Data with regard to the proportion of congenital malformations found at birth are presented from a survey of 657 pairs of twins of known zygosity and placentation delivered in Aberdeen and Northeast Scotland between 1968 and 1979. There was an excess of malformed individuals from monozygotic (MZ) pairs, but this did not reach statistical significance. The possible effect of monochorionic placentation in the causation of malformations in MZ twins in general is discussed. It seems that this type of placentation may be of less causative importance than has been previously suggested.

Two cases of holoacardius are presented. Both had a normal 46,XX female karyotype, identical to that of their respective cotwin. Data are presented supporting the concept that the placental vascular anastomoses are the primary agents in the formation of an acardiac. Cytogenetics, epidemiology and other theories of pathogenesis are discussed.

By dosing the serum-cholesterol in basal conditions in 5 0 MZ (25 ♂ ♂ and 25 ♀ ♀) and 50 DZ (16 ♂ ♂ 18 ♀ ♀ and 16 ♂ ♀ ) twin-pairs, the Authors found an intra-pair dissimilarity significantly different among the pairs of the two zygosity groups. On this basis, the genotypic origin of cholesterol biogenesis is pointed out. The twins, aged from 6 to 19, were all living with their respective families: nutrition and environment were thus the same for each pair.

Twins, more than any other home reared infants, experience intimate and usually continuous interaction with an age-mate from early infancy. The effects of this situation are examined and its implications for both the emotional and cognitive development are discussed in the light of evidence from two studies in which play sessions were video-recorded and subsequently analysed into behavioural categories. The first was a longitudinal study of a single pair of twins, while the second was a short-term study of interaction and cooperation in infants in the 9 to 20 months range. Five pairs of twins, 5 pairs of familiar peers, and the children's mothers were observed playing a structured cooperative game. The hypothesis is advanced that twins enjoy more emotional support but less intellectual stimulation than singletons and it is suggested that social enrichment of the environment of twins could foster social and cognitive development.

Although numerous genetic and anthropological markers are available for determining zygosity of twins, there is still a need for a more practical and informative method in zygosity diagnosis. Dinucleotide repeats or other short repeats (microsatellites) are highly variable between individuals and offer a simple, fast, cheap, and exact approach for zygosity determination. The feasibility of a set of microsatellites to be used for this purpose is demonstrated.

A simple path model applicable to twins and their parents and involving both cultural and genetic transmission in the presence of phenotypic assortative mating was extended to cover the bivariate case. The model allows for cross assortative mating as well as cross cultural transmission. It was applied to two correlated measures derived from a fear survey questionnaire given to 1000 subjects. In allowing for the impact of more than one variable, the model allows for a much more realistic picture of cultural transmission than provided by the univariate model. The logic of the model and an application are outlined. (The authors are indebted to Professor R.J. Rose for providing the illustrative data.)

The immediate global impressions of zygosity (based upon physical similarity) were recorded by the investigator at initial meetings with 105 pairs of twins. The accuracy of these ratings, as well as classifications provided by two objective procedures (physical resemblance questionnaire; dermatoglyphic analyses) and two subjective procedures (parental impressions; physician's impressions), were evaluated by comparison with results from bloodtyping for 53 pairs. The judgments of the investigator furnished the most accurate indication of zygosity (94-96% accuracy). Laboratory tests were repeated for five pairs when the results proved incompatible with the investigator's ratings. In all five cases, the investigator's judgments were confirmed, indicating that a laboratory error had occurred. It appears that the opinion of a skilled observer of twins can provide a convenient and highly effective alternative to bloodtyping.

An up-to-date review of literature on the distribution of haemoglobinopathies in the Indian subcontinent has been presented. Haemoglobins S, D, E, J, K, L, M, Q, β-thalassaemia major, β-thalassaemia minor, α-thalassaemia minor, haemoglobin H, haemoglobin Lepore and persistence of foetal haemoglobin have been reported from India, β-thalassaemia in combination with haemoglobins S, E, D, J, K, Q has also been reported. Haemoglobin S is found to be prevalent among tribal populations in various regions of India, while haemoglobin D is prevalent among Sikhs, Pathans and Gujratis. Haemoglobin E is present in high frequency among populations of Assam, Bengal and Veddahs of Ceylon. β-thalassaemia major is widespread in various populations. There is paucity of data regarding distribution of β-thalassaemia minor and α-thalassaemias in this region.

