Six new cases of monoamniotic twins are reported. All had true knots of the cord, the result being double survival in 5 cases and 11 normal children. The mechanism of twin development is described and monoamniotic twins are placed in accordance with the two theories from the literature. A short review of both the Scandinavian and the world cases is presented.

The authors have studied the frequency and nature of associated malformations in a series of 1000 patients with congenital heart disease. Etiological factors have been statistically analyzed, too.

A definition of efficiency is presented to aid a researcher in the choice of the genetic systems for the diagnosis of twin zygosity. An expression is given that produces a numeric value that indicates which systems do the most effective job of discriminating DZ twins from a sample of twin pairs. This definition of efficiency is illustrated with eight blood groups commonly used in twin zygosity diagnosis.

The Author reports the case of two MZ twins concordantly affected by a neoplasm of the testicle. The various etiopathogenetic theories of this form, the data reported in the literature and the genetic interpretation are discussed.

Two unrelated patients with multiple congenital malformations and severe mental retardation were found to be carrier of a non-mosaic ring 22 chromosome abnormality. These observations further confirm the phenotypical variability of r(22) expression, which makes unreliable the attempts to delineate a clinical profile of the syndrome

We examined the neonatal outcome of 644 twins weighing 500 g or more and 656 singletons, born in the years 1984-1986 in the Soroka Medical Center, Beer-Sheva, Israel. There was nearly a four-fold risk of antepartum death in twins vs singletons, which disappeared when birth weight was controlled for. The risks for intrapartum and early neonatal mortality were not raised in this population. A statistically significant relative risk for congenital heart malformations in twins vs singletons remained (RR = 5.0, 95% CI = 1.5-16.3), after controlling for maternal age. Significantly higher rates of hyalin membrane disease, hypoglycemia, hyperbilirubinemia, anemia and septicemia were found in twins. Controlling for the confounding of the association between twinning and mortality or morbidity caused by differences in distributions of mode of delivery or gestational age between twins and singletons, was not as efficient as the controlling for birth weight. Thus, adjustment for birth weight removed all the excess risks detected except in hypoglycemia. Our findings suggest that the-lower birth weight of twins, which is so intimately associated with multiple gestations, is probably the single most important factor associated with neonatal problems found in twin births.

A concordance study of 6 infectious diseases of childhood has been carried out in a sample of 656 twin pairs classified by sex and zygosity. A new approach is proposed to estimate the respective influence of heredity and of common environment. The estimates thus obtained range from 86% hereditary component in the case of measles to 100% environmental component in the case of scarlet fever.

In an attempt to improve our understanding of the factors that affect human twinning, we further developed the models given by Hellin (1895) and Peller (1946). The connection between these models and our own model (“Fellman's law”) were studied. These attempts have resulted in a more general model, which was then applied to data from Åland Islands (1750-1939), Nîmes (1790-1875), Stuttgart (about 1790-1900) and Utah (1850-1900). The product of the mean sibship size and the total twinning rate can be considered as a crude estimate of the expected number of sets of twins in a sibship. The same can be said about the twinning parameter in our model. These estimates are in good agreement. If we consider twinning data only, we obtain the geometric distribution, and log (Nk), where Nk is the number of mothers with k twin maternities, is a linear function of the number of recurrences. Graphically, this property can easily be checked. For sibships containing three or more sets of twins, all four populations show higher values than expected, particularly the populations from Stuttgart and Utah, which data also show poor agreement according to a χ2-test. A more exact model would demand more detailed demographic information, such as distribution of sibship sizes, age-specific twinning rates and temporal variations in twinning.

The osberved number of mothers in Åland with several recurrences of multiple maternities shows a considerable excess over the expected number as predicted by Peller's rule. The parameters in our model can be estimated by the maximum likelihood method and the obtained model fits the data better then Peller's model.

The results of the Bene Anthony Family Relations Test on 53 pairs of twins and 30 older siblings, as well as an analysis of personality inventories from their parents, are presented. It is shown that weight and birth order can affect not only personality and the way in which twins interact with each other, but can also affect family interaction. Particular attention has been given to the effect on and of the preceding sibling.

Total intrauterine volume (TIUV) was determined serially in 27 twin pregnancies from which a nomogram was constructed. At the beginning of the second trimester, TIUV was similar to that in singleton pregnancy; by the 18th week it had become twice as large, and remained so until late in pregnancy. Labor was preceded more frequently by a slowing than by an acceleration of uterine growth.

A multiple birth frequency of 5.59% has been found in 25 Klinefelter sibships, which is statistically significantly higher than expected (P < 0.001).

A family investigation of a patient with Klinefelter's syndrome, who is a triplet himself, revealed 5 twin births and 2 triplet births in three generations.

The mortality of the 19 children from the 9 multiple births born in the 25 Klinefelter sibships was 53% up to the age of 3. Such a high mortality indicates that some of the twins and triplet births might have suffered from chromosome disorders contributing to the high mortality.

If the comparatively high frequency of twins in the sibships with Klinefelter's syndrome as well as Turner's syndrome is confirmed by further studies, this will indicate a relation between the cause of sex chromosome non-disjunction and multiple births or between the predisposition to non-disjunction and multiple births.

