With the exception of disorders related to gestation and birth, twins are usually found to be medically representative of the general population; diseases occur among twins with the same frequency as among singletons. As a corollary, in a large series of twins, if members of each pair are assigned randomly to two groups, A and B, both groups will be representative of the population. For example, if a disease occurs in 1% of the population, then we can expect it in 1% of twin-group A and in 1% of twin-group B.

Tab. I diagrams a hypothetical population of 15000 twin pairs in which 1% of all persons have the disease in question. Twins A include, in the left column, 150 sick persons. Each of these has a partner, twin B, who may be either sick or well, and so the total number of pairs in the left column is 150. The figures are the same when the table is read down or across; that is, the table is symmetrical with respect to twins A and twins B because assignment to A and B was random.

A sample of 122 twin pairs and 166 singletons from the urban Punjabi population of Chandigarh was studied. The observed frequency of double occipital hair whorls was higher in twins than in singletons (P < 0.01). The possible association between twinning and double occipital hair whorls is discussed.

Two cases of antepartum demise of one fetus in a twin pregnancy are reported. Obstetrical management and fetal outcome are discussed with particular respect to changes in maternal coagulation parameters.

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.

A certain amount of material is provided for studying the heredity of fissures of the iris, and it seems that such a heredity is probably not of monohybrid nature. The relationship between fissures and pigmentation of the iris has been confirmed and noted also by earlier researches. This connection may be explained by the fact that fissures of the iris-appearing even earlier, during embriological development—leave fewer chromatophores in the stratum limitans anterior for the formation of pigment. It thus follows that the colour of the iris, which is determined by comparing it with table of colours of the eye, would not be of monohybrid heredity. It is doubtful even when both colour and structure of the iris are contemporaneously studied in order to realise a polysymptomatic resemblance—as for the diagnosis of monovular twins of the diagnostic of paternity—since the logical basis for establishing a polysymptomatic comparison of resemblance is, in fact, given by the independance of partial signs connected one with another. This goes for all cases, whether or no statistical methods are used for the critical examination of such researches, such as, for example, Essen-Möller's formula (3,4) (Wichmann 22, 23).

Campimetric pictures in several cases of chiasmal syndrome are presented. Comparison between white light and laser visual field data shows that earlier and more precise evidence of visual field defects is obtained with the latter method.

Three pairs of male twins concordant for muscular dystrophy are presented. Two of these pairs are shown to be monozygotic and affected with the Duchenne form of muscular dystrophy. The third pair is probably dizygotic. The clinical and genetical features of each pair of twins are discussed. Pedigrees with photographs of the twins are presented.

Duodenal ulcer is a rather common disease, showing a clear family concentration. The aim of this work was to detect, in the etiology of this disease, possible genetic components, and to study their characteristics. The survey has been carried out by analyzing the data collected in 100 families, assembled by starting from a corresponding number of « index cases », and utilizing for comparison and also further analyzing the data by Doll and Buch (1950) and by Doll and Kelloch (1951). The " secondary cases " found, as a whole, were 109 (90 males and 19 females). The results may be summarized as follows:

1) the frequency of the disease in the sibships, independently from sex and corrected for age, is of 32.5% ± 2.5;

2) the frequency is, however, different in the two sexes. The estimate of this difference has been obtained calculating the relative risk (x) to contract the disease in the males, as compared to the females. Considering all the families together, such risk is x =5.8. However, x =16.7 in the 39 families with both healthy parents; x = 4.0 in the 45 families with the father affected; x = 2.9 in the families with the mother affected;

3) though the analysis of segregation ratios would not allow, by itself, definitely excluding the hypothesis of a monofactorial recessive inheritance, many reasons lead to think of a possible case of « simulated mendelism »;

4) the application of the methods of analysis proposed and worked out, in their theoretical basis, by Penrose (1953) and by Edwards (1960) for the genetic study of common diseases, makes much more probable the hypothesis that the genetic basis of duodenal ulcer is multifactorial. This does not exclude, however, that some particular environmental factors may increase the phenotypical correlation among the brothers — at least in the material under consideration.

Therefore, the hypothesis now allowing the best interpretation of the observations is that duodenal ulcer could be controlled by a number of additive genes with a weak effect.

The acute form of twin-to-twin transfusion syndrome is caused by rapid transfer of blood from one of the twins to another via placental anastomoses. Usually, this only occurs during the second stage of labour as a result of a sudden relative rise of blood pressure in one of the fetal circulations. This can result in the sudden intrauterine death of a fetus (or both, as in our case). Currently, there is no reliable means of identifying such an at-risk pregnancy by means of ultrasound antenatally. We would classify this as TTTS Type III.

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 case is described of uncommon vertebral malformation simulating a duplication of the spine. Pathogenesis, etiology and terminology of such an anomaly are briefly discussed.

Most of the cases of Amalric's pigmentary macular dystrophy associated with deafmutism seem to be due to a rubella or to another viral infection of the mother during pregnancy. This association can sometimes probably also be due to an infectious disease of the early childhood. Heredity seems to be a rather exceptional factor.

Three familial cases of epibulbar dermoids, associated with lid ptosis, antimongoloid aspect of the palpebral fissure and nasal lobuloschisis are reported.

The hereditary character of this malformation is interpreted in terms of polimery.

The possibilities of early diagnosis of multiple pregnancy are shown, which helps to prevent and reduce the incidence of premature deliveries.

Four boys, born in 1954 as uniovular quadruplets, live in Wroclaw, Poland. This is a unique case in the world. Their uniovularity was proved as a result of serological analyses.

Despite the fact that one of the boys was seriously injured during the labor (he suffered from one-sided paresis and deafness) and lived in special conditions, all the boys are identical and they develop identically.

Genetic factors seem to be more important, in the shaping of the phenotype, than the influence of environment.

Current antenatal technologies have improved the obstetrician's ability to assess fetal well-being as well as to diagnose fetal compromise. These technologies have given rise to very difficult ethical issues in the management of compromised twin pregnancies: for example, a choice must be made between putting a healthy twin at risk due to preterm delivery for the sake of a compromised cotwin or of allowing the compromised twin to die in order to buy time for the healthy twin. Though each case is unique, good medical practice requires a standard of care by which consistent patient management can be proposed. In the pluralistic environment of Northwestern Memorial Hospital, our staff has favored a standard of care based on patient autonomy. This approach demands: 1) a practitioner who offers a thorough explanation of the diagnosis and possible treatment approaches; 2) time for the patient and her partner to assimilate this information and test treatment options against their personal value system; 3) a third, but disinterested, party to facilitate patient understanding and value clarification; 4) a practitioner either willing to support the patient's decisions or refer her to another practitioner who will.

To determine the small population as a specific group, several quantitative indexes have to be taken into consideration. The author, making use of these indexes, differentiates three types of small populations: the isolate, the « dem » and the « large dem ». The central type — the dem — is a group including 1,500 to 4,000 individuals with an increase of more than 20-25 per cent in 25 years (one generation); a high proportion, about 80 per cent, of the marriages is concluded inside the group and an insignificant part of the members have descended from allogenous ancestors. The inhabitants of a rural locality constitute in most cases a « dem ». Theoretical calculations as well as genealogical studies have shown that the preponderant part of adult members of a « dem » appear to be related to each other in the eighth degree (according to Tab. 2 - cousins thrice removed).