Twins and odd‐ones‐out: a twin study of genetic contributions to variability in personal colour space

BACKGROUND

Individuals differ in the biological substrate of vision, often as a result of genetic differences. There are also subtle variations within the normal population in aspects of colour behaviour (for example, colour naming and unique-hue judgements) but it is surprisingly hard to connect these to the genetic variation. Perceptions of inter-colour similarities (and variations in the structure of colour space, reconstructed from them) may show a closer link to the biological basis of vision.

METHODS

To quantify the spectrum of variation, each subject used caps from the D-15 panel test to make 70 odd-one-out triad judgements. Analysis yielded the parameters of individual observers' colour structure: specifically, the weights they placed on the axes of a standard colour space. Similarities between 19 pairs of monozygotic twins and between 16 pairs of dizygotic twins were compared.

RESULTS

Monozygotic pairs were significantly more concordant than dizygotic or unrelated pairs.

DISCUSSION

The procedure provides sufficiently precise measurements to replicate earlier findings from more complex, time-consuming methods. By extension to other family relationships, the triadic procedure can clarify the genetic contribution. Weighting of colour axes is an important form of variation among normal individuals, with a contribution to these weights from genetic factors.

Colour perception in twins: individual variation beyond common genetic inheritance

BACKGROUND

The twin method was used to examine the genotype/phenotype relationship in colour vision, by determining concordance in colour perception within pairs of monozygotic (MZ) twins and dizygotic (DZ) twins. For MZ twins, whose photopigments are genetically identical, higher concordance in colour perception was expected; conversely, differences within each MZ pair would indicate a non-genetic contribution.

METHOD

Ratings of dissimilarity between successively presented colours were elicited from four MZ and three DZ twin pairs. A non-twin sibling pair and three unrelated normal trichromats were enrolled, for comparison. Concordance for each twin (sibling) pair was estimated by Spearman correlations (rs) between data matrices and by Procrustes distances (gl) between colour spaces, reconstructed from individual data using multidimensional scaling (MDS).

RESULTS

For MZ twins, rs) values (0.94-0.97) were comparable to intra-individual variability and significantly higher than those for DZ twins and siblings (0.72-0.82). Further, colour spaces for MZ co-twins were less discordant, with gl values (0.008-0.029) lower than for DZ co-twins (0.073-0.079) and siblings (0.052). Finally, concordances among all pairs of subjects were summarised by a geometrical 'subject space': the mean distance between MZ co-twins was 29 per cent of that between DZ and sibling pairs.

DISCUSSION

Lower concordance rates in DZ twins and siblings can be attributed to differences in the inherited arrays of photopigment genes. The high concordance for MZ twins is in line with their shared photopigment genotype, placing an upper limit on contributions to discordance from possible individual variations in non-genetic factors. Potential photoreceptor, ocular and cognitive sources of inter-twin variation are discussed.

Sex Differences and Heritability of Two Indices of Heart Rate Dynamics: A Twin Study

We investigated whether women show larger heart rate variability (HRV) than men after controlling for a large number of health-related covariates, using two indices of HRV, namely respiratory sinus arrhythmia (RSA) and approximate entropy (ApEn). In a twin design, the heritability of both indices was examined. The covariation between RSA and ApEn, a measure of heart rate dynamics derived from nonlinear dynamical systems theory, was decomposed into genetic and environmental components. Subjects were 196 male and 210 female middle-aged twins. Females showed larger HRV than men before (ApEn: p < .001; RSA: p = .052) and after adjustment for covariates (ApEn: p < .001; RSA: p = .015). This sex difference was confirmed by significant intrapair differences in the opposite-sex twin pairs for both ApEn (p < .001) and RSA (p = .03). In addition to sex, only heart period and age (both p < .001) were found to be independent predictors of ApEn, whereas RSA was also influenced by respiration rate and smoking (both p < .001). Age explained 16% and 6% of the variance in RSA and ApEn, respectively. Oral contraceptive use and menopausal status had no effect on HRV. Genetic model fitting yielded moderate heritability estimates for RSA (30%) and ApEn (40%) for both males and females. The correlation between RSA and ApEn (r = .60) could be attributed to genetic factors (48%), environmental factors (36%) and age (16%). The present study found support for a gender difference in HRV with women having greater HRV than men even after controlling for a large number of potential confounders. Indices of heart rate dynamics derived from nonlinear dynamical systems theory are moderately heritable and may be more sensitive than traditional indices of HRV to reveal subtle sex differences with important implications for health and disease.

Inflammation is related to coronary flow reserve detected by positron emission tomography in asymptomatic male twins

OBJECTIVES

This study sought to examine the relationship between inflammation and coronary microvascular function in asymptomatic individuals using positron emission tomography (PET) and assessment of coronary flow reserve (CFR).

BACKGROUND

Coronary microvascular dysfunction is an early precursor of coronary artery disease (CAD) thought to result from endothelial cell activation and inflammation, but data are limited.

METHODS

We examined 268 asymptomatic male monozygotic and dizygotic twins. Plasma biomarkers of inflammation and endothelial cell activation included C-reactive protein (CRP), interleukin (IL)-6, white blood cell count (WBC), vascular cell adhesion molecule (VCAM)-1, and intercellular adhesion molecule (ICAM)-1. Blood flow quantitation was obtained with [¹³N] ammonia PET at rest and after adenosine stress. CFR was measured as the ratio of maximum flow to baseline flow at rest; abnormal CFR was defined as a ratio < 2.5. A summed stress score for visible perfusion defects was calculated.

RESULTS

In within-pair analyses, all biomarkers, except VCAM-1, were higher in twins with lower CFR than their brothers with higher CFR (p < 0.05). This was observed in the entire sample, as well as within pairs discordant for a CFR of <2.5. Associations persisted after adjusting for summed stress score and CAD risk factors. In contrast no biomarker, except IL-6, was related to the summed stress score of visible defects.

