Zygosity Diagnosis Research Articles

Heterokaryotypic Monochorionic Twin Pregnancy: New Perspective

AbstractMonozygotic twins are thought to be identical since they are created from a single fertilized egg, yet there may be differences in their congenital defects, birth weight, and genetic makeup. Asymmetric X chromosome inactivation, unequal gene imprinting, and postzygotic mitotic mistakes including nondisjunction and anaphase lag can all result in heterokaryotypic monochorionic twins. We report a monochorionic twin pregnancy that exhibited stigmata associated with trisomy 18 on postnatal examination despite a low risk of common aneuploidy (trisomy 18) on noninvasive prenatal screening. Short tandem repeat markers were used for postnatal examination to confirm high-grade mosaicism. These markers indicated mosaic trisomy 18 in twin II and normal in twin I, ruling out uniparental disomy and establishing monozygosity in both fetuses. Twin sac amniocentesis is a prenatal diagnostic procedure that can be used to identify discrepant monochorionic twins because chorionic villus sampling, single sac amniocentesis, or cordocentesis may not be able to rule out aneuploidy in the second fetus and may yield a false-negative result. For prompt zygosity diagnosis, chromosomal complement, genetic counseling, and referral for selective fetal reduction, twin sac amniocentesis is recommended.

Unusual twinning: Additional findings during prenatal diagnosis of twin zygosity by single nucleotide polymorphism (SNP) array.

To evaluate the incidence and characteristics of unusual twinning by using single nucleotide polymorphism (SNP) array to identify twin zygosity. This study reviewed 386 twin pairs who were seen for prenatal or postnatal diagnosis and underwent SNP array to detect zygosity. The incidence of monozygotic (MZ) twins was 11.36% (25/220) in the assisted reproductive technology (ART)-conceived group. Monochorionic dizygotic twins represented 3 of 24 monochorionic ART-conceived twin pairs (3/24, 12.50%) but none in the spontaneous twin pairs. Among 4 single-embryo transfer twin pairs, 3 represented unusual twinning, including 2 MZ twin pairs with discordant karyotypes and 1 dizygotic twin pair of the same gender. Of the pregnancies with 2 or more embryos transferred, 7.77% (15/193) were MZ. Additionally, there was a dichorionic monozygotic twin pair with placental vascular anastomoses from a day-5 blastocyst transfer. Single nucleotide polymorphism array can provide zygosity diagnosis in addition to chromosomal copy number variation and uniparental disomy detection. ART twin pregnancies have a risk of unusual twinning, such as monochorionic dizygotic, single-embryo transfer twin pairs with discordant karyotypes or dizygotic, and dichorionic monozygotic with vascular anastomoses from day-5 transfer.

The role of environmental versus genetic factors on tooth and dental arch dimensions in a twin sample

Background: Genetic variability and heritability were estimated for mesiodistal tooth diameters and dental arch dimensions to evaluate the relative influence of genetics and environment on them and to determine if monozygotic and dizygotic twin can be differentiated on the basis of comparison between these parameters. Materials and methods: A sample of 32 pairs of monozygotic twin (13 males and 19 females) and 30 pairs of dizygotic twin (12 males and 18 females), both groups age range was 13-23, were selected. Both mesiodistal crown dimension and dental arch parameter were identified and the data was analyzed through two main quantitative genetic approaches, simple variance analysis (f ratio) and classical correlation analysis (falconer's formula). Results: the maxillary left central incisors was the common tooth which has high genetic variability and strong heritability estimate in total twin sample and in male and female twin pairs, A potential source of common environment contribution to tooth size was found in male twin pairs while females were appeared to have an impact on dental arch dimension. Conclusions: the maxillary left central incisors may be useful as an additional tool for zygosity determination along with other trait, while dental arch dimensions are not recommended for zygosity diagnosis. Common environment revealed an effect on teeth in males and on dental arch dimension in females

Zygosity Diagnosis: When Physicians and DNA Disagree.

