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.     

Cooperation and Competition as a Function of Zygosity in 7- to 9-Year-Old Twins

A study of 31 monozygotic (MZ) and 24 dizygotic (DZ) Singapore twin pairs (aged 7.6 to 9.7 years) is reported. MZ and DZ twins differed in cooperative and competitive behaviors measured by within-pair reactions to payoff structures designed to elicit simultaneous cooperation (SCO) or reciprocal altruism (RA) on a marble-pull task. Reciprocation entailed turn-taking in rewards. DZ twins showed greater competition than MZ twins when competing for marbles in the RA condition, as predicted. Unexpectedly, however, MZ twins were markedly more competitive than DZ twins under the SCO condition, when success on the task always rewarded both twins. The competition arose over which end of the apparatus was to be used on a given trial. This interaction between condition and zygosity could not be related to differences in parents’ reported treatment of their children or zygosity beliefs, nor did it appear to be related to differences in factors such as time spent together and shared activities. An explanation in terms of uncertain or unstable dominance in MZ twinships is suggested.     

Parent ratings of temperament in twins: explaining the 'too low' DZ correlations.

Twin studies of child temperament using objective measures consistently suggest moderate heritability for most dimensions. However, parent rating measures produce unusual patterns of results. Intraclass correlations for identical (MZ) twins are typically high, whereas fraternal (DZ) twin intraclass correlations are much lower than would be predicted from an additive genetic model. The 'too low' DZ correlations can be explained by parent-rating biases that either exaggerate the differences between DZ twins (contrast effects) or that inflate the similarity of MZ twins (assimilation effects), or by the presence of non-additive genetic variance. To evaluate the three possible explanations, we used model-fitting procedures applied to parent-rating data averaged across 14, 20, 24, and 36 months of age in a sample of 196 twin pairs participating in the MacArthur Longitudinal Twin Study. The data were best described by a model that included contrast effects. Implications for non-twin research are discussed.     

Fourier analysis of facial profiles of young twins

Twins studies provide a powerful approach to determining the relative contribution of genetics and environment to observed variation. Such studies assume trait differences in monozygous (MZ) twins are due to environmental factors and those in dizygous (DZ) twins are due to both genetic and environmental factors. This study quantitated facial profiles of twins using Fourier equations, determining their value in profile analysis and the assessment of the genetic contribution to facial shape. Standardized profile slide photographs of 79 pairs of 4-6 year-old twins (37 MZ pairs, 42 DZ pairs) were scanned and x and y coordinates were extracted from each profile using sellion and Camper's plane as references. The coordinates were subjected to Fourier analysis and the normalised vertex projection coefficients were studied. The means of the differences between coefficients for MZ co-twins did not differ significantly from that of DZ co-twins, although the DZ group showed higher mean differences in the higher harmonics. Subjective examination of superimposed reconstructions showed wider variation between DZ co-twins than MZ co-twins. Correct classification of twins by discriminant function analysis using Fourier coefficients was similar for both groups (MZ: 70.3%; DZ: 73.8%). Fourier analysis could quantitate facial profiles of young children and differentiate some details, but was unable to discriminate between genetic and environmental influences, and any possible interactions between these influences, on their overall facial profiles at this developmental stage.     

Potential bias regarding birth weight in historical and contemporary twin data bases.

In this study we examine the hypothesis that monozygotic (MZ) twins in historical databases are less discordant for birth weight due to negative selection of severely discordant MZ twins. Furthermore, we test the hypothesis that MZ twins are less discordant for birth weight when comparing a volunteer based twin registry with a population based twin registry, due to selective registration. Data were available on 3927 twin pairs from the volunteer Australian Twin Registry born before 1964, 3059 volunteer twin pairs from the Netherlands Twin Register born 1987-1989 and 454 Belgian twin pairs from The East Flanders Prospective Twin Survey born 1987-1989. Intrapair relative birth weight differences (RBWD) were computed for MZ and dizygotic (DZ) twins from each twin registry. Comparing birth weight differences between MZ and DZ twins provides support for the hypothesis that MZ twins are subject to a negative selection in historical databases. Furthermore, Australian MZ twins have a lower RBWD compared to Dutch MZ twins when corrected for the RBWD of Australian and Dutch DZ twins, indicating circumstances which only affect MZ twins. Our hypothesis that MZ twins are less discordant for birth weight in a volunteer based twin registry compared to a population based twin registry had to be rejected. We suggest that investigators using historical databases to test the fetal origins hypothesis should be aware of this increased likelihood of selective exclusion of individuals with extreme morphometric parameters at time of birth.     