CONCLUSIONS

Even in asymptomatic subjects, a decrease in coronary microvascular function is accompanied by a systemic inflammatory response, independent of CAD risk factors. Our results, using a controlled twin design, highlight the importance of coronary microvascular function in the early phases of CAD.

Preliminary evidence that estradiol moderates genetic influences on disordered eating attitudes and behaviors during puberty

BACKGROUND

Puberty moderates genetic influences on disordered eating attitudes and behaviors, with little genetic influence before puberty but large (50%) genetic effects during and after puberty. To date, however, nothing is known about the mechanisms that underlie these effects. Estradiol is a particularly promising candidate, as estrogens become elevated at puberty and regulate gene transcription within neurotransmitter systems important for eating-related phenotypes. The aim of this pilot study was to examine whether estradiol levels moderate genetic influences on disordered eating during puberty.

METHOD

Participants included 198 female twins (ages 10-15 years) from the Michigan State University Twin Registry. Disordered eating attitudes and behaviors were assessed with the total score, weight preoccupation, body dissatisfaction and binge eating/compensatory behavior subscales of the Minnesota Eating Behavior Survey (MEBS). Afternoon saliva samples were assayed for estradiol levels. Moderation of genetic effects was examined by comparing twin correlations in low versus high estradiol groups.

RESULTS

In the low estradiol group, monozygotic (MZ) and dizygotic (DZ) twin correlations for all MEBS scales were similar, suggesting little genetic influence. In the high estradiol group, the MZ twin correlation was more than double the DZ twin correlation, indicating the presence of genetic effects. Findings could not be accounted for by age, body mass index or the physical changes of puberty.

CONCLUSIONS

Estradiol may be one important moderator of genetic effects on disordered eating during puberty. Larger twin studies are needed to replicate this pilot work and quantify the extent of genetic moderation.

Traces of embryogenesis are the same in monozygotic and dizygotic twins: not compatible with double ovulation

Common knowledge of over a century has it that monozygotic and dizygotic twinning events occur by unrelated mechanisms: monozygotic twinning 'splits' embryos, producing anomalously re-arranged embryogenic asymmetries; dizygotic twinning begins with independent ovulations yielding undisturbed parallel embryogeneses with no expectation of departures from singleton outcomes. The anomalies statistically associated with twin births are due to the re-arranged embryos of the monozygotics. Common knowledge further requires that dizygotic pairs are dichorionic; monochorionicity is exclusive to monozygotic pairs. These are fundamental certainties in the literature of twin biology. Multiple observations contradict those common knowledge understandings. The double ovulation hypothesis of dizygotic twinning is untenable. Girl-boy twins differ subtly from all other humans of either sex, absolutely not representative of all dizygotics. Embryogenesis of dizygotic twins differs from singleton development at least as much as monozygotic embryogenesis does, and in the same ways, and the differences between singletons and twins of both zygosities represent a coherent system of re-arranged embryogenic asymmetries. Dizygotic twinning and monozygotic twinning have the same list of consequences of anomalous embryogenesis. Those include an unignorable fraction of dizygotic pairs that are in fact monochorionic, plus many more sharing co-twins' cells in tissues other than a common chorion. The idea that monozygotic and dizygotic twinning events arise from the same embryogenic mechanism is the only plausible hypothesis that might explain all of the observations.

Does having a twin brother make for a bigger brain?

OBJECTIVE

Brain volume of boys is larger than that of girls by approximately 10%. Prenatal exposure to testosterone has been suggested in the masculinization of the brain. For example, in litter-bearing mammals intrauterine position increases prenatal testosterone exposure through adjacent male fetuses, resulting in masculinization of brain morphology.

DESIGN

The influence of intrauterine presence of a male co-twin on masculinization of human brain volume was studied in 9-year old twins.

METHODS

Magnetic resonance imaging brain scans, current testosterone, and estradiol levels were acquired from four groups of dizygotic (DZ) twins: boys from same-sex twin-pairs (SSM), boys from opposite-sex twin-pairs (OSM), girls from opposite-sex twin-pairs (OSF), and girls from same-sex twin-pairs (SSF; n=119 individuals). Data on total brain, cerebellum, gray and white matter volumes were examined.

RESULTS

Irrespective of their own sex, children with a male co-twin as compared to children with a female co-twin had larger total brain (+2.5%) and cerebellum (+5.5%) volumes. SSM, purportedly exposed to the highest prenatal testosterone levels, were found to have the largest volumes, followed by OSM, OSF and SSF children. Birth weight partly explained the effect on brain volumes. Current testosterone and estradiol levels did not account for the volumetric brain differences. However, the effects observed in children did not replicate in adult twins.

CONCLUSIONS

Our study indicates that sharing the uterus with a DZ twin brother increases total brain volume in 9-year olds. The effect may be transient and limited to a critical period in childhood.

Dizygotic monochorionic twin pregnancy conceived following intracytoplasmic sperm injection treatment and complicated by twin-twin transfusion syndrome and blood chimerism

We report a case of a dizygotic monochorionic twin pregnancy preceded by intracytoplasmic sperm injection treatment including assisted hatching. On ultrasound examination at 25 weeks' gestation the twins, which had been assumed to be monochorionic, were found to be of different sexes. Karyotyping and zygocity determination were performed on amniotic fluid and showed the twins to be dizygotic with normal female and male karyotypes. There were clinical and sonographic signs of twin-twin transfusion syndrome (TTTS), and Cesarean delivery was performed at 32 weeks' gestation. At birth the twins were phenotypically a normal male and a normal female. Histology of the placenta showed it to be monochorionic diamniotic. Blood chimerism was found postnatally as both infants had the karyotypes 46,XX[13]/46,XY[17]. Chimerism was not found in cells from a buccal swab at 6 months of age. This is one of only a few reported cases of dizygotic monochorionic twins. Nearly all of these cases have been conceived after assisted reproductive technology procedures. It is of clinical importance to be aware of this rare phenomenon in relation to TTTS, prenatal screening and parental counseling.

