A twin population model is proposed, and two different definitions of the rate of concordance are investigated within the context of this model. It is found that the statistic formed from the ratio of the number of affected twins that come from concordant pairs to all affected twins has certain advantages over the traditional definition of the rate of concordance. This twin population model is extended to nontruncated sample of twins. The parameters of this situation are estimated. Finally, it is shown how these parameters relate to the two usual estimates of the proportion of DZ. twins in a twin population, and the efficiency of these methods is computed.

The polymorphisms of constitutive heterochromatin regions, present on chromosomes 1, 9, 16 and Y, are inherited in a Mendelian fashion. The C-band heteromorphism has been reported to be associated with various types of cancer. Heterochromatin is considered to play a role in protecting genome against the mutagens. Changes in the quantity and proportion of the different types of satellite DNA might increase the genetic susceptibility in people with heterochromatic variations, which in turn cause chromosome instability and predispose the individual to cancer. We report a case of bilateral retinoblastoma with complete absence of pericentromeric heterochromatin on one of the chromosomes number 9. A similar deficiency of pericentromeric heterochromatin on chromosome number 9 and 16 has been reported in a phenotypically normal individual and a Down syndrome case, respectively. This deficiency was found to be inherited from the father in all the three cases. Complete absence of pericentromeric heterochromatin of chromosome 9 is not being reported in association with cancer syndromes. Further studies are necessary to understand the role of this factor in normals and in those with cancer susceptibility, specially with retinoblastoma and the paternal origin of this deficiency.

Data of 986 twins from the records of the Clinica Ostetrica of the University of Rome have been analysed as regards the distributions, means and standard deviations of mother's age, order of birth, birth weight and length of gestation time in groups of like and unlike sex.

The chief characteristic of these data compared with similar data from English hospitals is the inclusion of cases of low birth weight and short gestation time. In the English data there are 9 cases out of 1137 of the group of weight under 680 g; in the present data there are 57 out of 1942 which fall into the lowest group under 500 g.

Similarly in length of gestation the English data have only 2 cases out of 926 under 170 days compared with 33 of the Italian out of 945.

The correlations between twin weights are respectively .8030 for like sex and .7604 for unlike sex; for weight and gestation period they are .3814 (like sex) and .3606 (unlike sex).

The regression of weight on length of gestation runs as a smooth curve in accordance with previous findings. The mean weights for given gestation times are about the same as, or a little higher than, those for the English data.

I am greatly indebted to Dr. Norma McArthur for giving me a copy of these data.

Perinatal mortality in twins is found to vary widely in six developed countries for which national data are available. The influence of factors such as birth weight, maternal age, and parity is examined, but no consistent trend is found.

Multivariate analyses are used to study the structural patterns of ridge counts of fingers. There is a significant difference between the ridge counts of two hands, chiefly due to the first and fourth fingers. There is asymmetry in the covariance matrix of the ridge counts. The correlations between fingers are significant and the decrease in correlation with increasing distance between fingers is not significant.

A case of probably MZ twin girls is reported, one of which presenting a serious gonadal hypoplasia, macroschelia and a number of slight malformations, while the other appeared perfectly normal. Both twins presented positive sex-chromatine and a normal karyotype, i. e. 44 A + XX.

After a review of twin cases of the « rudimentary gonad syndrome » — which may differ as to its origin, according to the various cases — the present case, the anomalies exhibited by one of the twins are probably the result of the action of a number of noxae, in the course of embryological or fetal life.

In order to examine the genetic variance and heritability of serum total cholesterol, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C) and triglycerides, a total of 349 pairs of same-sexed twin neonates born in four major general teaching hospitals in Taipei City were studied. Based on the placental pattern and 12 red blood cell antigens, 271 monozygotic (MZ) and 78 dizygotic (DZ) twin pairs were identified. There was a significant genetic variance for total cholesterol, HDL-C, LDL-C, and triglycerides both unadjusted and adjusted for sex, gestational age and placentation. The unadjusted heritability of total cholesterol, HDL-C, LDL-C, and triglycerides was 0.59, 0.30, 0.25 and 0.75, respectively; while the corresponding adjusted heritability was 0.74, 0.38, 0.31, and 0.49, respectively. Intrapair variance of serum lipids was not significantly different between monochorionic and dichorionic MZ twins.