Physicians and other medical professionals do not always provide new parents with an accurate diagnosis of their twins' zygosity. An overview of this problem is presented, supplemented by an interview with a mother who recently learned that her 2-year-old 'dizygotic (DZ)' twin girls are actually 'monozygotic (MZ)'. Reviews of two case studies, one of twins with sex-discordance and chimerism and the other of twins with congenital amegakaryotic thrombocytopenia, follow. Two additional studies, one a twin analysis of attractiveness to mosquitoes and the other a study of twins coping with crisis, are also described. Several articles and letters from the popular media, concerning less favored twins, paternity issues surrounding superfecundation, twins with late-onset Tay-Sachs disease, and triplets admitted to MIT are informative and insightful.

Validity of the Zygosity Questionnaire and Characteristics of Zygosity-Misdiagnosed Twin Pairs in the Healthy Twin Study of Korea

Determining valid zygosity is a basic and important requirement in a twin study, because misdiagnosing zygosity leads to biased results. The Healthy Twin Study has collected data from adult like-sex twins and their families since 2005. In the study, a questionnaire to determine zygosity was developed comprising four questions; one concerning the degree of resemblance, and three concerning the degree of confusion by the resemblance. Among 2,761 individuals (624 twin pairs) of twin and their families, 406 pairs of twins (mean age 38.3, 63.5% women) with both questionnaire and genotype information were selected to examine the validity of the zygosity questionnaire using 16 short tandem repeat markers. We first determined individual zygosity including undetermined category, and then decided the zygosity of a twin pair using a decision tree. Sensitivity of questionnaire diagnosis was 98.8% for monozygotic (MZ) and 88.9% for dizygotic (DZ) twins, and positive predictive value was 97.2% for MZ and 95.0% for DZ. When we compared correctly and wrongly diagnosed twin pairs, misdiagnosed DZ twins (nine pairs) showed striking similarity in stature or obesity even exceeding that of true MZ twins. Our finding suggests that a parsimonious questionnaire method of diagnosing the zygosity will be useful, and adding physical or physiological measurements to a questionnaire of zygosity diagnosis will either confound the correct diagnosis or reduce the efficiency of the study compared with using questionnaire alone or with introducing genotyping.

Early Research on Human Genetics Using the Twin Method: Who Really Invented the Method?

The twin method consists of a formal comparison between the resemblance between identical (monozygotic, MZ) twins and the resemblance between fraternal (dizygotic, DZ) twins for some trait of interest. It was developed between 1900 and about 1940, as more accurate tools for diagnosis of zygosity and for statistically analyzing the resemblance between relatives were built. Its early use was in the demonstration that a trait was inherited or that part of the causation of a trait was genetical, but it has now evolved to the point that twin registries constitute an important resource for the identification of specific genes and their interactions both with other genes and with the internal and external environment. Who really invented the method is still an unsettled question, which this article explores.

Zygosity and Chorionicity in the Prognosis of Triplet Pregnancies: Contribution of Microsatellites

We assess the prognostic values of zygosity and chorionicity in triplet pregnancies (TP) with the assistance of microsatellites analysis. 53 consecutive TP whose prenatal care and delivery occurred in our maternity hospital were included in this prospective study. Zygosity of all sets of triplets (alive or stillbirth, after 22 weeks of gestation) was determined by PCR-amplified microsatellites markers analysis. Chorionicity was determined by placental analysis in our fetopathology referral department and validated by molecular analysis of zygosity. Placental conformations, obstetrical and neonatal outcomes of TP were studied according to their zygosity and chorionicity. Monozygotic and to a greater extent, dizygotic TP were associated with an increase in placental ischemic injuries, velamentous cord insertions, twin-twin transfusion syndromes, fetal anomalies, and perinatal mortality when compared with the trizygotic TP (p < 0.05). Monochorionic and more significantly, dichorionic TP presented with the same increases as trichorionic TP (p < 0.05). Thus chorionicity easily determined by ultrasound evaluation during the first trimester of the pregnancy must remain the main prognosis criterion in TP. Taking into account the rare indications of prenatal diagnosis of zygosity, it became apparent that chorionicity has a greater impact than zygosity when distinguishing high-risk groups of TP.