Genetic loci linked to pituitary-thyroid axis set points: a genome-wide scan of a large twin cohort

OBJECTIVE

Previous studies have shown that circulating concentrations of TSH, free T4, and free T3 are genetically regulated, but the genes responsible remain largely unknown. The aim of this study was to identify genetic loci associated with these parameters.

DESIGN

We performed a multipoint, nonparametric genome-wide linkage scan of 613 female dizygotic twin pairs. All subjects were euthyroid (TSH 0.4-4.0 mU/liter) with negative thyroid peroxidase antibodies and no history of thyroid disease. The genome scan comprised 737 microsatellite markers supplemented with dinucleotide markers. Data were analyzed using residualized thyroid hormone data after adjustment for age, smoking, and body mass index.

RESULTS

Multipoint linkage analysis gave linkage peaks for free T4 on chromosome 14q13 and 18q21 [logarithm of odds (LOD) 2.4-3.2]; TSH on chromosomes 2q36, 4q32, and 9q34 (LOD 2.1-3.2); and free T3 on chromosomes 7q36, 8q22, and 18q21 (LOD 2.0-2.3).

CONCLUSIONS

This study has identified eight genomic locations with linkage of LOD of 2.0 or greater. These results should enable targeted positional candidate and positional cloning studies to advance our understanding of genetic control of the pituitary-thyroid axis.

The influence of twinning on cardiac development

The risk for a cardiac anomaly in a twin pregnancy is increased, particularly in monochorionic twins. This is relevant in terms of fetal diagnosis as well as for the management of the pregnancy; there are also implications for the neonatal period and possibly beyond. The risk for a cardiac abnormality depends on the type of monochorionic twin as determined by the timing of embryonic division. Prenatal identification of twin type and the relative risks for a cardiac anomaly are discussed along with theories for the aetiology of the different cardiac lesions.

Relation of birth weight, body mass index, and change in size from birth to adulthood to insulin resistance in a female twin cohort

CONTEXT AND OBJECTIVES

Because an adverse intrauterine environment is thought to induce insulin resistance, our objective was to investigate the relationships between birth weight, BMI, and change in body size over the life course and insulin resistance. SETTING, DESIGN, AND PARTICIPANTS: We conducted a cross-sectional study in a cohort of 1194 female twins aged 18-74 yr. The relationship between birth weight and insulin resistance was analyzed using a regression method allowing for a simultaneous estimation of within- and between-pair influences. The approach allows the influence of individual fetal nutrition on adult insulin resistance to be distinguished from effects that are mediated by confounding factors in the maternal environment.

MAIN OUTCOME MEASURES

Insulin resistance was measured by the homeostasis model assessment.

RESULTS

Individual level regression analyses showed no significant relationship between birth weight and insulin resistance. There was a significant positive relationship between insulin resistance and current body mass index (BMI) (a 26% increase in insulin resistance per sd increase in BMI; confidence interval, 22.6-29.5%). This significant relationship was accounted for in equal parts by individual-specific effects and by confounding factors in the shared environment of the twins. The relationship with birth weight became significant only after adjustment for BMI and was mediated only through between-pair differences.

CONCLUSIONS

These results suggest that insulin resistance is influenced more by current body size than birth weight and that postnatal growth is potentially more important than fetal growth in the subsequent development of insulin resistance.

Shared environmental factors associated with telomere length maintenance in elderly male twins

During aging, chromosome ends, or telomeres, gradually erode or shorten with each somatic cell division. Loss of telomere length homeostasis has been linked to age-related disease. Remarkably, specific environmental assaults, both physical and psychological, have been shown to correlate with shortened telomeres. However, the extent that genetic and/or environmental factors may influence telomere length during later stages of lifespan is not known. Telomere length was measured in 686 male US World War II and Korean War veteran monozygotic (MZ) and dizygotic (DZ) twins (including 181 MZ and 125 DZ complete pairs) with a mean age of 77.5 years (range 73-85 years). During the entire process of telomere length measurement, participant age and twin status were completely blinded. White blood cell mean telomere length shortened in this elderly population by 71 base pairs per year (P < 0.0001). We observed no evidence of heritable effects in this elderly population on telomere length maintenance, but rather find that telomere length was largely associated with shared environmental factors (P < 0.0001). Additionally, we found that individuals with hypertension and cardiovascular disease had significantly shorter telomeres (P = 0.0025 and 0.002, respectively). Our results emphasize that shared environmental factors can have a primary impact on telomere length maintenance in elderly humans.

Sex Ratios: Human Twins and Fraternal Effects

Historical data from Finnish populations reveals that, for females, exposure to a male twin in the womb may have significant, life-long, effects on subsequent fitness, with profound implications for the evolution of sex ratios and brood size.

Twins in metabolic and diabetes research: what do they tell us?

PURPOSE OF REVIEW

The purpose of this review is to provide a critical update of the impact of the 'thrifty phenotype hypothesis' on metabolism, and its implications for heritability estimates as obtained in so-called classical twin studies.

RECENT FINDINGS

Our recent studies demonstrated a complex age or time-dependent relationship between different and independent markers of an adverse intrauterine environment, including birth weight, twin and zygosity status on one side, and distinct defects of insulin secretion and glucose metabolism on the other side.

SUMMARY

These novel findings may, to some unknown extent, influence and perhaps bias the heritability estimates of type 2 diabetes-related traits obtained using classical twin analyses. The studies add to the increasing evidence of the intrauterine environment as a - or the - key player in the cause and pathophysiology of type 2 diabetes and the metabolic syndrome. We and others have shown that monozygotic twins represent a powerful tool to demonstrate nongenetic associations between low birth weight and different phenotypes of the metabolic syndrome, including type 2 diabetes, insulin resistance and, recently, hypertension. Future studies of discordant monozygotic twins may provide novel and crucial mechanistic explanations of the link between low birth weight and the metabolic syndrome.

