The WHO Meeting of Investigators on Methodology of Twin Studies was held in Geneva from 24 October to 1 November 1965. The meeting was opened by Dr L. Verhoestraete, Director, Division of Health Protection and Promotion, who welcomed the participants. He said that this meeting of geneticists, epidemiologists and clinicians had been called to review the possible contribution of twin studies, a recognized tool in genetic research, to the broader field of epidemiological investigations, where so far they had found comparatively little use.

Professor D. D. Reid was elected Chairman; Professor L. Gedda, Vice-Chairman and Dr R. Cederlöf, Rapporteur.

Studies of general population characteristics include vital statistics such as mortality and morbidity data which are obtained on a routine basis. These data constitute one of the best sources of raw material for epidemiological studies and provide easily accessible information for basic research and for programme planning in control of disease. Very often they raise questions concerning the epidemiology of chronic disease, which may help delineate the relevant factors and provide guiding hypotheses for further research in this field. A deeper understanding of disease may also be arrived at through the study of individual characteristics in cross-sectional, retrospective and prospective studies, and through experimental investigations. A cross-sectional study can be used not only in determining prevalence of disease in a population, but it can also serve as the base-point for a longitudinal study of chronic diseases.

The Twin Clinical Method (Gedda, 1963) has been applied to the study of dental pathology. Four cases of MZ female twin pairs have been studied, concordantly affected by dental agenesis (case I), mirror-imaging dental agenesis (case II), diastema (case III), and dental caries and enamel dystrophy (case IV). The test has also been applied, and for the first time, to a set of MZ female triplets, concordantly affected by dental caries and malposition.

In all the above cases, the test has permitted to reject hypothesis of a concordance being due to chance, while postulating the existence of a causality.

The latter is ascribed to the common heredity characterizing MZ cotwins.

Discordant schizophrenic monozygotic twin pairs were examined in order to clarify the view interpreting schizoid disorders as attenuated or incomplete expressions of schizophrenic heredity. The aggregate observations indicate that typical schizophrenia can be paired with all types and degrees of schizophrenic or schizoid illness, including clinical normality. These intra-pair combinations seem to occur more or less at random. The reviewed schizoid states showing evidence of schizophrenic psychopathology represent one of the developmental phases of schizophrenic process. Yet the psychopathologic connection with schizophrenia is not evident in all deviations and non-deviant personality patterns. Thus the seeming continuum of schizophnic psychosis — schizoid disorder — neurotic character — normal personality may consist of two discrete series: one with underlying schizophrenic psychopathology, and the other independent of the psychotic process. This division might reflect a threshold effect, dependent upon a switch mechanism.

When the result of the reaction to an antiserum is known for only one twin, it is nevertheless possible to calculate the probability that the twins be monozygotic, making allowance for that partial result.

The probability of monozygosity is calculated in the hypothesis of complete concordance.

This too high probability is then combined with the probability of concordance between sibs for the trait considered.

A new case has been presented of MZ twin girls, both showing albinism of the incomplete generalized type. Such cases are quite rare. Two elder siblings and the parents presented various degrees of diascleral translucency of the irides.

A sibship is reported, including four female children, two of which MZ twins, all affected by retinoblastoma (the lesion being bilateral in three of them, and unilateral in a fourth one). The literature on the heredity of the condition is considered; the latter does not appear to be inherited as a regular dominant. Finally its genetic interpretation in the observed sibship is considered.

37 DZ and 12 MZ twin pairs underwent a clinical and ECG heart examination. As to their zygosity, it is suggested that a concurrent similar clinical finding, very similar ECG and a mean intrapair difference of corresponding unipolar chest lead waves d ≤ 1.5 definitely support monozygosity.

Paired analysis of the ECG curves shows that the difference between MZ and DZ twins is best characterized by Tv1-6 and, to a certain extent, by Rv1-6. The same applies — in partial agreement with Mathers et al. — for PQ and QTc intervals.

Considering the intrapair differences in both MZ and DZ twins, it was found that they tend to increase in the course of one cardiac cycle, thus indicating a gradual prevalence of peristatic factors.

A case of probably MZ twin girls is reported, one of which presenting a serious gonadal hypoplasia, macroschelia and a number of slight malformations, while the other appeared perfectly normal. Both twins presented positive sex-chromatine and a normal karyotype, i. e. 44 A + XX.

After a review of twin cases of the « rudimentary gonad syndrome » — which may differ as to its origin, according to the various cases — the present case, the anomalies exhibited by one of the twins are probably the result of the action of a number of noxae, in the course of embryological or fetal life.

Two DZ monoplacental twins are reported, one affected by mongolism and the other normal. Chromosome examination in the former indicated trisomy 21, while the latter presented a mosaicism consisting, at the age of 13 days, in 70% normal and 30% 21-trisomic cells; at the age of 5 months, in 95% normal and 5% trisomic cells; while, at the age of one year, all the cells appeared to be normal.

The possibility of mosaicism, and, according to clinical and hematological findings, that of feto-fetal blood transfusion are excluded.

The hypothesis is suggested of a progressive loss of the reproductive capacity of « chimaera » of abnormal lymphocytes.