It is the aim of twin studies to obtain results which are not only valid for twins, but apply to the whole population. Therefore the following questions have to be answered first: do twins differ from non-twins in the trait under study? Do different nongenetic factors act upon MZ and DZ twins which alter the probability of manifestation of a trait, even before birth? There are important differences in embryogenesis and placental blood flow in mono- and dichorionic twins; this can influence the normal fetal development. Therefore the value of twin studies alone in analysing the genetic component in the etiology of congenital malformations is rather ambiguous. Twin studies beyond the newborn period can be classified into four approaches: (1) Case reports; (2) Accumulated case reports; (3) Limited representative sample; (4) Unlimited representative sample. The most frequent systematic method in medical genetics is the establishment of all twins in a defined population of probands (3). Another successful application in the last few years has been in pharmacogenetics. Although no simple mode of inheritance could be discovered, it was possible to estimate the genetic component within the interindividual variability of the metabolism of certain drugs (nortriptyline, antipyrine, phenylbutazone, ethanol). Now, additional non-twin research is needed to work out single factors within the observed polygenic systems.

Records of two hospitals and of an association of mothers of twins were analysed for the years 1924-1980, finding a total of 656,109 and 1185 twin births, respectively. Twinning rates were higher in summer or fall, in general, but these seasonal variations occasionally changed within a short period of years, as in 1973-75 in the Association group. The twinning rate was also affected by the month of mother's birth. It was low among the mothers born around May-July in maternities up to 1960, and these mothers did not show seasonal variation of twin births. The unlike-sexed twinning rate of the mothers born in May-July was also lower than that of the other mothers, and constantly low even at higher maternal ages. A possible explanation for these phenomena is discussed.

In his recent book Dr. Roy J. Schepherd states that:

“Further information of genetic effects is likely to be derived from comparisons of identical (monozygotic) and fraternal (dizygotic) twins. Gedda (1961) has already shown that identical twins are more likely to have a similar interest in sports than are fraternal twins. It would be interesting to carry out comparisons of maximum oxygen intake and other objective measurements of endurance fitness on the two classes of twins, but this crucial experiment has yet to be completed”.

We are very glad to be able to announce here that Dr. Schepherd's wish is being fulfilled within the scope of a wider research project undertaken jointly by the Gregor Mendel Institute for Medical Genetics and Twin Research and the Italian Olympic Committee's Institute of Sports Medicine under the auspices of the Italian Sports Medicine Federation.

The studies we have undertaken aim to assess the respective roles of genetic and environmental factors as related to selection and training in sports and physical education.

The twin method affords, through a comparison of MZ and DZ within-pair concordances, an assessment of the degree of genetic conditioning in any individual trait.

It is quite obvious that the possible influence of training on any given trait is inversely proportional to the degree of genetic conditioning of the same trait.

If a given trait, required for a certain type of activity, is completely or almost completely conditioned by the genotype, we should seek those who have inherited it from their parents without wasting time and effort in useless training sessions.

As we can see, the scientific information we are seeking has great practical importance in the selection and training of athletes in every single field of sports and physical education.

Perhaps half of all patients with multiple abnormalities have known, recognized syndromes. The other half represent unknown entities that need to be further delineated. The significance of syndrome delineation cannot be overestimated. As an unknown syndrome becomes delineated, its phenotypic spectrum, its natural history, and its risk of recurrence become known, allowing for better patient care and family counseling.

The process of syndrome delineation is discussed in terms of unknown genesis syndromes of the provisionally-unique and recurrent-pattern types, and known-genesis syndromes of the pedigree, chromosomal biochemical-defect, and environmentally-induced types. Several special syndrome categories are defined, including the chance syndrome, variant additive syndrome, association syndrome, exceptional chromosomal syndrome, and exceptional monogenic syndrome. Finally, a population definition of a syndrome is developed.