Heritability of anterior cingulate response to conflict: an fMRI study in female twins

Interference processing requires increased focus on relevant dimensions of environmental stimuli and selective allocation of attentional resources, in order to filter extraneous information and inhibit non-adaptive responses. This process is important in everyday life and is necessary for responding to novel and challenging situations. Dorsal anterior cingulate cortex (dACC) is involved in this process, and behavioral twin studies indicate that performance on interference processing tasks is highly heritable. However, the extent to which dACC activation related to such tasks is influenced by genetic factors has not been reported. In the current study, 10 pairs of monozygotic and 10 pairs of dizygotic female twins performed a validated interference processing task during fMRI. There were three main results: (1) increased dACC activation for incongruent (INC) minus congruent (CON) trials was observed; (2) dACC activation for INC minus CON trials was both moderately heritable and significantly correlated with the difference in reaction time (RT) between INC and CON trials; (3) RT for INC trials was moderately heritable. RT for CON trials and the latency difference between INC and CON trials were not influenced significantly by genetic factors. The current study provides the first functional imaging evidence that dACC activation during interference processing is significantly influenced by genes. These results suggest an endophenotype that may be applied to various psychiatric disorders that are both highly heritable and associated with altered dACC function.

Associations between long-term physical activity, waist circumference and weight gain: a 30-year longitudinal twin study

BACKGROUND AND OBJECTIVE

Physical activity level and obesity are both partly determined by genes and childhood environment. To determine the associations between long-term leisure-time physical activity, weight gain and waist circumference and whether these are independent of genes and childhood effects.

DESIGN AND SUBJECTS

The study design is a 30-year follow-up twin study in Finland. For this study, 146 twin pairs were comprehensively identified from the large Finnish Twin Cohort. These twin pairs were discordant for both intensity and volume of leisure physical activity in 1975 and 1981 and were healthy in 1981. At follow-up in 2005, both members of 89 pairs were alive and participated in a structured telephone interview. In the interview self-measured weight and waist circumference, and physical activity level for the whole follow-up were assessed. Paired tests were used in the statistical analyses.

MAIN OUTCOME MEASURES

Waist circumference at 30-year follow-up (2005) and change in weight from 1975 to 2005.

RESULTS

In the 42 twin pairs discordant for physical activity at all time points during the 30-year period, the mean weight gain from 1975 through 2005 was 5.4 kg (95% confidence interval (CI) 2.0-8.9) less in the active compared to inactive co-twins (paired t-test, P=0.003). In 2005, the mean waist circumference was 8.4 cm (95% CI 4.0-12.7) less in the active compared with inactive co-twins (P<0.001). These trends were similar for both monozygotic and dizygotic twin pairs. Pairwise differences in weight gain and waist circumference were not seen in the 47 twin pairs, who were not consistently discordant for physical activity.

CONCLUSION

Persistent participation in leisure-time physical activity is associated with decreased rate of weight gain and with a smaller waist circumference to a clinically significant extent even after partially controlling for genetic liability and childhood environment.

Male twins reduce fitness of female co-twins in humans

In mammals, including humans, female fetuses that are exposed to testosterone from adjacent male fetuses in utero can have masculinized anatomy and behavior. However, the reproductive consequences of such prebirth sex-ratio effects for offspring and their implications for maternal fitness remain unexplored. Here we investigate the effects of being gestated with a male co-twin for daughter lifetime reproductive success, and the fitness consequences for mothers of producing mixed-sex twins in preindustrial (1734-1888) Finns. We show that daughters born with a male co-twin have reduced lifetime reproductive success compared to those born with a female co-twin. This reduction arises because such daughters have decreased probabilities of marrying as well as reduced fecundity. Mothers who produce opposite-sex twins consequently have fewer grandchildren (and hence lower fitness) than mothers who produce same-sex twins. Our results are unlikely to be a consequence of females born with male co-twins receiving less nutrition because such females do not have reduced survival and increases in food availability fail to improve their reproductive success. Nor are our results explained by after-birth social factors (females growing up with similarly aged brothers) because females born with a male co-twin have reduced success even when their co-twin dies shortly after birth and are raised as singletons after birth. Our findings suggest that hormonal interactions between opposite-sex fetuses known to influence female morphology and behavior can also have negative effects on daughter fecundity and, hence, maternal fitness, and bear significant implications for adaptive sex allocation in mammals.

Gender differences in plasma ghrelin and its relations to body composition and bone - an opposite-sex twin study

BACKGROUND

Ghrelin, a peptide hormone that plays a role in the regulation of appetite and body adiposity, may also play a role in bone metabolism.

OBJECTIVES

We used the opposite-sex twin model to study associations of plasma ghrelin levels with measures of bone mass and body composition, and determine how such associations were influenced by gender and age.

PATIENTS AND MEASUREMENTS

We measured total plasma ghrelin by radioimmunoassay (RIA) and bone mass/body composition parameters by dual energy X-ray absorptiometry in 79 pairs of opposite sex twins (n = 158 subjects). To examine the effect of age, the study population was divided by median age into two groups: under 51.2 years (38 pairs) and over 51.2 years (41 pairs).

RESULTS

Women had higher plasma ghrelin levels than men (median 1063 vs. 869 ng/l, P < 0.01). Age was a significant predictor of plasma ghrelin levels after adjustment for gender, fat mass and body size. In the older age group, plasma ghrelin levels were inversely associated with fat mass measures in men and women, but there were gender differences in the nature of these associations. In women, plasma ghrelin correlated inversely with body mass index (BMI, r = -0.32), total fat mass (r = -0.30) and fat mass/lean mass ratio (r = -0.42), whereas in men associations with abdominal fat mass (r = -0.31) and fat distribution index (r = -0.33) were observed. Plasma ghrelin was associated with alcohol consumption in older men and women. In the obese subgroup (BMI > 30) no significant gender differences in plasma ghrelin were found. Plasma ghrelin levels were not significantly associated with bone mineral density (BMD) generally, except for hip BMD in younger women (r = -0.39).