The risk of monozygotic twins after assisted reproductive technology: a systematic review and meta-analysis

It is estimated that there is at least a 2-fold rise in the incidence of monozygotic twinning after assisted reproductive technology compared with natural conception. This can result in adverse pregnancy outcomes. We searched MEDLINE, EMBASE and SCISEARCH for studies that estimated the risk of monozygotic twinning and its association with any particular assisted reproductive technique. Monozygotic twinning was defined by ultrasound or Weinberg criteria. A meta-analysis of the proportion of monozygotic twins was performed using both fixed and random effects models. The search revealed 37 publications reporting on the incidence of monozygotic twins after assisted reproductive techniques. Twenty-seven studies met the inclusion criteria and were included in the meta-analysis. The summary incidence of monozygotic twins after assisted conception was 0.9% (0.8-0.9%). The incidence of monozygotic twins in natural conception is 0.4%. Blastocyst transfer and intracytoplasmic sperm injection are associated with 4.25 and 2.25 times higher risk of monozygotic twins. The risk of monozygotic twins in assisted conception is 2.25 times higher than the natural conceptions. Larger studies reporting on monozygotic twinning following single-embryo transfer or after post-natal confirmation of zygosity with DNA analysis are warranted before definitive conclusions can be drawn and guidelines produced. In order to provide adequate pre-conceptional counselling, it is important to monitor the incidence of monozygotic twins in both natural and assisted conceptions. We suggest building a national multiple pregnancy database based on accurate diagnosis of zygosity.

Comparing Sensory Experience in Bitter Taste Perception of Phenylthiocarbamide within and between Human Twins and Singletons: Intrapair Differences in Thresholds and Genetic Variance Estimates

Quantitative genetic studies revealed that not all of the phenotypic variance in PTC taste perception is heritable. To study quantitative variations in PTC tasting ability in twins and to estimate heritability of PTC taste perception on the taste of twin data on males and females sexes separately. The data for PTC taste sensitivity following the classic method of Harris & Kalmus (1949) were collected on a sample of 141 twin pairs (66 MZ and 75 DZ) and 275 singletons (128 males and 147 females) from Chandigarh, India. Genetic analyses were performed following Christian (1979), Donner (1986) and Sham (1998). Frequency of non-tasters was similar in twins (33 %) and singletons (32 %), but significant sex differences were observed. No differences were found between zygosities for mean thresholds. Similarly, no evidence of variance heterogeneity and environmental covariance was seen between zygosities. Since no basic assumption of the twin method was found violated, within-pair estimates of genetic variance would be unbiased. These estimates were highly significant in both males and females. However, dominance and additive components of genetic variance were found to differ between sexes. PTC thresholds do not seem to be significantly affected by environmental factors as no variance inequality was observed between twin zygosities. Intensity of bitterness (scalar dimensions) of PTC is a separate trait having no commonality with the genetic basis of recognition threshold for PTC tasting ability. The receptors recognizing bitter taste are different from the receptors determining intensity of taste. The absolute difference between co-twins in PTC thresholds can be used as a simple tool in the twin zygosity diagnosis. The results show that none of the MZ co-twins had manifested difference of more than 3 in their PTC threshold.

Determination of Zygosity by Questionnaire and Physical Features Comparison in Chinese Adult Twins

AbstractThis study reports on the determination of zygosity in Chinese adult twins by simple questionnaire and physical features comparison. The subjects were 511 twin pairs from two cities and their town areas, consisting of 371 monozygotic (MZ) and 140 same-sex dizygotic (DZ) pairs, identified by ABO blood group and multiplex polymerase chain reaction of several polymorphic short tandem repeat markers. The twins themselves responded to 8 questionnaire items, 4 items on twin similarity, and 4 items on the frequency of mistaking one twin for another by parents, relatives, teachers and strangers when they were 6 to 13 years old. Research assistants responded to 20 items regarding twins' physical features at the moment of interview. A parsimonious model established using stepwise logistic regression analysis of the 28 items showed that the total accuracy of zygosity diagnosis was 90.1%. The accuracy was 89.2% when using only the items dealing with the confusion of twins and 85.4% using only similarity. In the questionnaire, ‘facial appearance’, ‘mistaken by teachers’ and ‘mistaken by strangers’ had stronger discriminating power between MZ and DZ twins. Two physical features — ‘eyelid’ and ‘middigital hair’ — were informative to some extent. There was no statistically significant sex and area difference in the validity of such questionnaire and physical features comparison-based classification. In conclusion, questionnaire-based zygosity assessment in this Chinese adult twin sample could still be regarded as a valid and valuable classification method. Physical features comparison, however, could only provide limited information for zygosity determination.