The object of this study was to develop a theory of the relationship between a mother and her unborn twins using a qualitative grounded theory methodology. Ten women participated in interviews during the last trimester of a twin pregnancy and again during the early postpartum period. Data provided by these women was analysed using the constant comparative method. Two additional informants were sampled in order to formulate and verify the tentative theory. For women in this study, the findings indicate that mothers form a relationship with their unborn twins within the wider context of accommodating the twin pregnancy in their lives. A woman accomplishes this process by immediately seeking information about twin pregnancy, by making room, in all senses, for two babies instead of one in her life, and by appraising her own pregnancy risk. A mother engages in self-protective behaviors and in behaviors designed to protect the health of her unborn twins as a method of coping with the perceived risk of the twin pregnancy and with the twin pregnancy itself. As the twin pregnancy progresses, women move toward accepting the idea of being pregnant with twins. For women pregnant with twins, the birth of two infants confirms the reality of the pregnancy.

Rates of congenital anomalies of the cardiovascular system were compared between twins and singletons in a population-based study in Northern Ireland during the period 1974-1978. Multiple sources of ascertainment were used. As in previous studies, the rate of anomalies of the cardiovascular system in twins (91.0/10,000) was higher than the rate in singletons (66.4/10,000). The excess was confined to twins from pairs of like sex and, in the main, anomalies of the circulatory system other than of the heart itself were involved. Problems in the interpretation of this excess are discussed. No twins were concordant for congenital cardiovascular anomalies of any type.

Maternal plasma IGF-I and hPL concentrations were examined in 10 singleton pregnancies and in 11 twin gestations near term. hPL concentrations were higher in the twin pregnancies (14.4 ± 2.4 μg/1 vs 6.9 ± 0.9 μg/1, P < 0.02). In contrast, plasma IGF-I concentrations were similar in the singleton and twin pregnancies (533 ± 45 μg/1 vs 572 ± 60 μg/1, respectively). IGF-I concentrations failed to correlate with hPL concentrations in either group separately or when all subjects were considered together. These data do not support the hypothesis that maternal IGF-I secretion is an hPL-dependent process.

In a large population-based study in Northern Ireland during the period 1974-1979, the rate of anencephalus in twins (9.1/10,000) was found to be less than that in singletons (24.3/10,000). This finding is in contrast with most other studies and the possibility of underascertainment of twin cases is considered, but it is concluded that chance is the likeliest explanation. The rate of spina bifida in twins (36.4/10,000) was similar to that in singletons (31.9/10,000). All of the twins with anencephalus were female and from pairs of like sex. Rates of spina bifida in twins from pairs of the two sex types were similar but, unusually, there was a male preponderance. As in previous studies, the great majority of twins with NTDs had unaffected cotwins.

Three pairs of monozygotic twins were ascertained during a general survey of language disabilities conducted among schoolchildren of Porto Alegre, Brazil. Two of them were concordant for dysgraphia, dysorthographia, dyslexia, and speech defects, while the other was concordant for dysorthographia and dyslexia, but discordant for dysgraphia. Two of the mothers and two sibs also presented language problems, but of a type that was not completely similar to those of the twins. Concomitant neurological and psychological studies, as well as the family histories, helped to understand the similarities and dissimilarities observed.

The present paper reviews the research lines which have been explored to evaluate to what extent genetic factors are intervening on the mechanism of resistance and susceptibility to leprosy.

It presents a critical discussion of the investigations on the familial association of leprosy, familial association of leprosy types, intrafamilial contagion of leprosy, concordance of leprosy in twinpairs, racial differences on leprosy prevalence and lepromatous rate, pedigree studies, association of leprosy to genetic markers, Australia antigen, and dermatoglyphic patterns. Space was also allotted to review family and twin-pair studies on the Mitsuda reaction, as well as to the investigation on the in vitro behaviour of blood macrophages against killed M. leprae.

Some areas in which further research on leprosy and genetics may be considered as prioritary are outlined with some detail.