CONCLUSION

Plasma ghrelin levels are associated with age, gender, alcohol intake and fat mass measures but only weakly to bone mass measures.

A longitudinal behavioral genetic analysis of task persistence

Change in task persistence was assessed in two annual assessments using teachers', testers', and observers' ratings. Participants included 79 monozygotic and 116 same-sex dizygotic twin pairs who were in Kindergarten or 1st grade (4.3 to 7.9 years old) at the initial assessment. Task persistence was widely distributed and higher among older children and girls. Overall, there was modest growth in persistence over time, and moderate stability of individual differences. Most of the stability was accounted for by genetic influences, whereas most of the change was accounted for by nonshared environment, including an association with observed differential maternal warm supportive behavior.

Genetic overlap between P300, P50, and duration mismatch negativity

Mismatch Negativity (MMN), P300, and P50 suppression event-related potential (ERP) components measure intermediate stages of information processing but little is known of how they relate to each other genetically. The present study used multivariate genetic model fitting analytic techniques in 46 monozygotic and 32 dizygotic twin pairs. P300, P50 suppression, and MMN were recorded using a 19-channel electroencephalograph (EEG). Zygosity was determined using DNA genotyping. Little evidence for either genetic or environmental association between each of the three ERP paradigms was found. This result suggests that P300, MMN, and P50 suppression serve to evaluate different brain information processing functions that may be mediated by distinct neurobiological mechanisms which in turn are influenced by different sets of genes. Within paradigm, P300 amplitude and latency shared about half of their genetic effects.

The intrauterine environment as reflected by birth size and twin and zygosity status influences insulin action and intracellular glucose metabolism in an age- or time-dependent manner

According to the "fetal origins hypothesis," monozygotic (MZ) twins may be more prone to develop various metabolic abnormalities compared with dizygotic (DZ) twins, and twins all together may be more predisposed to metabolic defects compared with singletons. To determine the impact of twin and zygosity status as well as birth size on in vivo measures of glucose metabolism, we examined 123 young (aged 22-31 years) and 103 elderly (aged 57-66 years) MZ and DZ twins and age-matched singleton control subjects. All participants were born at term with available birth records. Peripheral and hepatic insulin action and intracellular glucose partitioning was determined by a euglycemic-hyperinsulinemic clamp using tritiated glucose combined with indirect calorimetry. In elderly subjects, zygosity status influenced nonoxidative glucose metabolism, while twin status per se was associated with elevated hepatic glucose production during both steady-state periods. Birth weight was associated with nonoxidative glucose metabolism in a nongenetic manner within twins and with a high glucose and low lipid oxidation in singletons. In younger subjects, twin status influenced glucose and lipid oxidation rates. We demonstrate a complex age- or time-dependent relationship between independent markers of fetal environment and glucose homeostasis in twins. The documented differential programming effects associated with either low birth weight and twin or zygosity status all represent known defects of glucose homeostasis in type 2 diabetes.

Heritability and genetic covariation of sensitivity to PROP, SOA, quinine HCl, and caffeine

The perceived bitterness intensity for bitter solutions of propylthiouracil (PROP), sucrose octa-acetate (SOA), quinine HCl and caffeine were examined in a genetically informative sample of 392 females and 313 males (mean age of 17.8 +/- 3.1 years), including 62 monozygotic and 131 dizygotic twin pairs and 237 sib pairs. Broad-sense heritabilities were estimated at 0.72, 0.28, 0.34, and 0.30 for PROP, SOA, quinine, and caffeine, respectively, for perceived intensity measures. Modeling showed 1) a group factor which explained a large amount of the genetic variation in SOA, quinine, and caffeine (22-28% phenotypic variation), 2) a factor responsible for all the genetic variation in PROP (72% phenotypic variation), which only accounted for 1% and 2% of the phenotypic variation in SOA and caffeine, respectively, and 3) a modest specific genetic factor for quinine (12% phenotypic variation). Unique environmental influences for all four compounds were due to a single factor responsible for 7-22% of phenotypic variation. The results suggest that the perception of PROP and the perception of SOA, quinine, and caffeine are influenced by two distinct sets of genes.

Finger-length ratios show evidence of prenatal hormone-transfer between opposite-sex twins

Finger-length ratio (second to fourth finger; 2D:4D) has been associated with various measures thought to be related to prenatal androgens. In addition, hormone-transfer theory posits that hormones can transfer between twins. We examined 2D:4D in same-sex (SS) and opposite-sex (OS) dizygotic twins to test both propositions. Results show that 2D:4D is masculinized in OS females compared to SS females. This provides strong evidence that 2D:4D is laid down prenatally, and that hormones (likely androgens) can transfer from male to female fetuses. Implications for developmental timeframes for both hormone-transfer and 2D:4D are discussed.

Abnormal placental cord insertion may induce intrauterine growth restriction in IVF-twin pregnancies

BACKGROUND

This study determined whether twins conceived through assisted reproduction technology (ART) have an increased risk of perinatal complications compared with natural twin pregnancies and investigated potential associated major risk factors.

METHODS

This retrospective study consisted of 199 twins born between 1994 and 2003. There were four groups according to conception modalities: 97 twins after spontaneous pregnancy, 24 after induced ovulation, 28 after intrauterine insemination (IUI) and 50 after IVF with embryo transfer. Analysis included preterm birth, Caesarean delivery, weight discordance, intrauterine growth restriction (IUGR), low-birth-weight, Apgar score, chorionicity, gross placental pathology and placental umbilical cord insertion (UCI) site.

