Background: Although the clinical utility of categorically defined attention‐deficit hyperactivity disorder (ADHD) is well established, there is also strong evidence supporting the notion of ADHD as ...an extreme of a continuous trait. Nevertheless, the question of whether the etiology is the same for different levels of DSM–IV ADHD symptoms remains to be investigated. The aim of this study was to assess genetic links between the extreme and the subthreshold range of ADHD symptoms.
Method: Parents of all Swedish 9‐ and 12‐year‐old twins born between 1992 and 2000 were interviewed for DSM–IV ADHD symptoms and associated conditions. Two validated cutoff values were used for screening and assigning research diagnoses. Response rate was 80%. Twin methods were applied to investigate the extent to which ADHD is etiologically distinct from subthreshold variations in ADHD symptoms.
Results: Extremes analyses indicated a strong genetic link between the extreme and the subthreshold variation, with almost identical group heritability estimates around .60 for the diagnostic (prevalence 1.78%) and screening (prevalence 9.75%) criteria of ADHD.
Conclusion: A strong genetic link between the extreme and the subthreshold variation of DSM–IV based assessments of ADHD symptoms was found. The data suggest that ADHD is best viewed as the quantitative extreme of genetic and environmental factors operating dimensionally throughout the distribution of ADHD symptoms, indicating that the same etiologic factors are involved in the full range of symptoms of inattention, hyperactivity and impulsivity.
The mechanisms responsible for twinning and disorders of twin gestations have been the subject of considerable interest by physicians and scientists, and cases of atypical twinning have called for a ...reexamination of the fundamental theories invoked to explain twin gestations. This article presents a review of the literature focusing on twinning and atypical twinning with an emphasis on the phenomena of chimeric twins, phenotypically discordant monozygotic twins, mirror-image twins, polar body twins, complete hydatidiform mole with a coexistent twin, vanishing twins, fetus papyraceus, fetus in fetu, superfetation, and superfecundation. The traditional models attributing monozygotic twinning to a fission event, and more recent models describing monozygotic twinning as a fusion event, are critically reviewed. Ethical restrictions on scientific experimentation with human embryos and the rarity of cases of atypical twinning have limited opportunities to elucidate the exact mechanisms by which these phenomena occur. Refinements in the modeling of early embryonic development in twin pregnancies may have significant clinical implications. The article includes a series of figures to illustrate the phenomena described.
Autism spectrum disorders are considered to be among the most heritable mental disorders, a notion based on surprisingly sparse data from small clinical studies. Population-based studies of the ...heritability of other neuropsychiatric disorders and comorbidities among them have also been sparse. The authors sought to address both of these issues.
Parents of all Swedish 9- and 12-year-old twin pairs born between 1992 and 2000 (N=10,895) were interviewed regarding autism spectrum disorders and associated conditions (response rate, 80%). Concordance rates and structural equation modeling were used for evaluating causes for familial aggregation and overlap between conditions.
Monozygotic twins had higher concordance rates than dizygotic twins for autism spectrum disorders, attention deficit hyperactivity disorder (ADHD), developmental coordination disorder, and tic disorder. Genetic effects accounted for 80% (95% CI=29-91) of the variation in liability for autism spectrum disorders, 79% (95% CI=61-88) for ADHD, 70% (95% CI=35-83) for developmental coordination disorder, and 56% (95% CI=37-68) for tic disorder. Among monozygotic co-twins of children with autism spectrum disorders, the probability of having a diagnosis of ADHD was 44%, compared with 15% for dizygotic co-twins. Differences in cross-disorder effects between monozygotic and dizygotic twins were observed for most other comorbidities, and substantial proportions of the genetic variance for autism spectrum disorders was shared with each of the other disorders.
Different neuropsychiatric disorders seem to have a common genetic etiology, suggesting caution in the use of diagnostic entities and proband status in efforts to uncover genes predisposing to autism spectrum disorders.
The Danish Twin Registry: An Updated Overview Pedersen, Dorthe Almind; Larsen, Lisbeth Aagaard; Nygaard, Marianne ...
Twin research and human genetics,
12/2019, Volume:
22, Issue:
6
Journal Article
Peer reviewed
Open access
The Danish Twin Registry (DTR) was established in the 1950s, when twins born from 1870 to 1910 were ascertained, and has since been extended to include twins from birth cohorts until 2009. The DTR ...currently comprises of more than 175,000 twins from the 140 birth cohorts. This makes the DTR the oldest nationwide twin register and among the largest in the world. The combination of data from several surveys, including biological samples and repeated measurements on the same individuals, and data from Danish national registers provides a unique resource for a wide range of twin studies. This article provides an updated overview of the data in the DTR: First, we provide a summary of the establishment of the register, the different ascertainment methods and the twins included; then follows an overview of major surveys conducted in the DTR since 1994 and a description of the DTR biobank, including a description of the molecular data created so far; finally, a short description is given of the linkage to Danish national registers at Statistics Denmark and some recent examples of studies using the various data resources in the DTR are highlighted.
