Variation In Twins Research Articles

Genetic and environmental impact on variation in the palatal dimensions in permanent dentition: a twin study.

The objective of this study was to assess the relative contributions of genetic and environmental factors to variation in palatal parameters in twins with completed maxillary growth. The subjects of this study comprised digital dental casts of 50 monozygotic and 35 dizygotic twin pairs. The subjects' average age was 17.95 ± 2.83years. Zygosity determination was carried out using 15 specific DNA markers and an amel fragment of the amelogenin gene. The interdental distances were measured between selected dental landmarks at the occlusal and gingival planes. The palatal height, surface area and volume were measured between the gingival plane and the midpalate suture. High heritability estimates were observed for all transverse intra-arch measurements. The palate height (a2 = 0.8), dental arch width in the molar area (a2 = 0.86), palatal surface area (a2 = 0.61) and palate volume (a2 = 0.69) were under strong additive genetic control. Moderate genetic dominance was observed for dental arch widths at the gingival line in the canine (d2 = 0.5) and premolar regions (d2 = 0.78-0.81). Sexual dimorphism was shown, with males exhibiting a greater arch width, palate surface area and volume than females (p < 0.01). The majority of palate parameters variation in twins was controlled by genetic effects, and most were highly heritable.

Alloying regulation mechanism toward microstructure evolution of Sn-bi-based solder joint under current stress

Sn–Bi-based solder alloys have triggered substantial concern in development of low-temperature interconnecting materials of multi-level packaging electronic devices. However, with the high-density current treatment, dramatic atom migration occurs in Sn–Bi-based solder joint, which accelerates the evolution of solder joint microstructure, including the aggregation of Bi phase and the excessive growth of interfacial intermetallic compound (IMC). In this work, Sb element shows a unique influence mechanism on the electromigration (EM) resistance of Sn-Bi-based solder joint. Doped Sb in Sn58Bi-1Sb (SB-Sb) solder massive precipitate as Sn-Sb and Bi-Sb nano precipitations under current stress. These nano precipitates show the ability to capture the migrating atoms, inhibit the migration of Bi atoms and Sn atoms, and enhance the EM resistance of solder joint. Importantly, Bi twin crystal structure is observed in Sn-Bi-Sb solder for the first time. The variation of twins' phase orientations in different positions of solder joint is associated with atomic migration paths induced by the current stress and would eliminate the overgrowth of the Bi rich phase and IMCs. The presented study provides the valuable insights into the regulates mechanism of alloy elements on atom migration in Sn-Bi-based solder joint.

Monozygotic Does Not Always Mean Identical: An Investigation of Anatomic Variations in Identical Twins

Identical (monozygotic) twins develop from a single zygote. As such, it is often assumed that monozygotic twins have identical genotype and phenotype; however, epigenetic and environmental influences may play a significant role in development of monozygotic twins. Anatomic variations are a result of genetic and environmental influences and are often attributed to developmental factors. It is currently unknown whether identical twins with the same genotype develop anatomic variations compared to each other and the general population; therefore, the purpose of this investigation is to document and compare neurovascular anatomic variations in monozygotic twin cadaveric donors. Monozygotic twin (XY) cadaveric donors were dissected using two techniques. An en mass autopsy protocol was utilized to dissect the viscera of the neck, thorax, abdomen, and pelvis, while a regional approach was employed to dissect the upper and lower limbs. Neurovascular anatomic variations were macroscopically visualized, and morphometric data recorded and compared between the donors. Fifteen total neurovascular variations (14 paired structures; one vein, four nerves, ten arteries) were noted in the twin donors (Table 1). Among variations, ten were identified in both donors, while two variations were only found in twin A and three found only in twin B. All ten variations in both donors were paired structures. Six of the ten paired structures demonstrated bilateral variation in both twins, one bilateral variation in twin A (unilateral in twin B), two bilateral variations in twin B (unilateral in twin A), and one unilateral variation in both twins. Of the five variations found in only one donor, four were paired structures, but the variation was only noted unilaterally. Neurovascular variations are not consistent between monozygotic twins. While the mechanism of twinning in these donors is unknown, this study is the first of its kind to demonstrate unique anatomic variations in identical twins. To our knowledge this is the first study to examine gross anatomical neurovascular variations among monozygotic twin cadaveric donors. This study provides a glimpse into the prevalence and inconsistency of anatomical variations in identical twins, which sheds new light on the environmental influences on anatomical variation. Future studies in organ morphologies and histology will further elucidate variations and potentially different pathologies between monozygotic twins.