The concordance of physiological and pathological times in human identical twin pairs induced the authors to postulate the existence of a hereditary biological time.

Having formulated the hypothesis that the information of each gene has a given period of existence and that, therefore, every gene has its own inherited temporal dimension, the authors report on five different experimental studies intended to verify their hypothesis.

In the first study (cf II. 1) a twin research on bone age and dental age is performed. The chronological study of the appearance of ossification nuclei in carpal bones and of mineralization of the gems of permanent dentition, in 20 MZ and 20 DZ human twin pairs, indicates that these wellknown “biological timetables” exhibit about 70% of genotypical control.

In order to verify whether biological time is a function of the genotype as a whole, or a property of each individual gene, the authors carried out an experimental study on the mean lifespan in different strains of Drosophila melanogaster whose genotypes were fully known (cf II.2). Their results indicate that the specific information of certain genes controls the insect's lifespan; it may also be inferred that the differential persistence of its specific information is an attribute of each individual gene. This chronological dimension of the gene is called chronon, which the authors also define as “the period during which the original information of the gene remains unchanged” — whether it is used for transcription or duplication, or it remains at the potential stage.

The determination of alkaline phosphatase activity in the same strains of D. melanogaster (cf II.3) affords an estimate of the amount of genie information (intensity of the individual trait) and the variation thereof during the gene's chronon. The authors observe that the amount of information decreases gradually during the gene's chronon, suggesting that this be due to the gradual exhaustion of a given specific energy. The decrease in the amount of information in the longitudinal study of chronon leads the authors to identify a further fundamental parametric unit of the gene which they call ergon.

Ergon is defined as “the degree of stability of a gene”.

In the fourth study (cf II.4) the twin test is applied to the chromosome association index in subcultures of lymphocytes from MZ and DZ twins at age 6 and age 60. This study affords a parallel estimate of chronon (i.e., duration of information) and ergon (i.e., stability of information).

Chronon and ergon are found to be interrelated; they may be considered as variables in a dimensional equation of the gene. Thus, the existence of the Ergon/Chronon (E/C) system is postulated.

Nine parameters of development and of senescence (first smile, first word, first steps, first pubic hair, menarche, first white hair, first loss of a permanent tooth, first use of reading glasses, onset of menopause) are studied in an experimental population of 666 twin pairs of either zygosity, leading the authors to formulate several conclusions concerning the characteristics of the E/C system (cf II.5).

The interpretation of their experimental findings leads the authors to consider the ergon (energy of stability) of a gene as the total result of the stabilities of all the nucleotides making up the DNA sequence of that gene. Since it is well known that the stability of adenine-thymine (AT) bonds exceeds the stability of guanine-cytosine (GC) bonds, and that different combinations of codons (differing in at least one nucleotide) may provide the same information, it is clear that identical polypeptide chains may be produced under the control of genetically different ergons resulting in genetically different chronons.

The authors summarize these concepts in the following two aphorisms: “one gene, one stability” (ergon) and “one gene, one time” (chronon).

Biological time, development, senescence, homeostasis and disease are interpreted by the authors in the light of the E/C system.

Despite some evidence that genotype-environment interaction (G×E) effects may be involved in the variation observed in behavioral and biological traits, few attempts have been made to detect and quantify this component of genetic variation in humans. We propose that one way to achieve this goal is to challenge several genotypes in a similar manner, submitting both members of several MZ twin pairs to an ethically acceptable experimental treatment capable of inducing an adaptative response. In this situation, the G×E effect can be assessed with a two-way analysis of variance for repeated measures on one factor, the treatment effect. In this design, twins are considered nested within the pair, whereas the treatment effect is considered a fixed variable. The intrapair resemblance in the response to the treatment is quantified with an intraclass correlation coefficient computed with between-sibhips and within-sibhips means of squares. To illustrate this approach, changes induced by long-term endurance training were studied in 10 MZ twin pairs. Significant intrapair resemblance in the response of maximal oxygen uptake was observed, with about 7 to 8 times more variance between pairs than within pairs. This design with MZ twins may be helpful in the study of human variation for multifactorial phenotypes.