Determination of Zygosity by Questionnaire and Physical Features Comparison in Chinese Adult Twins

This study reports on the determination of zygosity in Chinese adult twins by simple questionnaire and physical features comparison. The subjects were 511 twin pairs from two cities and their town areas, consisting of 371 monozygotic (MZ) and 140 same-sex dizygotic (DZ) pairs, identified by ABO blood group and multiplex polymerase chain reaction of several polymorphic short tandem repeat markers. The twins themselves responded to 8 questionnaire items, 4 items on twin similarity, and 4 items on the frequency of mistaking one twin for another by parents, relatives, teachers and strangers when they were 6 to 13 years old. Research assistants responded to 20 items regarding twins' physical features at the moment of interview. A parsimonious model established using stepwise logistic regression analysis of the 28 items showed that the total accuracy of zygosity diagnosis was 90.1%. The accuracy was 89.2% when using only the items dealing with the confusion of twins and 85.4% using only similarity. In the questionnaire, 'facial appearance', 'mistaken by teachers' and 'mistaken by strangers' had stronger discriminating power between MZ and DZ twins. Two physical features--'eyelid' and 'middigital hair'--were informative to some extent. There was no statistically significant sex and area difference in the validity of such questionnaire and physical features comparison-based classification. In conclusion, questionnaire-based zygosity assessment in this Chinese adult twin sample could still be regarded as a valid and valuable classification method. Physical features comparison, however, could only provide limited information for zygosity determination.

Zygosity Diagnosis in Young Twins by Questionnaire for Twins' Mothers and Twins' Self-reports

The present study deals with the determination of zygosity in twins of childhood age by simple questionnaire. The subjects were 224 twin pairs and their mothers, consisting of 159 monozygotic and 65 same-sex dizygotic pairs, identified by genetic markers including DNA samples. Mothers of twins responded to 19 questionnaire items dealing with twin similarity in 16 items about physical features and 3 items about the degree of similarity and frequency of being mistaken (confusion of identity) when twins were about 1 year of age. The twins themselves responded to three questionnaire items dealing with only confusion of identity items. The results of stepwise logistic regression analysis were as follows: the total accuracy of the mothers' questionnaire was 91.5% when using only the items dealing with confusion of identity. This accuracy was slightly lower than that obtained by twins' self-reports dealing with nearly the same question items of confusion of identity, answered by both twins separately with 93.3% accuracy. The total accuracy of mothers' questionnaire responses rose to 95.1% when we used all 19 items. In addition to ‘the frequency of being mistaken‘, two physical features, namely ‘shape of fingers’ and ‘shape of eyebrow’, were very informative. In conclusion, twin zygosity can be estimated by the use of the mothers' simple questionnaire with sufficient accuracy even in very young twins about 1 year of age.

Failure to obtain reliable retrospective determination of chorion type using parent information: confirmation with French data.

Following the Derom et al. paper recently published in Twin Research (2003, 6, 19-21), we investigated our French sample, studying the retrospective validity of information given by parents to determine the chorion type of their monozygotic twins. The sample included 44 MZ pairs of known chorion type. Chorion type was established by a pathologist specialising in the study of placenta and zygosity diagnosis using molecular genetic techniques. Results show that of the parents who had information, there was a high level of uncertainty of the chorion type. It therefore seems impossible to obtain valid retrospective determination from parents and the only solution is to have accurate information in the birth records.

Zygosity diagnosis in young twins by questionnaire for twins' mothers and twins' self-reports.

