The Author reexamines in current terms the old problem of Constitution from the medical viewpoint. Modern medicine, over the last 100 years, has mainly stopped to consider the importance of the diseases’ imprint of the phenotype as well as the diseases’ exogenous causes; today it is specially called upon to focus its attention on the deseases’ endogenous causes and on the individual’s morbid destiny. Constitutionally minded medical schools have surveyed this field from the morphological, functional and neuroendocrinological viewpoints, but with little practical result. Only by basing Constitution studies on the laws of Genetics will it be possible to solve the important problem from the clinical point of view.

Analysis of the statistics of multiple births in England and Wales between July, 1938 and December, 1945 proves that the frequency of monozygotic twin births is virtually constant, at about 3 per 1000 pregnancies, for all ages of mothers. The frequency of dizygotic twinning, however, increases with maternal age from 2 per 1000 pregnancies at 17 to a maximum of 14 per 1000 pregnancies at 38, and then decreases gradually to 5 per 1000 maternities at 45. Comparison of the average numbers of previous children to mothers at each age shows that the twin pregnancies were associated with a greater number of previous children then were the single maternities. While the reasons for this difference cannot be deduced from this data, if the tendency to multiple births is governed entirely by heredity and is independent of a woman’s parity, this greater number of previous children to mothers of twins vould indicate that the probability of repetition of a twin birth is ten times as large as he total population frequency of twins births.

1. The uses of twins as a tool to estimate heritability are reviewed.

2. Comparisons of dizygotic twins with sibs provide a means of estimating how effective conditions in utero common to both types of twins may be in producing intra-pair differences.

3. Genetic variations in plasticity should be taken into consideration in the interpretation of data obtained from twins.

4. Methods for testing simple modes of inheritance by information obtained from dizygotic twins are reviewed.

The typical picture of mongolism has been observed in both members of a proved monozygotic twin-pair, aged 15. There is no indication in the family history of any hereditary predisposition, and both, the pregnancy and the birth were quite normal. The author reviews all cases previously reported, and expresses the opinion that mongolism is due to some phenomenon occurring during the germinating period, that is to say during the initial stages of development of the egg.

To demonstrate the effects of home environment and parent-child relationship on personality and development, the author tells the life story of a pair of monozygotic twin girls reared apart. One, who grew up as a member of a large closely-knit family group, with mother-love and parental security, has become well-adjusted and happy in spite of severe set-backs. The other, reared as an only child by a hard and restrictive step-mother, and always required to do much hard work, became a tense and instable person, addicted to drugs and Anally probably committed suicide.

Description of 3 cases of identical twins affected by concordant oxycephaly. In the first case, one of the twins had a son who was likewise afflicted by a typical scaphocephaly. In the second case, both children showed also a convergent concomitant strabismus, which was latent in one and manifest in the other, and moreover a concordant ptosis. In the third case, the oxycephaly was associated with cutis frontis gyrata homologous in both twins.

A confrontation of the cases of oxycephaly in identical twins based on the literature and our personal cases, shows clearly a majority in favour of concordance : 25 concordant pairs as compared to 10 discordant ones; the fraternal twins (7-8 cases) are all discordant as regards oxycephaly. The cases of discordant oxycephaly in monozygotic twins must therefore be interpreted as the result of an unstable gene of feeble penetrance. The conclusion of the genotypic origin of oxycephaly is corroborated by some pedigrees showing a transmission through 2 or more generations. Two reported cases with consanguinity of the parents suggest likewise the possibility of recessive inheritance.

The association of oxycephaly with cutis frontis gyrata, as observed in our third case, having not yet been described, we cannot for the moment decide whether this combination is due to chance or whether there exists a pathogenic correlation between both anomalies.

Observation of the behaviour of a pair of one-egg twins, incompletely concordant with resspect to Paget’s disease, is consistent with the courrent theory (based on 57 families in which the disease has occurred in more than one member of the family, and one other pair of one-egg twins), that the condition is of genetic origin, the gene responsible being an incompletely dominant autosomal gene with variable degrees of penetrance and expressivity.

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.

This article discusses the major differences which can be fuond from time to time in homozygotic twins. In some instances there may be differences in manifestation, as in the case of a pair of monozygotic twins, aged 12, with concordant dysplasia of the hipjoint but with different radiological pictures. Differences may also be due to particular incidents during intrauterine life, alterations in growth resulting from hormonal disturbances etc. In some cases the cause of the differences is not known and, as an example, the author cites a monozygotic twin-pair aged 10, one member of which is much retarded in growth while the co-twin suffers from coloboma of the iris.