Genetic and environmental influences on bilateral dental asymmetry in Northwest Indian twins

The role of genetic and environmental factors on dental asymmetry (in maximum crown dimensions) was examined using 58 pairs of twins (23 MZ and 35 DZ) from Chandigarh, India. The t'-test for equality of means by zygosity showed only one variable significantly different among 56: this is ascribable to Type 1 error. Heterogeneity of MZ-DZ total variance was observed in 42.9% of traits of the two types (fluctuating and directional) of bilateral asymmetry. In general, MZ twins showed higher total variance than DZ pairs. MZ twins also showed stronger environmental covariance for a majority of the traits. Dental asymmetry measures thus yielded consistently low genetic variance ratios and indicated predominantly complex environmental determinism. Since fluctuating asymmetry is widely believed to be an environmental stress indicator, this data set allows confirmation of methods for detecting unequal environmental influences on the zygosities which bias estimates of genetic variance and heritability.     

Genetic determination of NOR activity in human lymphocytes from twins

Summary Activity of nucleolar organizer regions (NORs) was studied in cultured blood lymphocytes from 20 monozygotic (MZ) and 20 dizygotic (DZ) twin pairs. The number of Agstained NORs, the degree of staining, and the frequency of acrocentric associations were used as criteria of the NOR activity, the acrocentric chromosomes being identified by G-banding. Analysis of intrapair concordance as well as of intrapair variance showed the number of Ag+NORs and the size of Ag-deposits to be highly heritable traits. Intrapair differences in acrocentric association frequency were not significantly higher in DZ compared with MZ twins.     

A twin study of structural chromosome aberrations in lymphocytes

Structural chromosome aberrations were analyzed in peripheral lymphocytes of eight monozygotic (MZ) and seven dizygotic (DZ) pairs of male twins. There was no significant intrapair difference in the variance of aberration frequencies among the MZ and DZ twins. Thus, there was no evidence of a major genetic influence on the development of structural chromosome aberrations. Although a genetic component could not be excluded, it was concluded that any chromosome aberrations observed were probably due mainly to environmental influences.     

The influence of zygosity and chorion type on fat distribution in young adult twins consequences for twin studies.

An adverse intra-uterine environment has been associated with abdominal fat distribution in singletons. Twins often have a low birth weight and a short gestation. Therefore, they may have an increased risk to develop abdominal obesity. Furthermore, monozygotic monochorionic twins (MZ MC) have a larger intra-pair birth weight difference compared to monozygotic dichorionic twins (MZ DC). If adult anthropometry is programmed in utero, this may affect the intra-pair correlations in adulthood and, consequently, also the results from the classic twin method to estimate genetic and environmental influences. In the present study, we compared the absolute values, the intra-pair differences, and the intra-pair correlations of body mass, height, BMI, and abdominal fat distribution of 424 MZ MC, MZ DC and dizygotic (DZ) twin pairs (aged 18-34 yrs). DZ, MZ DC and MZ MC twins did not differ for most anthropometric characteristics. Only MZ women tended (p = 0.03) to accumulate more abdominal fat compared to DZ twins. Overall, the contribution of zygosity and chorion type to adult anthropometry was rather low (< or = 1.7%). Although the intra-pair birth weight difference of MZ MC pairs (10.5% in men, 12.3% in women) was significantly larger compared to that of MZ DC pairs (6.9% and 9.2% resp.), the intra-pair differences in adult anthropometry were similar for both MZ twin types. Also the intra-pair correlations of MZ MC and MZ DC pairs were strikingly alike, suggesting no significant influence of the prenatal environment on adult concordance. In conclusion, the substantial difference in the prenatal environment of MZ MC and MZ DC twins did not result in a difference in intra-pair concordance of adult anthropometry and fat distribution. Therefore, we suggest that the chorion type of MZ twins does not bias the twin design and the estimation of the genetic contribution to adult anthropometry.     

Pets and Our Mental Health: The Why,the What and the How

ABSTRACT

There is growing evidence that pet ownership and human–animal interaction (HAI) have benefits for human physical and psychological well-being. However, there may be pre-existing characteristics related to patterns of pet ownership and interactions with pets that could potentially bias results of research on HAI. The present study uses a behavioral genetic design to estimate the degree to which genetic and environmental factors contribute to individual differences in frequency of play with pets among adult men. Participants were from the ongoing longitudinal Vietnam Era Twin Study of Aging (VETSA), a population-based sample of 1,237 monozygotic (MZ) and dizygotic (DZ) twins aged 51–60 years. Results demonstrate that MZ twins have higher correlations than DZ twins on frequency of pet play, suggesting that genetic factors play a role in individual differences in interactions with pets. Structural equation modeling revealed that, according to the best model, genetic factors accounted for as much as 37% of the variance in pet play, although the majority of variance (63–71%) was due to environmental factors that are unique to each twin. Shared environmental factors, which would include childhood exposure to pets, overall accounted for <10% of the variance in adult frequency of pet play, and were not statistically significant. These results suggest that the effects of childhood exposure to pets on pet ownership and interaction patterns in adulthood may be mediated primarily by genetically-influenced characteristics.     