The present study deals with the determination of zygosity in twins of childhood age by simple questionnaire. The subjects were 224 twin pairs and their mothers, consisting of 159 monozygotic and 65 same-sex dizygotic pairs, identified by genetic markers including DNA samples. Mothers of twins responded to 19 questionnaire items dealing with twin similarity in 16 items about physical features and 3 items about the degree of similarity and frequency of being mistaken (confusion of identity) when twins were about 1 year of age. The twins themselves responded to three questionnaire items dealing with only confusion of identity items. The results of stepwise logistic regression analysis were as follows: the total accuracy of the mothers' questionnaire was 91.5% when using only the items dealing with confusion of identity. This accuracy was slightly lower than that obtained by twins' self-reports dealing with nearly the same question items of confusion of identity, answered by both twins separately with 93.3% accuracy. The total accuracy of mothers' questionnaire responses rose to 95.1% when we used all 19 items. In addition to "the frequency of being mistaken", two physical features, namely "shape of fingers" and "shape of eyebrow", were very informative. In conclusion, twin zygosity can be estimated by the use of the mothers' simple questionnaire with sufficient accuracy even in very young twins about 1 year of age.

Failure to Obtain Reliable Retrospective Determination of Chorion Type Using Parent Information: Confirmation with French Data

Following the Derom et al. paper recently published in Twin Research (2003, 6, 19–21), we investigated our French sample, studying the retrospective validity of information given by parents to determine the chorion type of their monozygotic twins. The sample included 44 MZ pairs of known chorion type. Chorion type was established by a pathologist specialising in the study of placenta and zygosity diagnosis using molecular genetic techniques. Results show that of the parents who had information, there was a high level of uncertainty of the chorion type. It therefore seems impossible to obtain valid retrospective determination from parents and the only solution is to have accurate information in the birth records.

Age- and Sex-differences in the Validity of Questionnaire-based Zygosity in Twins

Questionnaire-based zygosity assessment in twins has generally been found to be valid. In this report we evaluate sex- and age-differences in the validity of such questionnairebased classification when using the four questions that have been the basis of zygosity assessment in The Danish Twin Registry for half a century. Three hundred and forty-two male and 531 female twin pairs were zygosity diagnosed using genetic markers and the results compared with the original questionnaire based classification. We found significant differences in the accuracy of questionnaire based zygosity diagnosis when stratifying the data for sex as well as age: males and monozygotic having the highest misclassification. However, even in the group with the highest misclassification rate the frequency was less than 8%. The overall misclassification rate was only 4%, with a clear tendency towards a higher proportion of misclassified monozygotic than dizygotic twins. The results demonstrate that questionnaire based zygosity diagnosis can still be regarded as a valid and valuable classification method for most purposes.

Age- and sex-differences in the validity of questionnaire-based zygosity in twins.

Questionnaire-based zygosity assessment in twins has generally been found to be valid. In this report we evaluate sex- and age-differences in the validity of such questionnaire-based classification when using the four questions that have been the basis of zygosity assessment in The Danish Twin Registry for half a century. Three hundred and forty-two male and 531 female twin pairs were zygosity diagnosed using genetic markers and the results compared with the original questionnaire based classification. We found significant differences in the accuracy of questionnaire based zygosity diagnosis when stratifying the data for sex as well as age: males and monozygotic having the highest misclassification. However, even in the group with the highest misclassification rate the frequency was less than 8%. The overall misclassification rate was only 4%, with a clear tendency towards a higher proportion of misclassified monozygotic than dizygotic twins. The results demonstrate that questionnaire based zygosity diagnosis can still be regarded as a valid and valuable classification method for most purposes.

Rapid prenatal diagnosis of aneuploidies and zygosity in multiple pregnancies by amniocentesis with single insertion of the needle and quantitative fluorescent PCR.