RESULTS

A significant difference was found in IUGR between the IVF group (7.0%) and spontaneous pregnancy group (14.9%). When maternal age was >30 years there was a 2.86-fold increase in the risk of IUGR. There was a 3.69-fold increased risk of IUGR in the presence of abnormal UCI (odds ratio 3.69, 95% CI 1.62-8.42) and a 2.18-fold increased risk of abnormal UCI in monochorionic twins when compared with dichorionic twins (odds ratio 2.18, 95% CI 1.30-3.66).

CONCLUSION

Twins conceived through ART are not at an increased risk of perinatal complications. A relationship has been found between abnormal UCI and IUGR.

Genetic influences on dynamic complexity of brain oscillations

Human electroencephalogram (EEG) consists of complex aperiodic oscillations that are assumed to indicate underlying neural dynamics such as the number and degree of independence of oscillating neuronal networks. EEG complexity can be estimated using measures derived from nonlinear dynamic systems theory. Variations in such measures have been shown to be associated with normal individual differences in cognition and some neuropsychiatric disorders. Despite the increasing use of EEG complexity measures for the study of normal and abnormal brain functioning, little is known about genetic and environmental influences on these measures. Using the pointwise dimension (PD2) algorithm, this study assessed heritability of EEG complexity at rest in a sample of 214 young female twins consisting of 51 monozygotic (MZ) and 56 dizygotic (DZ) pairs. In MZ twins, intrapair correlations were high and statistically significant; in DZ twins, correlations were substantially smaller. Genetic analyses using linear structural equation modeling revealed high and significant heritability of EEG complexity: 62-68% in the eyes-closed condition, and 46-60% in the eyes-open condition. Results suggest that individual differences in the complexity of resting electrocortical dynamics are largely determined by genetic factors. Neurophysiological mechanisms mediating genetic variation in EEG complexity may include the degree of structural connectivity and functional differentiation among cortical neuronal assemblies.

Genetic effects on physical health: lower at higher income levels

Given the robust finding that people in higher income groups tend to experience better physical health, there is interest in identifying mechanisms underlying this gradient. Using a nationwide sample of 719 twin pairs from the National Survey of Midlife Development in the United States, we investigated the possibility that gene-environment interaction underlies the income-health gradient. We observed that genetic variance associated with 2 measures of physical health, number of chronic illnesses and body mass index, each declined significantly with increasing income. This interaction effect could not be removed by adjusting income for the presence of health insurance coverage and education, suggesting that the interaction is not simply a result of differences in levels of those characteristics with income.

Twins born following fertility treatment: implications for quantitative genetic studies

The rate of multiple births is substantially elevated in women who have had assisted reproduction treatment (ART; approximately 26%) compared to the general population ( approximately 1%), and these offspring are usually included in twin studies. Several studies have attempted to identify possible consequences of undergoing ART on the subsequent offspring. However, most studies have only included singleton births. We first examined whether twins born by ART differed from other twins on measures of childhood psychopathology, putative risk factors and correlates, and secondly tested for differences in the degree of twin similarity for available outcome measures. From a population-based twin sample, 101 families with dizygotic (DZ) twins conceived via ART were identified and compared with 1073 naturally conceived (NC) control DZ twin pairs. Analyses performed were (1) univariate and multivariate comparisons of between-group mean differences; and (2) comparison of twin 1-twin 2 correlations between the groups. The groups differed significantly on demographic factors (parental age, family size and social class) and pregnancy variables (smoking during pregnancy and birthweight) but did not differ on family conflict scores or in the frequency of obstetric complications. Family cohesion was higher in the ART group but this was accounted for by demographic factors. For child psychopathology there was a difference between the groups only for teacher-rated ADHD (Attention Deficit Hyperactivity Disorder). Differences were also found between groups for twin correlations. The differences found between ART and NC twins on group means and twin correlations suggest that researchers should be aware that including ART twins may influence results from twin studies.

Age-related and individual differences in the performance of a delayed response task (the A-not-B task) in infant twins aged 7-12 months

The aim of the present work was to analyze the age-related dynamics and nature of interindividual differences in performing the A-not-B task, which addresses working memory in children aged 7-12 months. The cohort consisted of 150 children aged 7-12 months from mono- and dizygotic twin pairs. Working memory was assessed in terms of the maximum delay which the children could tolerate in a delayed response test (the A-not-B task). Performance of the task improved with age, such that a sharp change in the ability to tolerate the delay occurred between nine and ten months of age. Mental development as assessed using the Bayley scale showed a significant correlation with the duration of the delay in the task only after a threshold period (9-10 months of age) in the development of working memory. Analysis of intrapair correlations in mono- and dizygotic twins showed that interindividual differences in the A-not-B task delay at age 7-9 months were completely determined by individual environmental factors (including measurement errors), while at age 10-12 months the leading role in the interindividual variability in this measure was taken by the systematic environment. These results show that the abilities to tolerate delays at the beginning and end of the second six months of life are based on different mechanisms.

Genetic and environmental influences on birthweight in a sample of Korean twins

This study is the first report of genetic and environmental influences on birthweight using Korean twins. The sample consisted of 255 monozygotic (MZ) and 178 dizygotic (DZ) twin pairs drawn from the Seoul Twin Family Study. Intraclass twin correlations were computed for the twins' birthweights obtained from parents (typically mothers) of the twins. To estimate genetic and shared and nonshared environmental influences on birthweight, standard univariate model-fitting analyses were performed using a software, Mx. For each gender, MZ twin correlations were higher than DZ twin correlations, suggesting existence of genetic influences on birthweight; however, DZ twin correlations were higher than half the MZ twin correlations, indicating that shared environmental factors are also important. For each zygosity, twin correlations were not significantly different between males and females, implicating that genes and environments that cause individual differences in birthweight may not vary between males and females. Model-fitting analyses based on the data pooled across gender yielded estimates of 17% for genetic, 60% for shared environmental, and 23% for nonshared environmental influences on birthweight.

