7 in 1990 and 3.8 in 1995. For unlike-sexed propositus twins, the RF of MZ twins were 5.0 per 1000 couples in 1990 and 5.5 in 1995. The RF of DZ twins were 5.3 in 1990 and 4.6 in 1995. As for like-sexed propositus twins, the corresponding RFs were 6.2 and 5.5 for MZ twins, and 3.4 and 3.6 for DZ twins, respectively. In mothers who have experienced a twin maternity, the overall RF of twinning was 1.5 to 2 times as high as the average mother's chance of having twins. There was no RF for triplets for both census years. As for geographic variations of the overall RF, the rates in Okinawa (16.2) and Hokkaido (15.3) were significantly higher than those in the Tohoku (8.7), Kanto (8.0) and Kyushu (7.4) districts.

Twin–twin transfusion syndrome (TTTS) is an antenatal complication of monochorionic multiple gestations. There have been few studies exploring the role of laser photocoagulation or outcomes following treatment in monochorionic triplet pregnancies with TTTS. We present a case where TTTS and twin anemia–polycythemia sequence (TAPS) complicated a monochorionic triplet pregnancy. Following the laser photocoagulation to treat the TTTS between the triplets, an intra-uterine death occurred in one triplet and TAPS developed in the remaining two triplets. Intervention in this case resulted in a 2-week prolongation of pregnancy and a positive outcome for the remaining fetuses. This case and other published data reviewed in this article suggest that laser photocoagulation has a potential role for TTTS in monochorionic triplet pregnancies.

Temporal variations in the frequency of multiple maternities in many Western European countries have been described. However, within a single country, regional differences are observed. Urban industrialized regions and rural agricultural areas have experienced in recent decades a distinct decline in multiple deliveries, which in cases have been related to maternal age and parity changes. Research on multiple deliveries in Spain is scarce and none of the studies go back to the beginning of the 20th century or consider regional variation over an extended period of time. The present paper is a yearly study on multiple deliveries in Spain since 1900 including a geographical analysis. Rather than dealing with recent changes in multi-parity, this paper is concerned with Spain's long-term national variation (between 1900 and 2006). The changing pattern of double and triple deliveries was analyzed using data from the Spanish National Statistics Institute (INE). Twinning rates in Spain are low in comparison to those of equivalent periods in other countries, and the minimum rates correspond to the 1980s decade. Results were interpreted by taking into account the influence of age at maternity and reproductive variation up to 1990. A good fit between observed and predicted rates was obtained after the application of models, which besides maternal age and parity, include their interaction. Regarding territorial variability, the values corresponding to southern, northern and insular Spanish provinces are consistent with an earlier reduction of the crude birth rate in the north-east regions and latter in the southern regions and the Canary Islands.

The Seoul Twin Family Study seeks to investigate genetic and environmental influences on the development of cognitive abilities and other psychological traits among children and adolescents in Seoul, South Korea. In November, 2001 with the assistance of the Seoul Metropolitan Office of Education, we began to solicit all private and public schools in Seoul to obtain names, addresses, and telephone numbers of the twins enrolled in schools in Seoul. The vast majority of the recruited twins were between 6 and 18 years old. The recruited twins and their non-twin siblings and parents will undergo comprehensive assessment of cognitive abilities, interests, daily activities, personality, and familial and non-familial environments. We plan to reassess the twins and their families every two years as the twins progress through major changes that characterize childhood, adolescence, and adulthood. In addition, medical records of the twins during the prenatal period will be examined to determine the effects of intrauterine environment on individual differences in cognitive abilities and other psychological traits. We also plan to undertake molecular genetic studies to identify genes involved in specific cognitive abilities.

In the population-based FinnTwin16 study, proportions of genetic and environmental factors contributing to muscle dissatisfaction and muscle-enhancing substance use were assessed in 319 pairs of twin brothers: 141 monozygotic (MZ) and 178 dizygotic (DZ) pairs. In addition there were 86 twin individuals from pairs in which only one co-twin responded. Of all respondents, 30% experienced high muscle dissatisfaction. The corresponding proportion of muscle-enhancing substance use was 10%. The subjects were similar in age (23.8 years, 95% confidence interval [CI] 23.76–23.84), body mass index (23.7, 95% CI 23.5–23.9), and waist circumference (84.5 cm, 95% CI 83.7–85.2), independent of their muscle dissatisfaction or muscle-enhancing substance use status and independent of their zygosity. The MZ polychoric correlation for muscle dissatisfaction was .39 (95% CI .17–.58) and .27 for DZ pairs (95% CI .07–.46). The MZ tetrachoric correlation for muscle-enhancing substance use was .65 (95% CI .28–.87) and .56 for DZ pairs (95% CI .26–.78). The AE model, where additive genetic factors (A) accounted for 42% (95% CI .23–.59) and unique environmental factors (E) 58% (95% CI .41–.77) of the liability, provided the best fit for muscle dissatisfaction. The CE model, where common environmental factors (C) accounted for 60% (95% CI .37–.77) and unique environmental factors (E) 40% (95% CI .23–.63) of the liability, provided the best fit for muscle-enhancing substance use. Both genetic and unique (nonfamilial) environmental factors are involved in muscle dissatisfaction in the population. Nongenetic factors (both familial and non-familial) appear to best explain the use of muscle-enhancing substances.