A total of 63 twin pairs (22 MZ and 41 DZ), with one or both members affected by not organic stuttering and/or tics, have been drawn out of the Mendel Institute's twin file. Concordance of stuttering was observed in 10:12 MZ, but only in 2:19 DZ twin pairs; concordance of tics in 6:10 MZ and in 2:22 DZ twin pairs. Stuttering did not come out significantly associated either with other disturbances of language or with an IQ under the average; almost always it was possible to find out a precipitating factor. Significant relations between stuttering and tics could not be stated. Sex ratio, obtained regarding any affected subject separately, clearly turned in favour of males both in stuttering (32:12) and in tics (27:12). Stutterers and left-handers incidence in the families of stuttering subjects appeared significant in comparison with families of subjects with tics.

From 1971 to 1981, 16752 women were delivered at the University Women's Clinic in Heidelberg; 228 of these were twin pregnancies (1.36%). The relationships between sex, gestational age, birthweight, mortality, and fetal outcome were investigated. For the reason of grading, so-called χ2 curves are used. These curves show the strength and the course of relationship between two parameters. Mortality of the twin (8.3%) depended on birthweight and on sex of the infant, but not on the mode of delivery. All twins were ‘small-for-date’ (25th weight percentile of single births). Starting at the 35th week of gestation, the increase in weight decelerated additionally. So, immaturity as well as twin-related factors led to the high mortality rate for twins. Fetal outcome was evaluated in respect to the birthweight and to the delivery interval.

A new approach is proposed in twin research based on the study of twins who, though reared together, have subsequently lived apart for a period of at least five years. With respect to the more powerful study of twins reared apart, the twins living apart test has the advantages of being more realistic and affording easier access to sufficiently large samples. In this pilot study, the test has been applied to a sample of 92 monozygotic pairs now aged 35–45; 15 pairs were still living together and 77 had lived apart for over five years. As a first approach, comparisons have been made, in the cotwins of the two subsamples, with respect to the following traits: height, weight, presbyopia, presbyacusia, alcohol consumption, tobacco consumption, blood pressure (systolic and diastolic), dental caries, and hours of sleep.

The results of the small number of existing investigations of criminality among twins have mostly been considered as contributions to the discussion of the relative importance of heredity and environment. I attempt to look at the problem in a slightly different context, but find it useful to give a brief summary of the older investigations. The results of previous studies utilizing the twin method to throw light on the problem of crime appear from Tab. I.

I am sure you will remember Lange's conclusion from 1929: “ As far as crime is concerned, MZ twins on the whole react in a definitely similar manner, DZ twins behave quite differently. In accordance with the significance of the twin method we must conclude that innate tendencies play a preponderant part among the causes of crime”.

There is one fundamental assumption behind the conclusions about the heredity of criminal behaviour, which is not fulfilled. The environment of each of the two twins in a pair must be equally similar or equally different, irrespective of zygosity. However, the milieus of the cotwins in a DZ pair are in fact more different than in a MZ one.

The Danish study is carried out in cooperation with the Danish Twin Register (Harvald and Hauge, 1965). The material consists of data about nearly 6000 pairs of twins, i. e., all twins born on the Danish islands in 1881-1910, where both twins have survived to the age of at least 15 years. The source material may be considered complete. About 900 pairs have been entered into the Central Police Register and/or into the local penal registers.

In the years 1966-1976, 50 pairs of Warsaw twins (27 MZ and 23 DZ) were subjected to longitudinal investigations. The twins were aged 9-11 years at the beginning of the observations, and 19-22 at their conclusion. The observations consisted of comprehensive somatic measurements and of the evaluation of biological maturity. The rate of growth of the twins, defined by the magnitude of yearly increments of body height, proved to be more concordant in MZ than DZ pairs. The appearance of the adolescent spurt occurred in the same year in 81 % of MZ and ca. 43% of DZ pairs. A very slight relationship was found between the rate of growth and final body height. The attaining of subsequent stages of sexual maturity in MZ twins was concordant in 88%, and in DZ twins in 30-39%, according to the respective stage. The body height curves were represented be means of three numerical parameters.