Simultaneous monitoring of separate fetal magnetocardiographic signals in twin pregnancy

Fetal magnetocardiography (fMCG) allows the non-invasive recording of fetal cardiac electrical activity with increasing efficacy as gestation progresses. Many reports on the successful extraction of reliable fetal magnetocardiographic traces in singleton pregnancies exist in the literature, whereas there is only one report on the reconstruction of averaged fetal cardiac signals obtained in a twin pregnancy with the use of a double sensor array system. In this paper, we aimed at assessing the effectiveness of an ICA-based procedure to reconstruct the time course of fetal cardiac signals recorded with a single-shot multi-channel fMCG device in an uncomplicated twin pregnancy at 27 weeks. The evaluation of heart rate and beats synchronicity permitted the differentiation of fetal components; the quality of reconstructed fetal signals allowed visual inspection on single cycles and the simultaneous monitoring of separate fetal heart rate patterns. The proposed technique might be applied in twin pregnancies not only to characterize fetal arrhythmias, but also in all cases of discordant fetal growth, either in the case of intra-uterine growth retardation affecting one fetus, or in the case of twin-twin transfusion syndrome, a life-threatening condition where both fetuses are at risk of heart failure.

Implications of absence of measurement invariance for detecting sex limitation and genotype by environment interaction

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 contribution of risk factors to blood pressure heritability estimates in young adults: the East flanders prospective twin study

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.

Genetic covariation of pelvic organ and elbow mobility in twins and their sisters

A range 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 approximately .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.

Gender mix in twins and fetal growth, length of gestation and adult cancer risk

This study evaluated the effect of gender mix (the gender combinations of twin pairs) on fetal growth and length of gestation, and reviewed the literature on the long-term effects of this altered fetal milieu on cancer risk. In singletons, it is well established that females weigh less than males at all gestations, averaging 125-135 g less at full term. This gender difference is generally believed to be the result of the effect of androgens on fetal growth. The gender difference in fetal growth is greater before the third trimester and less towards term, with males growing not only more, but also earlier than females. Plurality is a known risk factor for reduced fetal growth and birthweight. Compared with singletons, the mean birthweight percentiles of twins fall substantially (by 10% or more) below the singleton 10th percentile by 28 weeks, below the singleton 50th percentile by 30 weeks, and below the singleton 90th percentile by 34 weeks. In unlike-gender twin pairs, it has been reported that the female prolongs gestation for her brother, resulting in a higher birthweight for the male twin than that of like-gender male twins. Other researchers have demonstrated that females in unlike-gender pairs had higher birthweights than females in like-gender pairs. Analyses from our consortium on 2491 twin pregnancies with known chorionicity showed longer gestations and faster rates of fetal growth in both males and females in unlike-gender pairs compared with like-gender male or female pairs, although these differences were not statistically significant. The post-natal effects for females growing in an androgenic-anabolic environment include increased sensation-seeking behaviour and aggression, lowered visual acuity, more masculine attitudes and masculinising effects of the auditory system and craniofacial growth. In contrast, there is no evidence to suggest that there might be a similar feminising effect on males from unlike-gender pairs. This hormonal exposure in utero may influence adult body size and susceptability to breast cancer.

Determinants of birthweight and intrauterine growth in liveborn twins

We explored the relationship of umbilical cord insertion and fusion of placentas with birthweight in monozygotic monochorionic (MZ MC), monozygotic dichorionic (MZ DC), and dizygotic (DZ) twins. In addition, we evaluated some of the possible factors responsible for the restricted intrauterine growth of twins compared with singletons. The birthweight of 4529 liveborn twin pairs of the East Flanders Prospective Twin Survey was prospectively recorded, placentas were examined, and site of umbilical cord insertion was determined after delivery. Birthweight of 76 490 liveborn singletons was obtained from the Study Centre for Perinatal Epidemiology (SPE). Infants with a peripheral cord insertion weighed 150 g less (P < 0.001) than infants with a central cord insertion. DZ infants had a significantly (P < 0.001) higher incidence of central cord insertion than MZ DC and MZ MC infants. MZ DC infants with fused placentas and a peripheral cord insertion weighed on average 300 g less (P < 0.01) than infants with separate placentas and a central cord insertion. In DZ infants, fusion of the placentas did not affect birthweight. Twins gain less weight per week of gestation than singletons from 32 weeks onwards (twins: 128 g, 156 g, 75 g and singletons: 118 g, 251 g, 149 g, weeks 27-31,32-36, 37-42 respectively). From week 32 onwards, parity, birth rank, cord insertion and number of placentas also influenced birthweight of twins. We conclude that the difference between the birthweights of DZ, MZ DC, and MZ MC infants may originate from the least favourable antenatal situation, namely fused placentas with a peripheral cord insertion, which occurs most frequently in MZ twins. Gestation is the main determinant of birthweight. Other placental and maternal factors have a modest but significant influence on prenatal growth.

Gender mix: does it modify birthweight--outcome association?

Gender mix, especially the supposed hormone transfer in utero of the male fetus to his female co-twin, is a highly debated controversial subject. It occurs in animals (free-martin syndrome in the cow) but its existence in man has not been convincingly demonstrated. Two aspects of gender mix effects in man, birthweight and cognitive development, were studied in the Belgian East Flanders Prospective Twin Survey, a large population-based registry of multiple maternities, characterised by accurate data on pregnancy, placental structure and zygosity. The birthweight of the female member of the pair is not influenced by the male co-twin but, unexpectedly, the female twin enhances to a slight degree the birthweight of her male co-twin by prolonging the gestation for a few days. Also unexpectedly, in an opposite direction, the cognitive development, as measured by the IQ (WISC-R) of the female twin rises as compared with controls if her birthweight exceeds that of her male co-twin.

