The program of any individual's vital cycle encoded in the genetic information is executed according to a time schedule that is the object of study of Chronogenetics. MZ twins provide the best evidence of the existence of this hereditary biological time and they represent, therefore, the natural test of Chronogenetics. Twin research can thus serve medicine in developing its most modern perspective, individual preventive medicine. Moreover, through approaches such as the Twins-Living-Apart Test, twin research can help to identify the influences of the environment on the health of man.

The study of developmental processes in twins furnishes a powerful resource for examining the role of gene-action systems in guiding the course of growth [2,9].

While there is a steady and rapid progression from birth onward, the growth rate is not entirely uniform for a given child, but rather moves in episodes of acceleration and lag. The timing of the growth spurts follows a distinctive pattern for each child, and consequently a child who may be smaller than average at one age may then enter a phase of rapid growth, and ultimately catch up with or surpass his peers at a later age.

The effect of such individualized patterns of growth is that many children may change in relative size from one age to the next; and in this sense it may appear that the underlying developmental processes are erratic, rather than coherent. But if there is an underlying ground plan, a chronogenetic pattern, then the distinctive developmental gradients should unfold in synchrony for twins sharing the same genetic make-up. Episodes of acceleration and lag in growth would then occur in parallel for both twins and would presumably represent the activity of timed gene-action systems, which switch on and off according to a predetermined plan.

Although Francis Galton was the first to use twins for comparative genetic studies in a scientific way, there had been considerable interest and speculation about twinning from very early times. This has been referred to in numerous publications in the past, and indeed, Professor Gedda, Founding President of our Society, has written extensively on this [9]. The earlier writings referred almost entirely to the twins themselves, and there seemed to have been little attention paid to the mothers, except that they were either considered to have had intercourse with two men, or indeed, with a deity. Galton also referred in his publication of 1876 [8] to the largeness of the families into which twins are born, but a graduate of Aberdeen University, James Matthews Duncan, had pointed out some of the characteristics of mothers having twins ten years earlier. In 1865 he published his paper “On the Comparative Frequency of Twin-Bearing in Different Pregnancies” [7] and in this he showed that mothers of twins tended to be of higher parity and older. This very accurate observation has since been confirmed, but is was not clear whether age and parity were separate factors until Anderson, again working in Aberdeen, showed in 1956 that the twinning incidence increased up to a peak at age 35 and thereafter fell [1].

The ongoing comprehensive study of the updated population-based Norwegian Twin Panel (like-sexed twin pairs born since 1915) has already given results of interest to the research on coronary heart disease and its risk factors. Significantly more dizygotic (DZ) than monozygotic (MZ) pairs are discordant for death between 40 and 60 years of age. Presumably, several of the cases must have been coronary heart disease deaths. In pairs where both members are alive, concordance rate for coronary heart disease before the age of 60 years is significantly higher in MZ than in DZ pairs. Concordance rate for reported hypertension is significantly higher in MZ than in DZ pairs. These findings are compatible with a significant genetic effect on premature death, coronary heart disease and hypertension.

There is a strong genetic effect on serum level of apoB, apoA-I and apoA-II, a weaker effect on cholesterol level and a doubtful effect on triglyceride level. Genes belonging to several normal genetic polymorphisms may participate in the control of environmentally/dietary caused variability in lipid and lipoprotein parameters. The study of MZ twins that was conducted to detect these effects holds considerable promise for the detection of gene control of many kinds of quantitative parameters. Further work with this twin panel may provide more definite answers to several questions raised during the present investigation. Application of more sophisticated models for twin family analysis on several normal and pathological traits may be very informative. Also, this updated Norwegian Twin Panel should in the long run make it possible to estimate the predictive value for the second member of a twin pair of having a twin contracting coronary heart disease (or any other reasonably frequent disease) by a given age. Finally, the subsample that is subjected to extensive laboratory analyses will provide useful data for genetic linkage analyses since in many cases, offspring of two members of a MZ pair can effectively be considered as one single (more informative) sibship.

A multivariate procedure for estimating heritable components from twin data was applied to ridge counts obtained from the entire dermatoglyphic system. Covariance matrices of MZ and DZ within-pair differences were used to estimate genetic correlation matrices for 20 finger ridge counts, 6 palmar interdigital counts, 20 toe counts, 4 hallucal counts, and 6 sole interdigital counts. The proportion of genetic variation was found to be greater in ridge counts of patterns than in ridge counts of interdigital areas. On digits, finger counts are more highly heritable than toe counts. Each of the dermatoglyphic areas yielded several independent genetic components, ranging from general to specific. Environmental variation was found to be local and to frequently involve reciprocal interaction between twin pairs.