The perinatal mortality rate for twin gestation is in the range of 15 %, and this is due predominantly to prematurity, although twins may also be born growth retarded. Ritodrine HCl, a beta sympathomimetic drug, has been shown to be effective, both in stopping premature labor and in preventing intrauterine growth retardation. With this in mind, a double-blind study using ritodrine HCl or placebo was begun in order to study its effect on premature labor, intrauterine growth retardation, and the perinatal mortality rate in twins. Thus far, 30 patients have delivered and have been followed to 6 weeks postpartum. Although the results on individual patients have remained blinded to the investigators, an initial evaluation of the ritodrine and placebo groups have revealed no difference with respect to gestational age, birth weight, or perinatal mortality. These preliminary results are not significant. However, it appears that ritodrine HCl is a safe oral agent for the antepartum gravida and her fetus. The study will be continued until approximately 100 patients have been enrolled.

Numerous reports in the literature suggest that hormones may transfer from one fetus to another, in humans as in animals. In a large sample of over seven thousand Australian adult twins, it was found that opposite-sex females showed a statistically significant tendency to hold more masculine attitudes than did same-sex female twins. This may be due to post-natal social interaction, but could also be caused by the transfer of testosterone from the male to the female fetus in opposite-sex twins.

Immune-mediated diseases affect up to 5% of the population and are a major cause of morbidity and mortality. These diseases can be organ specific, such as insulin-dependent diabetes (IDDM) and non-organ specific, such as Rheumatoid Arthritis (RA). Identical and non-identical twins have been used to establish whether these diseases are determined by genetic or environmental factors. The results of these studies have been collated in a new section of the Mendel Institute in Rome.

Diseases included in these studies included IDDM, RA, Systemic Lupus Erythematosus (SLE), Multiple Sclerosis (MS) and Myasthenia. Striking differences in concordance rates between identical and non-identical twins in all these studies suggest that genetic factors are important in causing these diseases. All the diseases are known to be associated with HLA genes on chromosome 6 which may account for some or all of the genetic susceptibility. However, in the majority of pairs the affected twin has an unaffected co-twin. These observations suggest that non-genetically determined factors, probably environmental factors and not somatic mutations, are critical. The study of unaffected co-twins, who are at high disease-risk, has allowed the identification of changes which precede and predict the clinical disease. The immune-mediated destruction in many of these diseases is probably caused by T-lymphocytes. Twin studies have shown the importance of genetic factors in determining T-cell responses. Identical twins should, therefore, provide the perfect test bed to assess the role of T-cells in immune-mediated diseases.

The results of a multihospital study involving a total of 588 twin pairs born in Chicago in 1970–1975 are reported, with special respect to differences in mortality between first and second twins by time as well as by cause of death. Mortality was higher in second than in first twins and most commonly occurred after delivery and was the result of immaturity and of respiratory distress syndrome.

As part of the National Heart and Lung Institute multicenter study of twins, 190 twin pairs (93 MZ, 97 DZ) residing in California and aged 44-55 years were comprehensively studied for all risk factors for coronary heart disease and for Behavior Pattern Type A or B. Psychological tests included MMPI, CPI, 16 Cattell PF, Cough Adjective Check List (ACL), Thurstone Temperament Schedule (TTS). Heritability at statistically significant levels was found only for scales of TTS. Twelve scales of ACL and five scales of TTS showed significant correlations with Behavior Pattern Type A-B as well as with several risk factors, including blood pressure and serum lipids. Multiple regression equations were constructed for those characteristics accounting for significant and unique portions of the total variance of Type A-B Behavior. The use of certain scales of ACL and TTS may provide a new questionnaire for screening purposes in the assessment of Behavior Pattern Type A-B.