After elaborating the data supplied by 351 questionnaires of a survey addressed to 1,241 twin-pairs, the Author shows differences in the behavior of MZ and DZ twin-pairs as to the sports phenomenon. By a first criterium of analysis, relative to sports practice in general, intra-pair dissimilarity is found to correspond to 6% in the case of MZ and to 85% in the case of DZ twin pairs. A second criterium of analysis concerned the kind of sport practiced by pairs concordant as to sports practice in general: the observed concordance as to the kind of sport practiced was more frequent in MZ then in DZ twin-pairs. By a third criterium concerning individual role and performance, the Author has been able to demonstrate that the distribution of DZ pairs, as to concordance, follows opposite patterns than that of MZ pairs. The Author concludes that sports activity, besides exogenous influences, (social environment, training, economic level etc.) is certainly also conditioned by genotypic, hereditary factors.

An epidemiological study of all primigravid twin pregnancies delivered in Aberdeen between 1950 and 1969 was performed to determine the pattern of arterial blood pressure changes. There is a greater fall from non-pregnant levels in diastolic blood pressure by mid pregnancy and a greater rise of diastolic pressure by delivery. These changes are independent of age, body size and rate of weight gain during pregnancy. The expected increased incidence of proteinuric pre-eclampsia is also independent of rate of weight gain when defined for twin pregnancies.

1. The probability that a mother who has already given birth to twins will repeat the twin birth has been shown to be 3.63 ± 0.24%.

a) The increase in the frequency of binovular twin births of 4,55 ± 0,90% is statistically significant.

b) The increase in the frequency of monovular twin births of 1.43 ± 0,90% is not statistically sure in this data because of the relatively high standard error.

2. Althrough it cannot by proved statistically, there is some indication that, amongst the later births to women who have initially given birth to binovular twins, there is an increase in the frequency of monovular twins, and vice-versa.

3. Hormonal disturbances, such as a slow secretion of folliculin which would prevent ovulation, may be the cause of binovular twinning. Mon ovular twinning is probably due to an inherent tendency of the egg to divide. It is pos sible that if this tendency manifests itself before the reduction division and two separate eggs are formed binovular twinning may result. With this hypothesis is mind, an attempt has been made to discover some correlation between monovular and binovular twinning.

Traditional twin studies have resulted in higher concordance rates for premature coronary heart disease (CHD) in MZ than in DZ twin pairs. This is in agreement with strong evidence from several other studies, that genetic factors are of importance in the etiology of early onset CHD. Also, in a study of 291 Norwegian twin pairs the concordance rate for hypertension wa 0.36 in MZ and 0.08 in DZ pairs. Relationship between diseases and traditional gene markers have been extensively studied and several associations have been uncovered for CHD. Our group has developed a method to examine a possible permissive or restrictive effect of single genes on the degree of variation that environmental and/or life style factors can cause in a given parameter. This method for studying gene-environment interaction is based on the fact that MZ twins are identical with respect to genes, so that any difference between the two members of an MZ pair must necessarily be caused by environmental or life style factors. The possibility that a given gene influences the degree of variability in a parameter such as cholesterol is examined by comparing the within-pair difference in cholesterol level between MZ pairs possessing, and MZ pairs lacking the gene in question, and results of such studies will be presented. New possibilities to study restriction fragment length polymorphisms (RFLPs) at apolipoprotein loci have added a new dimension to research on genetics of CHD and hyperlipidemias. Association between apolipoprotein B, cholesterol and fasting triglyceride levels on one hand and DNA variation at the apolipoprotein B locus on the other has been found. We have studied RFLPs at apolipoprotein loci in twin families. The results are presented.

The frequency of twins and triplets was significantly higher than expected among 56 patients with Klinefelter's syndrome (7.1%), compared with 1.58% in the population (P<0.001), and the frequency of multiple births in the 56 sibships (4.08%) was also significantly higher than expected (P<0.01).

One of 15 patients with the XYY syndrome was a twin, and there were 2 twin births among the 53 births in the 15 sibships.

The experiences and needs of mothers who lose a newborn twin were explored by sending semistructured questionnaires to 14 bereaved mothers. All mothers continued to think of the surviving child as a twin. Six had feelings of resentment towards the survivor. All felt their loss had been underestimated. Support could be improved by acknowledging the mother's grief and encouraging her to talk about the dead baby. Zygosity should be determined and reminders, such as photographs (of the babies together) and ultrasound scans, provided. All parents should be offered counselling and the opportunity to meet similarly bereaved parents.