The behavior of 120 full term and 160 preterm newborn twins was assessed in several key areas: Irritability, soothability, reactivity, reinforcement value, and activity level. Infants were assessed during various situations so that aggregate, or summary, scores representing each behavioral area could be obtained. Evaluations were made during a feeding period, an active sleep period, an awake alert period during which orienting and interactional behaviors were observed, a quiet period during which reactivity to stress was observed, and any fussy periods during which irritability and soothability were assessed. Significant within-twin-pair concordance of behavior was obtained for each of the areas of behavior, suggesting the influence of constitutional variables on neonatal behavior. Analysis of these data by sex resulted in significant within-pair concordance of behavior for same-sex but not for opposite-sex twins. The actual ratings on these behaviors differentiated between full term and preterm infants. Increased behavioral deviance was observed with increasing prematurity in the areas of soothability and activity level. The results are discussed in terms of increased risk factors for preterm twins.

Francis Galton was born on 16th February 1822 (the same year as Mendel). His mother Violetta (1783-1874) was the daughter of Dr. Erasmus Darwin (1731-1802), a medical practitioner in Derby who had scientific interests, particularly in plants, and produced various mechanical inventions. He was also grandfather to Charles Darwin. Galton's father, Samuel Tertius Galton (1783-1844), was a Birmingham banker but possessed a number of scientific instruments. His father (Francis Galton's grandfather), Samuel Galton (1735-1832), also had scientific interests, including colour vision, and was elected a Fellow of the Royal Society.

Francis Galton became a medical student in Birmingham in 1838, subsequently attending King's College London, Cambridge and St. George's Hospital. However, he gave up his medical studies in 1844 after the death of his father [5,17]. Later he travelled in Egypt and South Africa about which he wrote various articles and books, including “The Art of Travel” (1855) [6] of which a total of eight editions were published. His scientific work from these expeditions won him his first medal, the gold medal of the Royal Geographical Society awarded in 1853. Subsequently he wrote further on scientific matters, mainly concerning geography, travel and meteorology. He worked on stereoscopic maps and problems associated with wind currents and sailing ships and introduced the word “anticyclone”. He was elected a Fellow of the Royal Society in 1856 and later to the Council of the Royal Geographical Society, becoming Secretary of the British Association for the Advancement of Science in 1863 [4,19].

In order to estimate genetic variance and heritability of systolic blood pressure (SBP), diastolic blood pressure (DBP), serum cholesterol and triglyceride levels, a total of 235 (79 male and 82 female MZ, 41 male and 33 female DZ) twin pairs, recruited from 12 junior high schools in Taipei city, were studied. Statistically significant genetic variance observed for SBP, DBP, serum cholesterol and triglycerides persisted after adjustment for age and anthropometric characteristics. However, further adjustment for dietary preference, beverage consumption, and other host and environmental factors gave different results: genetic variance of adjusted SBP and DBP was still significant, while significance was found only in males for cholesterol and in neither males nor females for triglycerides. Heritability estimates of unadjusted SBP, DBP, cholesterol and triglycerides were 0.27, 0.45, 0.21 and 0.41, respectively, for males, and 0.15, 0.42, 0.41 and 0.82, respectively, for females. After adjustment for age, anthropometric characteristics, host and environmental factors, the heritability estimates of SBP, DBP and cholesterol were 0.64, 0.72 and 0.50, respectively, for males, and 0.40, 0.60 and 0.37, respectively, for females.

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.

It is estimated here that, at the time of conception, the mean coital rate of young women who subsequently bear DZ twins is about 6% higher than that of the general population of young married women. This differential seems too small to account wholly for the greater promptness in conceiving DZ twins. Accordingly, it is suggested that coital rate is associated with DZ twinning in two ways. The major link seems to be indirect and mediated by erotic response which causes an increase in gonadotrophin levels and thus in double ovulation. In a minority of cases, frequent coitus may give rise to DZ twins via superfecundation, but it seems that, in human beings, the corpus luteum of one ovum will usually inhibit fertilization of any further ova after a short interval. The evidence presented here, though indirect, seems to suggest that, under particularly erotic conditions, double ovulation is sometimes induced. It seems reasonable to infer that coitus also occasionally provokes or accelerates single ovulations, thus impugning some rhythm methods of contraception. But there seems no very good evidence that rape induces ovulation. Some notes are added on the implications of induced ovulation for estimates of fecundability and of the length of the fertile period.

Physiological adaptation including expansion in plasma volume is exaggerated in women with twin pregnancies. In singleton pregnancy and multiparous twin pregnancies there is an association between plasma volume expansion and birth weight, but this is not so in primigravid twin pregnancies. Women with twin pregnancies have a similar dietary intake to singleton pregnancies, but it is not known whether there are differences with parity or zygosity. Absorption and utilisation of nutrients may be increased to meet demands for extra fetal growth. Nutrient supply and plasma volume expansion will be further discussed and their association with birth weight presented.