Unsafe Chewing: Choking and Other Risks

Unsafe Chewing: Choking and Other Risks

A longitudinal study of epigenetic variation in twins

DNA methylation is a key epigenetic mechanism involved in the developmental regulation of gene expression. Alterations in DNA methylation are established contributors to inter-individual phenotypic variation and have been associated with disease susceptibility. The degree to which changes in loci-specific DNA methylation are under the influence of heritable and environmental factors is largely unknown. In this study, we quantitatively measured DNA methylation across the promoter regions of the dopamine receptor 4 gene (DRD4), the serotonin transporter gene (SLC6A4/SERT) and the X-linked monoamine oxidase A gene (MAOA) using DNA sampled at both ages 5 and 10 years in 46 MZ twin-pairs and 45 DZ twin-pairs (total n=182). Our data suggest that DNA methylation differences are apparent already in early childhood, even between genetically identical individuals, and that individual differences in methylation are not stable over time. Our longitudinal-developmental study suggests that environmental influences are important factors accounting for interindividual DNA methylation differences, and that these influences differ across the genome. The observation of dynamic changes in DNA methylation over time highlights the importance of longitudinal research designs for epigenetic research.

Genetic Covariance Between γ-Glutamyl Transpeptidase and Fatty Liver Risk Factors: Role of β2-Adrenergic Receptor Genetic Variation in Twins

Plasma levels of gamma-glutamyl transpeptidase (GGT) are associated with risk factors for nonalcoholic fatty liver disease (NAFLD), such as dyslipidemia, insulin resistance (IR), and hypertension. Limited data exist on whether there is genetic covariance between plasma levels of GGT and NAFLD risk factors. Variants of beta2-adrenergic receptor gene (ADRB2) have been associated with dyslipidemia, IR, and hypertension, but its effect on GGT secretion is not known. We estimated the heritability of GGT using a twin-study design and examined the genetic covariance between GGT levels, IR, hypertension, levels of low-density lipoproteins and triglycerides, and ADRB2 variants. We studied phenotypes of 362 twins; the heritabilities of increased GGT activity and genetic covariance with NAFLD risk factors were estimated by variance-component methodology. ADRB2 genotype associations with plasma GGT activity were examined using generalized estimating equations to account for intra-twinship correlations. GGT activity was heritable at 49% +/- 8% of the twin cohort and had significant covariance with IR; insulin, triglyceride, and uric acid levels; and diastolic blood pressure. In generalized estimating equation models, the most common haplotype of ADRB2 was significantly associated with plasma GGT activity. Five single nucleotide polymorphisms in ADRB2 were associated with levels of GGT; ADRB2 haplotypes displayed pleiotropic effects on GGT and triglyceride levels. In a twin study, GGT shared genetic codetermination with traits of metabolic syndrome. The ADRB2 gene had pleiotropic effects on plasma levels of GGT and triglycerides, indicating linked pathways (eg, adrenergic) between genetic susceptibility to NAFLD and metabolic syndrome.

Epigenetic variation in twins: heritability and the role of body fatness

Epigenetic variation in twins: heritability and the role of body fatness

Dizygotic twinning

The tendency to conceive spontaneous dizygotic (DZ) twins is a complex trait with important contributions from both environmental factors and genetic disposition. Twins are relatively common and occur on average 13 times per 1000 maternities, though the twinning frequency varies over time and geographic location. This variation is mostly attributed to the differences in DZ twinning rate, since the monozygotic twinning rate is relatively constant. DZ twinning is in part under genetic control, with mothers of DZ twins reporting significantly more female family members with DZ twins than mothers of monozygotic twins. Maternal factors such as genetic history, advanced age and increased parity are known to increase the risk of DZ twins. Recent research confirmed that taller mothers and mothers with a high body mass index (30>) are at greater risk of DZ twinning. Seasonality, smoking, oral contraceptive use and folic acid show less convincing associations with twinning. Genetic analysis is beginning to identify genes contributing to the variation in twinning. Mutations in one of these genes (growth differentiation factor 9) are significantly more frequent in mothers of DZ twins. However, the mutations are rare and only account for a small part of the genetic contribution for twinning.

Developmental origins of low mathematics performance and normal variation in twins from 7 to 9 years.