Can we generalise from findings in twins?

A number of recent studies have used data from twins to shed light on the causal pathways underlying the observed association between birthweight and cardiovascular risk factors or coronary heart disease. The issue of whether findings from twin studies are generally informative, or whether factors associated with twinning preclude generalisation, is considered here. It is concluded that the association between birthweight and later health may differ quantitatively between twins and singletons, but evidence regarding blood pressure suggests it may not differ qualitatively. However, more information is needed on a number of gestational and maternal factors, and on measures of health other than blood pressure. Placentation and issues relating to infertility and its treatment need to be recorded and, together with gestation length, may need to be taken into account in analyses.

Genetic Study of the Height and Weight Process During Infancy

Longitudinal height and weight data from 4649 Dutch twin pairs between birth and 2.5 years of age were analyzed. The data were first summarized into parameters of a polynomial of degree 4 by a mixed-effects procedure. Next, the variation and covariation in the parameters of the growth curve (size at one year of age, growth velocity, deceleration of growth, rate of change in deceleration [i.e., jerk] and rate of change in jerk [i.e., snap]) were decomposed into genetic and nongenetic sources. Additionally, the variation in the estimated size at birth and at 2 years of age interpolated from the polynomial was decomposed into genetic and nongenetic components. Variation in growth was best characterized by a genetic model which included additive genetic, common environmental and specific environmental influences, plus effects of gestational age. The effect of gestational age was largest for size at birth, explaining 39% of the variance. The differences between monozygotic and dizygotic twin correlations were largest for size at 1 and 2 years of age and growth velocity of weight, which suggests that these parameters are more influenced by heritability than size at birth, deceleration and jerk. The percentage of variance explained by additive genetic influences for height at 2 years of age was 52% for females and 58% for males. For weight at 2 years of age, heritability was approximately 58% for both sexes. Variation in snap height for males was also mainly influenced by additive genetic factors, while snap for females was influenced by both additive genetic and common environmental factors. The correlations for the additive genetic and common environmental factors for deceleration and snap are large, indicating that these parameters are almost entirely under control of the same additive genetic and common environmental factors. Female jerk and snap, and also female height at birth and height at 2 years of age, are mostly under control of the same additive genetic factor.

Heritability of ambulatory heart rate variability

BACKGROUND

Reduced heart rate variability (HRV) is a prognostic factor for cardiac disease and cardiac mortality. Understanding the sources of individual differences in HRV may increase its diagnostic use and provide new angles for preventive therapy. To date, the contribution of genetic and environmental factors to the variance in HRV has not been investigated during prolonged periods of ambulatory monitoring in a naturalistic setting.

METHODS AND RESULTS

In 772 healthy twins and singleton siblings, ambulatory ECG was recorded during 24 hours. Two time domain measures of HRV were used: the standard deviations of all normal-to-normal intervals across 5-minute segments (SDNN index) and the root mean square of successive differences between adjacent normal RR intervals (RMSSD). Multivariate genetic analyses across 4 periods of day (morning, afternoon, evening, night) yielded significant estimates for genetic contribution to the mean ambulatory SDNN index (ranging from 35% to 47%) and the mean ambulatory RMSSD (ranging from 40% to 48%).

CONCLUSIONS

Ambulatory HRV measures are highly heritable traits that can be used to support genetic association and linkage studies in their search for genetic variation influencing cardiovascular disease risk.

Relative roles of heredity and physical activity in adolescence and adulthood on blood pressure

Part of the association between physical activity and low blood pressure (BP) may be a consequence of genetic selection. We investigated the association of genetic factors and physical activity in adolescence and adulthood with BP. BP was measured with a Finapres device in 71 monozygotic and 104 dizygotic male twin pairs using no antihypertensive medication. Subjects' mean age was 50.4 yr (range 40–72 yr). Subjects were interviewed about their lifetime exercise and other health habits. Exercise was classified as aerobic, power, or other, and these were further divided into adolescence (12–20 yr of age), the previous year, and lifetime. Genetic modeling was conducted to estimate genetic and environmental components of variance of systolic and diastolic BP. Aerobic exercise in adolescence and high-intensity aerobic exercise throughout the lifetime were associated with low diastolic BP in adulthood. Of the variance in diastolic BP, genetic factors accounted for 35% and aerobic exercise in adolescence for 5%. For systolic BP, genetic factors accounted for 39% of the variance. In turn, genetic factors accounted for 44% of the variance in aerobic exercise in adolescence. The genetic factors in part accounting for the variance in diastolic BP and those in part accounting for variance in aerobic exercise in adolescence were correlated. The association between aerobic exercise in adolescence and low diastolic BP in adulthood is a new finding, as is the observation that the factors partly share the same genes.

The Cognitive and Identity Development of Twins at 16 Years of Age: A Follow-up Study of 32 Twin Pairs

This is a follow up study of twins within the Stockholm area, including 32 families and their twins attending grade nine. The twins have been followed from birth to 16 years of age. The main purpose of this study from its inception was to assess mental and cognitive development at different ages. Another aim was to see how the twins who were born prematurely are developing during the school ages. A third aim has been to gain a deeper insight into the relationship between co-twins and the development of their identities, which is the focus of this paper. Several ability tests have been used, as well as questionnaires about interests, attitudes toward school, and leisure activities. At the 16-year follow-up, a psychological method, the Wartegg drawing test, designed to examine identity, ego strength, dependency, ambition, anxiety, willpower, creativity, empathy and coping strategies has been used. The results indicate that it is difficult for twins to develop independence and a positive identity, as they have to emancipate themselves both from their parents and from their co-twins. Some differences in identity, anxiety and ambition were observed between female and male twins, MZ and DZ twins, preterm and fullterm twins. Prematurity, sex and zygosity no longer had any relation to cognitive development at 16 years of age.