Asymmetry In Twins Research Articles

“You and me”: Parental perceptions on asymmetry in twins' development and their dominance relationship dynamics

AbstractObjectiveThis study investigated the role of nontypical development in the relative dominance in twins' relationships throughout childhood.BackgroundDominance dynamics, affecting siblings' well‐being, are different in twins than in singletons for whom age and development often dictate sibling hierarchy. These dynamics in twins, who share similar ages and developmental contexts, remain underexplored and demand further understanding.MethodA longitudinal study surveyed 1,547 mothers and 536 fathers of 322 monozygotic (sharing nearly 100% genes) and 1,199 dizygotic (sharing 50% genetic variance) twin pairs, aged 3 to 8–9. Both parents reported on the twins' relationships. Mothers reported whether either twin had a developmental condition.ResultsNo dominance difference was found in similar developmental conditions dyads, whether both twins had typical or nontypical development. However, in dyads where twins differed in the developmental condition, nontypically developing twins were less dominant than their typically developing cotwins. This dominance imbalance persisted throughout childhood, even if initial developmental issues were resolved.ConclusionFrom parents' perspectives, nontypical development does not, in itself, prevent children from demonstrating dominance behaviors in twinship, but it is more likely that the asymmetry in developmental conditions is associated with the relationship between the twins.Implications and RecommendationsAccording to parents' perceptions, twins with nontypical development might experience imbalances in their relationship throughout childhood when their co‐twin is a typically developing child. However, demonstrating dominance might be possible for them in other contexts. Understanding these dominance dynamics is vital for caregivers, informing tailored parenting strategies and interventions to support the well‐being of children.

Facial asymmetry and chewing sides in twins

Objective To resolve how the preferred chewing side (PCS) affects facial asymmetry in twins, whether there are differences between monozygotic (MZ) and dizygotic (DZ) twins, and whether the twins with PCS have more asymmetric faces compared to symmetrically chewing twins. Material and methods The study included 106 Lithuanian twin pairs of the same sex, 59 MZ and 47 DZ pairs. The data were analysed from facial 3D images and manually added landmarks. 3D images were analysed by Rapidform2006 software and statistical analyses were done by using the R software environment version 4.1.0. Results The contralateral effect of PCS and larger chin side was dominant among right and non-right side chewing twins. Being female increased the whole face symmetry. Conclusion The volume of the chin becomes larger on the side opposite to the twins’ habitual chewing side. As the results are quite similar in both twin types, functional factors are more prominent than heredity.

Primary tooth size asymmetry in twins and singletons.

To explore asymmetry values of antimeric deciduous tooth crown dimensions in three types of twins: monozygotic (MZ), dizygotic same-sex (DZ) and opposite-sex (OS) vs. single-born controls. Mesiodistal and labio-lingual crown dimensions of second deciduous molars and mesiodistal canine and first molar crown dimensions of 2159 children at 6-12 years of age were evaluated, originating from the US cross-sectional Collaborative Perinatal Study from the 1970s, including altogether MZ (n = 28), DZ same-sex (n = 33) and OS (n = 39) pairs. Single born (n = 1959) were used as controls. Dental casts were measured for comparison of variance relationships calculated from antimeric teeth, exhibiting fluctuating (FA), and directional (DA) asymmetry using anova. Significant differences appeared in MZ and OS girls in DA of deciduous canines, which gain size in the first and second trimester, and deciduous second molars, which finally stop crown growth during the early post-natal period. Significantly, increased FA values appeared for lower deciduous canines and second molars, indicating greatest environmental stress in OS girls, MZ girls and DZ boys. Twin girls had more fluctuating and directional crown asymmetry than twin boys, but in some dimensions, the twins were more symmetric than controls. Transmembrane hormonal influence between opposite-sex twins, and late gestational stress factors, caused by placental malfunction and/or monochorionicity, may be involved in asymmetric growth of antimers, during critical periods of crown size gain.

Fluctuating asymmetry in quantitative dermatoglyphic traits: twins and singletons

Fluctuating asymmetry for the digital quantitative value was analyzed in a sample of twins (mono and dizygotic) and singletons. The aim was to check if the influence of twinning on the development is expressed in a higher fluctuating asymmetry in twins than in singletons. The results have shown that significant differences exist among the three groups studied, and these differences are fundamentally expressed when the radial and ulnar counts are considered separately.