To investigate amniotic fluid (AF) samples retrieved in multiple pregnancies by single insertion of the needle, for rapid assessment of chromosome copy number, zygosity, and cross-contamination between fetuses, using Quantitative Fluorescent Polymerase Chain Reaction (QF-PCR) amplification of highly polymorphic microsatellite markers. Fifty-two multiple pregnancies were selected (47 twins, 5 triplets) and 108 samples of amniotic fluid were sampled between 12 to 20 weeks of gestation (mean 15.5) using the single-needle technique. Aneuploidy screening by QF-PCR amplification of short tandem repeats (STRs) on chromosomes X, Y, 21, 13, and 18 was carried out within 24 h of collection. Owing to the sampling procedure, the eventual presence of contamination between fetuses was also evaluated in every case. Normal and aneuploid fetuses were readily identified by QF-PCR. Fetal reduction was made available, for trisomic fetuses, without further waiting for completion of fetal karyotyping. In twin gestations, the ultrasound examination of chorionicity was always in agreement with the molecular assessment of zygosity. Contamination between fetuses due to the sampling procedure with a single puncture was never observed. Rapid prenatal diagnosis of aneuploidies by QF-PCR is a sensitive, efficient, and reliable assay. When applied in multiple pregnancies, it has the added value of allowing the assessment of zygosity in all cases, independently of chorionicity and fetal sex.

Zygosity Diagnosis in the Absence of Genotypic Data: An Approach Using Latent Class Analysis

For zygosity diagnosis in the absence of genotypic data, or in the recruitment phase of a twin study where only single twins from same-sex pairs are being screened, or to provide a test for sample duplication leading to the false identification of a dizygotic pair as monozygotic, the appropriate analysis of respondents' answers to questions about zygosity is critical. Using data from a young adult Australian twin cohort (N = 2094 complete pairs and 519 singleton twins from same-sex pairs with complete responses to all zygosity items), we show that application of latent class analysis (LCA), fitting a 2-class model, yields results that show good concordance with traditional methods of zygosity diagnosis, but with certain important advantages. These include the ability, in many cases, to assign zygosity with specified probability on the basis of responses of a single informant (advantageous when one zygosity type is being oversampled); and the ability to quantify the probability of misassignment of zygosity, allowing prioritization of cases for genotyping as well as identification of cases of probable laboratory error. Out of 242 twins (from 121 like-sex pairs) where genotypic data were available for zygosity confirmation, only a single case was identified of incorrect zygosity assignment by the latent class algorithm. Zygosity assignment for that single case was identified by the LCA as uncertain (probability of being a monozygotic twin only 76%), and the co-twin's responses clearly identified the pair as dizygotic (probability of being dizygotic 100%). In the absence of genotypic data, or as a safeguard against sample duplication, application of LCA for zygosity assignment or confirmation is strongly recommended.

Zygosity diagnosis in the absence of genotypic data: an approach using latent class analysis.

For zygosity diagnosis in the absence of genotypic data, or in the recruitment phase of a twin study where only single twins from same-sex pairs are being screened, or to provide a test for sample duplication leading to the false identification of a dizygotic pair as monozygotic, the appropriate analysis of respondents' answers to questions about zygosity is critical. Using data from a young adult Australian twin cohort (N = 2094 complete pairs and 519 singleton twins from same-sex pairs with complete responses to all zygosity items), we show that application of latent class analysis (LCA), fitting a 2-class model, yields results that show good concordance with traditional methods of zygosity diagnosis, but with certain important advantages. These include the ability, in many cases, to assign zygosity with specified probability on the basis of responses of a single informant (advantageous when one zygosity type is being oversampled); and the ability to quantify the probability of misassignment of zygosity, allowing prioritization of cases for genotyping as well as identification of cases of probable laboratory error. Out of 242 twins (from 121 like-sex pairs) where genotypic data were available for zygosity confirmation, only a single case was identified of incorrect zygosity assignment by the latent class algorithm. Zygosity assignment for that single case was identified by the LCA as uncertain (probability of being a monozygotic twin only 76%), and the co-twin's responses clearly identified the pair as dizygotic (probability of being dizygotic 100%). In the absence of genotypic data, or as a safeguard against sample duplication, application of LCA for zygosity assignment or confirmation is strongly recommended.