Color perception in twins

The classical twin method was used to examine the genotype--phenotype relationship in color vision. Suprathreshold color differences were assessed by 5 pairs of monozygotic (MZ) and 3 pairs of dizygotic (DZ) twins. The control group included 3 unrelated normal trichromats, a non-twin sibling pair, and a previously diagnosed deuteranomal. Concordance rates were calculated by Spearman's correlation coefficients (rs) and Procrustean distances (gl) between the reconstructed color spaces for each related pair of observers. For 4 pairs of the MZ twins, the rs values were comparable to intraindividual variability in the control normal trichromat; they were significantly higher (0.94-0.97) than those for the DZ twins and siblings (0.72-0.82). The gl values for the MZ twins (0.008-0.029) were lower than for the DZ twins (0.073-0.079) and siblings (0.053). The high concordance between each pair of the MZ twins suggests that their shared photopigment genome constrains a contribution of possible individual variations in nongenetic factors to variability of their color spaces. Lower concordance rates in the DZ twins and siblings can be attributed to differences in the inherited arrays of photopigment genes. Contributions to intrapair variation in color spaces of twins from cognitive factors such as perceptual-cognitive color categorization and decision-process variability are discussed.     

Utilizing Twins as Controls for Non-Twin Case-Materials in Genome Wide Association Studies

Twin registries around the globe have collected DNA samples from large numbers of monozygotic and dizygotic twins. The twin sample collections are frequently used as controls in disease-specific studies together with non-twins. This approach is unbiased under the hypothesis that twins and singletons are comparable in terms of allele frequencies; i.e. there are no genetic variants associated with being a twin per se. To test this hypothesis we performed a genome-wide association study comparing the allele frequency of 572,352 single nucleotide polymorphisms (SNPs) in 1,413 monozygotic (MZ) and 5,451 dizygotic (DZ) twins with 3,720 healthy singletons. Twins and singletons have been genotyped using the same platform. SNPs showing association with being a twin at P-value < 1 × 10^-5 were selected for replication analysis in 1,492 twins (463 MZ and 1,029 DZ) and 1,880 singletons from Finland. No SNPs reached genome-wide significance (P-value < 5 × 10^-8) in the main analysis combining MZ and DZ twins. In a secondary analysis including only DZ twins two SNPs (rs2033541 close to ADAMTSL1 and rs4149283 close to ABCA1) were genome-wide significant after meta-analysis with the Finnish population. The estimated proportion of variance on the liability scale explained by all SNPs was 0.08 (P-value=0.003) when MZ and DZ were considered together and smaller for MZ (0.06, P-value=0.10) compared to DZ (0.09, P-value=0.003) when analyzed separately. In conclusion, twins and singletons can be used in genetic studies together with general population samples without introducing large bias. Further research is needed to explore genetic variances associated with DZ twinning.     

A study of lactose absorption capacity in twins

Summary Lactose absorption capacity was determined by lactose tolerance tests with breath hydrogen determination in 102 healthy, adult, Hungarian pairs of twins in order to test monogenic Mendelian inheritance of the absorptive lactase phenotypes, lactose absorber and lactose malabsorber. Of the total, 52 pairs were monozygous (MZ) and 50 dizygous (DZ) twins of indentical sex. All MZ twins were concordant with respect to lactase phenotype. Among DZ twins, the distribution of lactase phenotypes was in agreement with Hardy-Weinberg expectations derived from the frequencies of the hypolactasia gene in DZ and MZ twins, and in the general Budapest population.In the second part of the study, three commonly used methods of lactose tolerance testing, the blood glucose, the blood galactose, and the breath hydrogen tests, were compared in 49 pairs of twins concordant for lactase phenotype. Blood galactose concentration showed the greatest and only significant difference between the intrapair correlation coefficients of MZ and DZ, and no overlap between lactose absorbers and lactose malabsorbers. The intrapair correlation coefficients of peak breath hydrogen concentration in MZ and DZ twins did not significantly differ from zero, but the resolution of lactase phenotypes was satisfactory. Differences in glucose absorption and concentration in lactose absorbers and malobsorbers overlapped considerably, and among lactose absorbers correlation coefficients in DZ were higher than in MZ twins. In MZ and DZ twins, the difference in concordance and constancy of lactose intolerance symptoms was not significant.     

Does higher concordance in monozygotic twins than in dizygotic twins suggest a genetic component?