The monozygotic (MZ) cotwin control method was employed to elucidate possible environmental determinants of systolic blood pressure (SBP), diastolic blood pressure (DBP), serum cholesterol and triglyceride levels. A population-based twin sample of 73 male and 77 females MZ twin pairs was recruited from 12 junior high schools in Taipei city. Intrapair differences in blood pressure were negatively associated with intrapair difference in vegetable preference, attaining significance for DBP in males and SBP in females. Cholesterol was positively associated with milk consumption and preference for sweets, fried foods, meat and fish. A negative association was also observed between choleserol and vegetable preference. These associations for cholesterol were significant in males only. Triglyceride level negatively associated with preferences for sweets and vegetable, attaining significance for vegetables in both males and females and for sweets in males only.

Parents of 702 twin pairs, ages 4 through 12, completed a sex-role behavioral preference questionnaire for each cotwin. Data were analyzed to determine the effects of gender, zygosity, and age on behavioral similarities and differences between cotwins. Among same-sex cotwins, male MZ pairs were reported to behave the most similarly. Girl-boy pairs were the most dissimilar. Sex and zygosity contributed significantly to co-twin behavioral differences, with female pairs varying more on sex-typed behaviors than male pairs and DZ pairs varying more than MZ pairs. Age of twins was not a major source of differences within twin pairs.

The statistical model for the analysis of twin-family data of Williams and Iyer is examined. The model uses a large number of redundant parameters. It does not lead to quantitative predictions for new relationships. It allows for epistasisbut not dominance. It makes assumptions about assortative mating which are inconsistent with any biologically plausible mechanism. It assumes that the environmental correlation between parent and offspring is due to the direct effect of the parental genotypes, not phenotypes, on offspring environment. Other models which avoid these problems are more appropriate for the analysis of extended twin-family data.

An individual's functional ability in physiological responsiveness is thought to be an interaction between his heredity and his environment. This hypothesis was tested to determine if different extragenetic influences would alter functional adaptability in a set of MZ triplets. After a 3-month aerobic physical fitness training program varying only in frequency, measured values for the triplets' maximum oxygen consumption (MaxVo2 ) were 59.1, 44.5, and 57.8 ml/min/kg, as compared to pretreatment values of 45.2,45.1, and 49.1 ml/min/kg respectively. These results clearly indicate intrapair differences in functional adaptability, stemming from difference in the training frequency program. The split-triplet design of this study indicates that environmental factors contribute substantially to the intrapair variance found among MZ siblings. Data extrapolation suggests that environmental stimulation of sufficient magnitude is likely to alter the functional adaptability in the individual set by his genotype.

A population-based sample of 73 male and 77 female monozygotic (MZ), and 41 male and 33 female dizygotic (DZ) Chinese adolescent twin pairs were studied to assess effects of gene-environment interactions of systolic blood pressure (SBP), diastolic blood pressure (DBP), serum cholesterol and triglyceride levels. Intrapair concordance in BP levels was found to be significantly associated with the interaction of zygosity and salty foods preference and also with that of zygosity and vegetable preference. A consistently positive and statistically significant association was observed between the intrapair difference in serum cholesterol and the interaction of zygosity and animal organ preference; while intrapair concordance in serum cholesterol was associated with the interaction of zygosity and milk consumption. Intrapair difference in serum triglycerides was associated with the interaction of zygosity and fish preference, and a significant association was also found between the intrapair concordance in serum triglycerides and the interaction of zygosity and sweets preference. These observations suggest that the impact of these environmental agents may be influenced by the genotype.

In the Louisville Twin Study, laboratory observations of twin infants' temperament at 12, 18, and 24 months were linked with parental ratings from temperament questionnaires. Core dimensions of temperament were extracted by factor analysis applied to each set of measures at each age. The laboratory temperament dimension was recurrently represented by emotional tone, social orientation, attentiveness, and reaction to restraint. The questionnaire temperament dimension was recurrently represented by mood, approach/withdrawal and adaptability. The laboratory and questionnaire dimensions were found to be correlated at each age (convergent correlations: 0.38 to 0.52) and to be stable across ages (stability correlations 0.37 to 0.66). The temperament dimensions were used to demonstrate that temperament profiles were more concordant for identical than for fraternal twin pairs. The results demonstrate the genetic influences on (a) the primary dimensions of temperament and (b) the developmental transformations of temperament.

The relation between spontaneous abortions and twinning was studied in the years 1980-1981 in 22 towns of the South Moravian Region, Czechoslovakia. Each town has more than 10,000 inhabitants and less than 750 births per year. The number of spontaneous abortions and the number of twin births were evaluated separately for each town and for a time unit of three months. The comparison indicated that the group with a higher number of spontaneous abortions also had a lower number of twin births. The relation between spontaneous abortions and twinning is discussed.