Male and female rats of three different strains were subjected to physical restraint, when it was demonstrated that there is a relationship between the strain of the rat and its susceptibility to gastric ulcer formation. Severity of ulceration was greatest in Wistar rats. Hooded Wistar rats were also ulcer-susceptible, but not to such a great extent as Wistar animals. Sprague-Dawley rats were resistant to ulcer formation. Sex differences in ulcer severity were found only with restrained Hooded Wistar rats.

The results of this investigation compare favourably with results published for individual strains of rats by other workers. It is possible that genetic influences may be responsible for the differences observed, but the way in which they operate is not understood.

Radioimmunoassay of FSH and LH serum levels has been carried out on 15 Nigerian women, 6 of them mothers of singletons and 9 mothers of twins (2 of two sets). Higher FSH levels have been found in mothers of twins, which supports the view that a higher production of pituitary gonadotropins, resulting in a higher incidence of multiple ovulation, may be responsible for the higher incidence of twinning in Negro populations.

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.

Among 2,005 Swedish men the percentage of “non-tasters” for PTC is 18.4 ± 0.9 and by 1,051 women 12.8 ± 1.0. The difference is 5.6 ± 1.4%. The difference of “non-tasters” between women who are nulliparae (437) and others (614) is 19.5 — 8.1 = 11.4 ± 2.2%.

The genetic and environmental effects on the levels of 17 serum biochemical quantitative traits (calcium, phosphorus, glucose, urea nitrogen, uric acid, total protein, albumin, total bilirubin, alkaline phosphatase, lactate dehydrogenase (LDH), glutamic-oxaloacetic transaminase (SGOT), total lipid, cholesterol, triglyceride, α-lipoprotein, pre-β-lipoprotein and β-lipoprotein) were estimated in 105 pairs of healthy twins of both sexes (57 MZ and 48 DZ) by path analysis. The genotype effect was significant for all traits (P < 0.001) and its value extended from 0.52 (α-lipoprotein) to 0.81 (alkaline phosphatase), whereas environmental effect was significant (P < 0.05) in only 10 traits of the 17 analyzed, with the maximum value of 0.13 (cholesterol). Correlations between genotypes of paired traits were estimated and, of 136 values, 47 were significant at the 5% levels, thus indicating partial and common genetic mechanisms.

Two cases of monoamniotic conjoined male twins, born at term after normal pregnancies, are reported. The first case, a bicephalus, shows hypoplastic and malformed left-side organs, absence of the left umbilical artery, and two communicating hearts, the left one with three cameras. The second case, a pygotho-racopagus, consists in a twin “parasite”, with no head but with two upper and two lower limbs, slightly less developed than those of the formed twin. The left eye of the formed twin is double than the right one and contains two eye apples — one well-formed and the other rudimentary. There is a rudiment of a second mouth on the left cheek. The umbilical cord contains five blood vessels — one umbilical vein and four umbilical arteries. The cytogenetic study of the pygothoracopagus reveals aneuploidy, more pronounced in the “parasite” than in the formed twin.

In 1980-82, a mailed questionnaire was completed by 3,810 pairs of adult twins enrolled on the Australian NH&MRC Twin Register. Twins were asked whether they had had their tonsils out and, if so, at what age. The sample was divided into four birth cohorts of approximately equal size, and only childhood tonsillectomy (to the age of 18) was considered. The prevalence of tonsillectomy differed markedly between cohorts, being highest in those born in the 1940s and early 1950s. Within each cohort, the prevalence was very similar in MZ and DZ twins, yet concordance was much higher in MZ twins, indicating the importance of genetic factors in predisposition to tonsillectomy. However, the proportions of variance in liability due to genetic and shared environmental factors differed markedly between cohorts. In the 1950s, when tonsillectomy was fashionable, shared environment accounted for 60% of variance and genetic factors for only 29%. However, by the early 1960s, when tonsillectomy was going out of fashion, heritability was up to 0.82 and shared environment accounted for only 10% of variance. Our results illustrate, once again, that heritability is not a constant, but depends on the precise characteristics of the population and the time at which it is studied.

The Danish Twin Register, which comprises all twins born in Denmark between 1870 and 1920, has been used for studies of selected problems in the field of epidemiology and public health. This report gives a survey of the preliminary results of an investigation of this type, which intends to amplify the present knowledge about the possible harmful effects of smoking on health, by taking advantage of a twin material which makes it possible to keep the genetic factors under control.

A series of 762 MZ and same-sexed DZ twin pairs, which had their smoking habits mapped in 1959, has been followed continuously, and the mortality which has occurred in this group has been analysed in relation to tobacco consumption. Neither MZ nor DZ pairs showed any tendency to increased mortality in the heavier smoking cotwins. When the material was subdivided according to cause of death, no conspicuous exception from the general pattern was found, but the subgroups were relatively small.

The distribution of selected diseases and symptoms, mainly of the cardiovascular and respiratory systems, was studied in this and a second series, followed since 1966, with a total of 1584 pairs.