It is widely regarded that twins can be used as a natural experiment to subject hypotheses to empirical testing regarding the contributions of genetic factors to phenotypic variability in human traits, especially behavioral traits. In genetic epidemiology, a higher concordance rate in monozygotic (MZ) twins than in dizygotic (DZ) twins is often taken as prima facie evidence for a genetic component. While twins studies have been used to estimate the contributions of genetic factors to phenotypic variability in human traits, the corresponding methodology that allows the estimation entails several crucial assumptions. The most critical is that MZ and DZ twins are equally similar environmentally. Although MZ twins are genetically more similar than DZ twins, they are often environmentally more similar. This paper demonstrates that, even in the complete absence of any genetic factor and of any biases, the greater environmental similarity alone in MZ twins can result in higher concordance rate in MZ twins than in DZ twins. This is especially true when there are multiple environmental factors, which may have multiple exposure levels and/or interact strongly, although each of them may be of low risk. This may serve as a sobering antidote to the uncritical reliance on twin studies without examining the validity of the underlying assumptions.     

Monozygotic and dizygotic twins' retrospective and current bereavement-related behaviors: an evolutionary perspective.

The present study compared bereavement responses of 325 monozygotic (MZ) and 176 dizygotic (DZ) adolescent and adult twins following the loss of their co-twins. A subset of twins completed the Grief Experience Inventory using a retrospective time frame, while a second subset completed it using a current time frame. It was hypothesized that MZ twins (in both retrospective and current groups) would report higher levels of grief-related behavior than DZ twins, consistent with Hamilton's (1964) concept of inclusive fitness. Discriminant function and profile analyses yielded supportive findings, but only for the retrospective MZ and DZ twin comparisons. Females in both groups expressed higher levels of bereavement-related behavior than males. Findings are discussed with reference to theoretical aspects of grief and mourning.     

Odontometric variations of permanent crown in Punjabi and U.S. twins

The study reports odontometric variations within and between Punjabi and U.S. twins. The data are based on 23 MZ and 35 DZ Punjabi twin pairs and 32 MZ and 43 DZ U.S. pairs. None of the t' tests indicates differences between MZ and DZ mean dental dimensions in either sample. Mean tooth dimensions of Punjabi twins are in general greater than those of U.S. twins with significant differences for about 25% of the traits. Coefficient of variation is highest in lateral incisors and canines and lowest in first molars in both samples. The two samples differ more in their variances than in their means. The results indicate that buccolingual dimensions of mandibular traits are under relatively greater selection pressure.     

Examining the dimensions of intimacy in twin and peer relationships.

While it is widely assumed that twinsh p involves high levels of intimacy to the detriment of other relationships, an examination of twinship using measures of intimacy has not yet been conducted. Participants were 214 individua twins (128 MZ, 62 DZ same-sex, and 24 DZ mixed-sex) between 16-73 years of age. Twins completed measures of intimacy in reference to their co-twin, their closest same-sex friend, and their closest other-sex friend. As expected, the highest level of intimacy was reported for the co-twin. Contrary to expectation, MZ twins did not report significantly higher levels of intimacy with their co-twin, or significantly lower levels of intimacy with their closest friends when compared to DZ same-sex or DZ mixed-sex twins. Furthermore, twins who reported high levels of intimacy with their co-twin did not report significantly lower levels of intimacy with their closest friends. Implications and directions for future research are discussed.     

Inheritance of finger pattern types in MZ and DZ twins

Digital patterns of a sample on twins were analyzed to estimate the resemblance between monozygotic (MZ) and dizygotic (DZ) twins and to evaluate the mode of inheritance by the use of maximum likelihood based variance decomposition analysis. MZ twin resemblance of finger pattern types appears to be more pronounced than in DZ twins, which suggests the presence of genetic factors in the forming of fingertip patterns. The most parsimonious model shows twin resemblance in count of all three basic finger patterns on 10 fingers. It has significant dominant genetic variance component across all fingers. In the general model, the dominant genetic variance component proportion is similar for all fingertips (about 60%) and the sibling environmental variance is significantly nonzero, but the proportion between additive and dominant variance components was different. Application of genetic model fitting technique of segregation analyses clearly shows mode of inheritance. A dominant genetic variance component or a specific genetic system modifies the phenotypic expression of the fingertip patterns. The present study provided evidence of strong genetic component in finger pattern types and seems more informative compared to the earlier traditional method of correlation analysis.     

Menstrual characteristics of mothers of twins

The menarcheal age and frequency of menstrual disorders in mothers of dizygotic (DZ) and monozygotic (MZ) twins born in Denmark in 1984 or 1985 were compared with a control sample of mothers of singleton infants born in the same period. Compared to control mothers, mothers of DZ twins had a lower menarcheal age and a lower frequency of menstrual irregularities--menstrual cycles exceeding 5 weeks duration, varying length of menstrual cycles and episodes of menostasia. Mothers of MZ twins had a higher frequency of intermenstrual bleeding compared to mothers of singletons. All results were adjusted for maternal age and parity. The findings are relevant to the interpretation of aetiological factors leading to twin births and support the hypothesis of differences in sex hormone activity of the mothers of DZ twins in comparison with other women.