Twin Crowns by Catherine Doyle (review) Tucker, Aaren
Bulletin of the Center for Children's Books,
04/2022, Volume:
75, Issue:
8
Journal Article, Book Review
There are variable reports of risk of concordance for progression to islet autoantibodies and type 1 diabetes in identical twins after one twin is diagnosed. We examined development of positive ...autoantibodies and type 1 diabetes and the effects of genetic factors and common environment on autoantibody positivity in identical twins, nonidentical twins, and full siblings.
Subjects from the TrialNet Pathway to Prevention Study (
= 48,026) were screened from 2004 to 2015 for islet autoantibodies (GAD antibody GADA, insulinoma-associated antigen 2 IA-2A, and autoantibodies against insulin IAA). Of these subjects, 17,226 (157 identical twins, 283 nonidentical twins, and 16,786 full siblings) were followed for autoantibody positivity or type 1 diabetes for a median of 2.1 years.
At screening, identical twins were more likely to have positive GADA, IA-2A, and IAA than nonidentical twins or full siblings (all
< 0.0001). Younger age, male sex, and genetic factors were significant factors for expression of IA-2A, IAA, one or more positive autoantibodies, and two or more positive autoantibodies (all
≤ 0.03). Initially autoantibody-positive identical twins had a 69% risk of diabetes by 3 years compared with 1.5% for initially autoantibody-negative identical twins. In nonidentical twins, type 1 diabetes risk by 3 years was 72% for initially multiple autoantibody-positive, 13% for single autoantibody-positive, and 0% for initially autoantibody-negative nonidentical twins. Full siblings had a 3-year type 1 diabetes risk of 47% for multiple autoantibody-positive, 12% for single autoantibody-positive, and 0.5% for initially autoantibody-negative subjects.
Risk of type 1 diabetes at 3 years is high for initially multiple and single autoantibody-positive identical twins and multiple autoantibody-positive nonidentical twins. Genetic predisposition, age, and male sex are significant risk factors for development of positive autoantibodies in twins.
Are genetic risk factors for current depressive symptoms good proxies for genetic risk factors for syndromal major depression (MD)?
In over 9000 twins from the population-based Virginia Adult Twin ...Study of Psychiatric and Substance Use Disorders, the occurrence of all nine DSM symptomatic criteria for MD in the last year was assessed at personal interview and then grouped by their temporal co-occurrence. The DSM criteria which occurred outside (OUT)
inside of (IN) MD episodes were then separated. We calculated tetrachoric correlations for OUT and IN depressive criteria in monozygotic (MZ) and dizygotic (DZ) pairs and fitted univariate and bivariate ACE twin models using OpenMx.
The mean twin correlations (±95% CIs) for IN depressive criteria were substantially higher than for OUT depressive criteria in both MZ +0.35 (0.32-0.38)
0.20 (0.17-0.24) and DZ pairs 0.20 (0.17-0.24)
0.10 (0.04-0.16. The mean IN-OUT cross-correlation in MZ and DZ pairs was modest +0.15 (0.07-0.24) and +0.07 (0.03-0.12). The mean heritability estimates for the nine In
Out depressive criteria was 0.31 (0.22-0.41) and 0.15 (0.08-0.21), in MZ and DZ pairs, respectively. The mean genetic correlation between the nine IN and OUT depressive criteria was +0.07 (-0.07 to 0.21).
Depressive criteria occurring outside depressive episodes are less heritable than those occurring within. These two ways criteria can manifest are not closely genetically related. Current depressive symptoms - most of which are occurring outside of depressive episodes - are not, for genetic studies, good proxies for MD.
Reply Myers, Chelsea E; Klein, Barbara EK; Klein, Ronald
American journal of ophthalmology,
07/2015, Volume:
160, Issue:
1
Journal Article
Peer reviewed
The notion of genetic factors explaining thicker central retina as determined by OCT is supported by a heritability estimate of 0.90 in a study using monozygotic and dizygotic female twin pairs.2 A ...different study of macular thickness found thicker retinas in younger persons, which led the authors to suggest that the differences between the sexes were due, in part, to gonadal hormone levels.3 The role of genes and environmental factors that determine retinal thickness remain to be further elucidated.
Heritability of endometriosis Saha, Rama; Pettersson, Hans Järnbert; Svedberg, Pia ...
Fertility and sterility,
10/2015, Volume:
104, Issue:
4
Journal Article
Peer reviewed
Open access
To estimate the relative contribution of genetic influences and prevalence on endometriosis.
Analysis of self-reported data from a nationwide population-based twin registry.
Not applicable.
A total ...of 28,370 women, female monozygotic (MZ) or dizygotic (DZ) twins, who participated in either of two surveys (1998-2002 or 2005-2006).
None.
Self-reported endometriosis, validated by medical records.
A history of endometriosis was reported by 1,228 female twins. The probandwise concordance was 0.21 for MZ and 0.10 for DZ twins. Higher within-pair (tetrachoric) correlation was observed among MZ (0.47) compared with DZ (0.20) twins. The best-fitting model revealed a contribution of 47% by additive genetic factors and the remaining 53% attributed to unique environmental effects.
Our findings suggest both genetic and unique (nonshared) environmental influences on the complex etiology of endometriosis and support the hypothesis that genes have a strong influence on phenotypic manifestations of endometriosis.
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