Definitive methods of zygosity determination in twins: relevance to problems in the biology of twinning

Comparing Facial 3D Analysis With DNA Testing to Determine Zygosities of Twins

The aim of this study was to compare facial 3D analysis to DNA testing in twin zygosity determinations. Facial 3D images of 106 pairs of young adult Lithuanian twins were taken with a stereophotogrammetric device (3dMD, Atlanta, Georgia) and zygosity was determined according to similarity of facial form. Statistical pattern recognition methodology was used for classification. The results showed that in 75% to 90% of the cases, zygosity determinations were similar to DNA-based results. There were 81 different classification scenarios, including 3 groups, 3 features, 3 different scaling methods, and 3 threshold levels. It appeared that coincidence with 0.5 mm tolerance is the most suitable feature for classification. Also, leaving out scaling improves results in most cases. Scaling was expected to equalize the magnitude of differences and therefore lead to better recognition performance. Still, better classification features and a more effective scaling method or classification in different facial areas could further improve the results. In most of the cases, male pair zygosity recognition was at a higher level compared with females. Erroneously classified twin pairs appear to be obvious outliers in the sample. In particular, faces of young dizygotic (DZ) twins may be so similar that it is very hard to define a feature that would help classify the pair as DZ. Correspondingly, monozygotic (MZ) twins may have faces with quite different shapes. Such anomalous twin pairs are interesting exceptions, but they form a considerable portion in both zygosity groups.

Twin zygosity diagnosis by mailed questionnaire below age twelve months. Le Groupe Romulus

Parents of a sample of 76 same sexed pairs of twins aged 3 to 9 months completed a mailed similarity questionnaire. It included the Bonnelykke et al.'s questionnaire and a four anthropological variable scale. To improve each of these two methods, three other combined methods were carried out and results were compared with the biological zygosity diagnosis. The Bonnelykke et al.'s classification combined with anthropological scale (method 4) gave only 1.2% misclassified in the whole sample. It is concluded that zygosity diagnosis using this type of procedure to distinguish MZ and DZ pairs would be important not only for epidemiological study but also for pediatricians and parents.

Prenatal exposure to disulfiram implicated in the cause of malformations in discordant monozygotic twins

Female monozygotic (MZ) twins were discordant for congenital structural anomalies: Twin A had a reduction defect of the right forearm; Twin B had a cleft palate. Both infants were small for gestational age. Specific prenatal exposures were identified at different times in the first trimester of pregnancy: crack cocaine, marijuana, disulfiram, heavy ethanol exposure, and cigarettes. The mother's hospitalization in a drug abuse program and incarceration allowed for identification of exposure timing. The cleft palate could have been related to either disulfiram or alcohol exposure; the limb abnormality most likely corresponded to the timing of disulfiram exposure. Discordance of anomalies in these twins may reflect differences in developmental timing, differences in susceptibility to one or more teratogens, or random events occurring within very complex developmental programs, with the thresholds for malformation affected by one or multiple teratogenic compounds.

Syndromes in twins

There is a higher incidence of perinatal mortality and morbidity in twins than in singleton pregnancies. The main reason for this increase is the higher incidence of preterm delivery in twins. There are special conditions unique to twins that also contribute to this increased perinatal morbidity and mortality. These conditions include monoamniotic twins, congenital anomalies unique to twins, ie, conjoined twins and acardia, intrauterine fetal demise, and twin-to-twin transfusion syndrome. These conditions are the subject of this review. The prenatal determination of chorionicity is discussed first, because this assessment is key to the diagnosis and management of many of these conditions.

Use of single- and multi-locus and polymerase chain reaction systems for zygosity determination--clinical application in twins with clefts of the lip and palate

Precision of zygosity determination in twins can be improved by the use of modern methods of DNA analysis. The clinical application of 4 single- (SLS) and 2 multi-locus (MLS), and 6 PCR (polymerase chain reaction) systems for zygosity determination in 12 twin pairs with oral clefts was compared with regard to the quality and quantity of sample material required and the probability of error in monozygosity determination. PCR systems proved to be superior to SLS or MLS, as DNA sampling is much more convenient, while its level of accuracy still fulfils clinical requirements. For this reason, PCR systems should be considered a basic method in modern clinical twin research.

Monozygotic twins discordant for gastroschisis: case report and review of the literature of twins and familial occurrence of gastroschisis

We describe a pair of monozygotic (MZ) female twins discordant for gastrochisis. To our knowledge, this is the first such case reported. The zygosity was verified by DNA analysis using highly polymorphic microsatellites. There was no family history of gastroschisis. During pregnancy there was no suspicion of any exposure responsible for the malformation. The number of twin cases described so far does not allow any conclusion as to hereditary factors in the cause of gastroschisis, but the number of families reported with familial gastroschisis suggests that the recurrence risk is higher than previously thought.

Clefts of the lip and palate in twins: use of DNA fingerprinting for zygosity determination

The study of twins is a well-established method for evaluating the relative roles of heredity and environmental factors in the etiology of diseases. Conclusions depend on zygosity determination and on the classification of minor forms of diseases. This paper reports on ten (5 mono- and 5 dizygotic) out of thirteen twin pairs among 1039 patients with cleft lip and palate (n = 677) or cleft palate (n = 362). Zygosity was determined using "DNA fingerprinting" on blood samples in all 10 pairs and on cleft-associated tissue in one pair. Including minor forms of clefting, two of five pairs of monozygotic and two of five pairs of dizygotic twins of the same sex showed concordance. "DNA fingerprinting" should be established as a definitive method for zygosity determination, and the calculation of concordance rates should always include minor forms of diseases.

The current status of Weinberg's differential rule

A reanalysis of the data presented in a recent large twin study suggests that opposite-sexed (OS) pairs may be not as exhaustively tested as same-sexed (SS) pairs on genetic markers. This is contrary to an assumption I made in estimating that there are about 8 SS dizygotic pairs to every 7 OS pairs (thus impugning Weinberg's differential rule). If this assumption is false also in regard to the samples I discussed, then that estimate is unsound and Weinberg's rule is unscathed by empirical data. However, regardless of such considerations, there are strong theoretical reasons for questioning the status of Weinberg's rule. It is based on two assumptions, namely that p (the probability that a dizygotic twin zygote is male) is equal and independent for all dizygotic twin zygotes. Data are adduced here to suggest that both assumptions are false. The upshot is that, at present, without testing, we cannot know, of any given population, whether the rule holds or not. Otherwise, though Weinberg's rule may be useful (like Hellin's law) as a rule-of-thumb, it cannot be assumed as a basis for serious scientific argument.