Genetic analysis of bilateral dermatoglyphic asymmetry in twins and their parents

The data for this study were collected on 64 twin pairs (30 MZ and 34 DZ) and their 128 parents. Two following hypotheses were evaluated: 1. Bilateral asymmetry is significantly genetically controlled; 2. The twinning phenomenon would affect the magnitude of bilateral asymmetry. The results revealed no statistically significant differences between mean values of MZ and DZ twins and their parents for the majority of the traits. Significant differences were recorded for only 6 of 96 comparisons (6%). Analysis of variance revealed separated sources of MZ, DZ and singleton variance. F-ratios, contrasting variances between different groups were significant for 26 of 96 comparisons (27%) showing heterogeneity of variance between zygosities and between twins and their parents. In addition, environmental covariance appeared to be larger for MZ than DZ with respect to directional asymmetry (DA) for all 16 traits and fluctuating asymmetry (FA) for 14 traits. These observations showed complex environmental determinism for bilateral asymmetry for the majority of dermatoglyphic traits. Significant genetic variance ratios (GVRs) were observed for four variables (25%) with respect to DA and three variables (18.75%) with respect to FA. All these significant GVRs were rendered insignificant because of evidence of greater environmental covariance for MZ twins, except possibly for DA for URC4.

The epigenesis of planum temporale asymmetry in twins.

Variation in hemispheric asymmetry of the planum temporale (PT) has been related to verbal ability. The degree to which genetic and environmental factors mediate PT asymmetry is not known. This study examined the heritability for planar asymmetry in 12 dizygotic (DZ) and 27 monozygotic (MZ) male twin pairs who were between 6 and 16 years of age. There was weak but positive evidence for heritability of planar asymmetry. Co-twin similarity for planar asymmetry and Sylvian fissure morphology increased when excluding twins discordant for writing hand and when excluding twins exhibiting birth weight differences >20% from the analyses. Birth weight differences were also related to twin differences in total cerebral volume, but not central sulcus asymmetry. These results suggest that exogenous perinatal factors affect the epigenesis of planar asymmetry development.

Hand Motor Performance and Degree of Asymmetry in Monozygotic Twins

To determine whether the absolute degree of asymmetry of hand motor performance (irrespective of direction of this asymmetry) may have a heritable component we examined 20 pairs of monozygotic (MZ) twins of whom 10 pairs were concordantly right-handed (MZRR) and 10 pairs discordant for handedness (MZ-RL). The tests comprised measurements of the maximum left and right hand tapping rate as well as a paper-and-pencil test of left and right hand motor proficiency. Intraclass correlations within MZ-RR and MZ-RL for absolute degree of hand motor asymmetry were not significant. In contrast, significant intrapair correlations emerged for overall hand motor performance, a measure unrelated to laterality. These results demonstrate that at least in MZ twins the degree of hand motor asymmetry is mainly determined by non-genetic factors, whereas overall hand motor skill is more likely to be influenced by the genome. In addition, the lack of a difference in overall hand motor performance between MZ twins and 40 singletons studied as controls would not support hypotheses explaining behavioural asymmetry in twins, or their discordance for asymmetry, by developmental dysfunction.

Fluctuating dermatoglyphic asymmetry in psychotic twins

Fluctuating asymmetry of bilateral morphological traits is the result of prenatal developmental instability and has been shown to be greater in organisms having more homozygous genotypes (aabb vs. AaBb, for example). This expected increase in fluctuating asymmetry has been found among individuals having a high degree of liability foe schizophrenia, as this disorder appears to have a polygenic basis. We tested the additional prediction that the greater genetic liability for schizophrenia necessary for concordance between twins should be associated with greater fluctuating asymmetry in twin pairs in which both twins are mentally ill compared to twin pairs in which one individual is normal. Our analysis of asymmetry for finger ridge counts from fingerprints of concordant and discordant pairs of twins supports this prediction and provides additional indirect support for the roles of polygenic transmission and prenatal epigenetic vulnerability in schizophrenia.