A previous publication reported the etiology of mathematics performance in 7-year-old twins (Oliver et al., 2004). As part of the same longitudinal study we investigated low mathematics performance and normal variation in a representative United Kingdom sample of 1713 same-sex 9-year-old twins based on teacher-assessed National Curriculum standards. Univariate individual differences and DeFries-Fulker extremes analyses were performed. Similar to our results at 7 years, all mathematics scores at 9 years showed high heritability (.62-.75) and low shared environmental estimates (.00-.11) for both the low performance group and the full sample. Longitudinal analyses were performed from 7 to 9 years. These longitudinal analyses indicated strong genetic continuity from 7 to 9 years for both low performance and mathematics in the normal range. We conclude that, despite the considerable differences in mathematics curricula from 7 to 9 years, the same genetic effects largely operate at the two ages.

Articles selected by Faculty of 1000: similar embryos from divergent genomes; gene expression variation in twins; the E. coli stress response network; adaptive immune system evolution; alloployploid tobacco evolution

Articles selected by Faculty of 1000: similar embryos from divergent genomes; gene expression variation in twins; the E. coli stress response network; adaptive immune system evolution; alloployploid tobacco evolution

Assessing natural variations in gene expression in humans by comparing with monozygotic twins using microarrays

Quantitative variation in gene expression in humans is the outcome of various factors, including differences in genetic background, gender, age, and environment. However, the extent of the influence of these factors on gene expression is not clear. We attempted to address this issue by carrying out gene expression profiling in blood leukocytes with 13 individuals (including 5 pairs of monozygotic twins) on 10,000 genes using HG-U95Av2 oligonucleotide microarrays. The proportion of differentially expressed genes between monozygotic twins was low (up to 1.76%). Most of the variations belonged to the least variable category. These genes, exhibiting "random variations," did not show clear preference to any functional class, although "signaling and communication" and "immune and related functions" generally topped the list. The extent of variation in gene expression increased in comparisons between unrelated individuals (up to 14.13%). Most of the genes (89%) exhibiting random variations in twins also varied in expression in unrelated individuals. As with twins, signaling and communication topped the list, and substantial variations were observed in all three categories: least variable, moderately variable, and most variable. An important outcome of this study was that the housekeeping genes were nearly insensitive to random variations but appeared to be more susceptible to genetic differences. However, the highly expressed housekeeping genes exhibited low variation and appeared to be insensitive to all known factors. Gene expression profiling in monozygotic twins can provide useful data for the assessment of natural variation in gene expression in humans.

Statistical analysis of the seasonal variation in the twinning rate

AbstractThere have been few secular analyses of the seasonal variation in human twinning and the results are conflicting. One reason for this is that the seasonal pattern of twinning varies in different populations and at different periods. Another reason is that the statistical methods used are different. The changing pattern of seasonal variation in twinning rates and total maternities in Denmark was traced for three periods (1855–69, 1870–94, and 1937–84). Two alternative methods of analysis are considered. The method of Walter and Elwood and a trigonometric regression model give closely similar results. The seasonal distribution of twin maternities for the periods in the 19th century showed highly significant departures. For both twin and general maternities, the main peaks can be seen from March to June and a local peak in September. During the spring– summer season the twinning rates were higher than the total birth rates, indicating a stronger seasonal variation for the twin maternities than for the general maternities. For 1937–84, there was a similar, but less accentuated, pattern. Studies of other populations are compared with the Danish results. The more accentuated seasonal variation of twinning in the past indicate that some factors in the past affected women during summer–autumn and around Christmas time, making them more fecund and particularly to be more prone to polyovulation and/or more able to complete a gestation with multiple embryos.

Genetic analysis of occlusal variation in twins

Orthodontists usually assume that genetic factors are of major significance to the development of malocclusion. Studies of twin similarities have lent support to this concept. Use of the twin model to estimate genetic variance, however, involves the assumptions that there are no mean or total variance inequalities between monozygotic and dizygotic twins. The environmental (including biologic) differences between monozygotic and dizygotic twin development may therefore constitute a source of bias. Genetic variance and heritability were estimated for a series of arch and occlusal traits in sixty twins using recently developed methods that are unbiased by variance heterogeneity between zygosities. Heterogeneity was demonstrated for twenty of the forty traits, suggesting considerable amounts of hidden environmental determinance. Tooth-displacement means differed between monozygotes and dizygotes, indicating bias in that trait's genetic variance analysis. Significant heritability could not be demonstrated for overbite, overjet, buccal segment relation, total tooth displacement, and occlusal discrepancies in arch shape. Arch size, individual tooth displacement scores, and cross-bite show significant genetic variance, averaging only about 36 percent of the total variance of these traits.

On the occurrence of sex-linked variations in twins

On the occurrence of sex-linked variations in twins