In 1941, I published a psychiatric study of 69 same-sexed pairs of twins. The index cases were derived from 8596 consecutive admissions to three mental hospitals and one psychiatric unit of a general hospital in South Sweden, every patient being checked for twin birth in the official birth registers. There was no pair with more than one index case. Upon examination, 21 pairs were considered more or less certainly MZ, and 48 DZ. The degree of certainty of the zygosity diagnosis was expressed by a special formula (Essen-Möller, 1941b).

Of the 21 index cases, 7 (N. 1-7 of the monograph) were at that time judged schizophrenic. The later course revaled that one more index case (N. 12) was a schizophrenic and should have been included into this diagnostic group, which I shall have to concentrate upon in this brief presentation.

Out of the 8 cotwins of the schizophrenic index cases, 5 had presented symptoms of mental disorder up to the time of my investigation, which took place almost thirty years ago. This corresponds so far to a concordance rate of 62%. However, all of the cotwin disorders were relatively mild and transient in nature and consisted mainly of depressive or anxiety states. Even if some of the disordered cotwins had spent some time in a nursing home or in a psychiatric ward of a general hospital, none of them had been admitted to a mental hospital. And, although several of the clinical pictures contained some single trait suggestive of schizophrenia — such as ideas of reference (N. 2 and 7) or hallucinations (N. 5) — in no case a proper diagnosis of schizophrenia could be made. The cotwin who came closest to this diagnosis was a man aged 35 (N. 1), who gave much thought to telepathic phenomena and the like; yet, he was comparatively open-minded and accessible to discussion of his ideas, and he had never been incapacitated by them. Thus, at the time of the first investigation, the correct rate of concordance for schizophrenia in fact was zero. This finding was the more striking as the two other samples published up to that time, those of Luxenburger (1928) and Rosanoff (1934), showed rates of concordance at about 70%, although Luxenburger (1934) later corrected his figure to 33%, for diagnostic reasons. Personally, I was inclined to attribute my zero finding to chance, the number of pairs being small, and also to the relatively short time of observation.

The A. presents the case of two sisters, both affected by mixed juvenile diabetes: diabetes mellitus and diabetes insipidus.

Three of their brothers died of diabetes millitus.

After discussing different etiological hypotheses, the A. indicates the hereditary one as the most likely.

Probably, in the case of the two sisters, there was an association of two hereditary processes, independent of one another, but coexistent.

Total birth records for the Bengalee Muslim population (BMP) and the Bengalee Hindu caste population (BHCP) for the period 1980-1988 were 17,720 and 119,107 respectively. Of these, the number of twin pairs were 363 BMP and 1,229 BHCP. These data were obtained from the registers of the following hospitals: Islamia Hospital, NRS Medical College and Hospitals and RG Kar Medical College and Hospitals, Calcutta and Medinipore Sadar Hospital, West Bengal, India. The twinning rates found were 20.48 and 10.57 per thousand deliveries in the BMP and BHCP respectively. The proportion of twins, 0.02048, in the BMP was significantly higher (Z = 12.38, p ≤ 0.01) than that in the BHCP, 0.01057. This finding of a higher twinning rate in the BMP is corroborated by the fact that available data on the Muslim population of Srinagar in Kammu and Kashmir, and Lucknow and Kanpur in Uttar Pradesh show higher twinning rates than the other populations of India. This increased twinning rate may be due to the greater amount of inbreeding in the BMP.

The Budapest Twin Register recorded 88.6% of the 1739 multiple births that occurred between 1970 and 1974. Of these, 31.2% were male-female pairs. Based on the examination of the placenta (19.8% monochorial) and of genetic markers, 406 pairs proved to be monozygotic and 645 dizygotic. Sex ratio of twins at birth showed a female preponderance (1000 F: 1009 M) in comparison to singletons (1000 F: 1071 M). Average birth weight of the twins was 2101 g. The frequency of stillbirths was about four times higher in twins than in singletons, twice as high in monozygotic than in dizygotic twins, and 2.4 times higher among second than first-born twins.