Twin and pedigree data are presented which indicate that relative length of hallux and adjacent second toe is based on simple Mendelian genetic factors. The normal anatomical trait involves two alleles and a single autosomal locus. The factor for relatively long hallux is recessive to the one for relatively long second toe. There is no evidence of sex influence. Penetrance is 100%. Expressivity is variable, and bilateral discordance is observed in a minority of heterozygous subjects. In every such case, the dominant genotype is expressed in the right foot. The recessive and dominant phenotypes occur, respectively, in 76% and 24% of the greater Cleveland area Caucasian population. Gene frequencies in this population are 87% and 13%, respectively, for the recessive and dominant allele.

A series of house-to-house surveys in Central Brazil revealed 32 cases of reduction deformities of the limbs (brachydactyly, synbrachydactyly, ectrodactyly, amely, adactyly, achiry, apody, etc.) among 58,761 births (0.54 per thousand, being 0.66 per thousand among Whites and 0.43 per thousand among Mulattoes and Negroes). No sibship was found with two or more equally affected singleton members. One case of concordance among twins is referred. There were no parity, inbreeding, race and sex effects. Both hands and both feet were also equally affected. Precocious mortality and twinning were significantly higher among affected individuals. The presence of cleft lip and palate in one of our index patients may also reflect an etiological relationship. Among the sibs, the total incidence of congenital malformations was similar to that generally accepted for general populations.

In vaginally delivered births (including a few cases where the mothers were under epidural anesthesia), differences between first and second born twins were compared according to presentation at delivery. Of twins with vertex/vertex presentation in 15 primipara cases and in 14 multipara cases, pO2 levels for the second born were significantly lower (both being p < 0.05) than for the first born. Of twins vertex/non-vertex presentation, the second born in 9 primipara cases showed significantly lower pH, pO2 and HCO3 levels (p<0.05). Also for multipara twins, the second born had significantly lower pH (p<0.05) and significantly higher pCO2 (p<0.05) values. Umbilical cord blood gas value findings indicate unfavourable conditions for second born twins.

Parents of a sample of 76 same sexed pairs of twins aged 3 to 9 months completed a mailed similarity questionnaire. It included the Bonnelykke et al.'s questionnaire and a four anthropological variable scale. To improve each of these two methods, three other combined methods were carried out and results were compared with the biological zygosity diagnosis. The Bonnelykke et al.'s classification combined with anthropological scale (method 4) gave only 1.2% misclassified in the whole sample. It is concluded that zygosity diagnosis using this type of procedure to distinguish MZ and DZ pairs would be important not only for epidemiological study but also for pediatricians and parents.

A case of 8 trisomy syndrome, identified on the basis of the heat denaturation technique, has been detected in a patient with mental retardation, congenital malformations, and peculiar ocular anomalies. This observation confirms that, even without evidence of mosaicism, this chromosome does not seriously upset the viability of cells and could be less deleterious to the individual than trisomy 13, 18, or 21.

The mental development is not too much impaired and the skeletal abnormalities are mild, as compared to those found in previously reported cases. On the other hand, the cardiovascular system is involved and the ocular lesions are complex. They include prominent eyes, epicanthus, hypertelorism, antimongoloid slant of palpebral fissures, ectopia of lacrimal point, megalocornea, enlarged iridocorneal angle, subatrophic papilla, tortuosities of retinal vessels, absence of deep perception.

The living habits of 289 twins in a one-year birth cohort beginning during pregnancy and followed up to adolescence were compared with those of 11,623 singletons and two sets of controls matched either by maternal factors and place of residence only or by these and perinatal morbidity, all from the same cohort. The twins went in for sports more often than the singletons or any kind of controls. A nonsignificant trend was found indicating that twins smoked less often than their matched controls. The twins also used alcohol less often than their controls. The intrapair similarities of twins were higher than the similarities of twins and either type of controls in all four variables tested: sports, smoking, use of alcohol, as well as having been drunk.

Two cases are reported of MZ twin pairs with a history of retarded speech and universal stammering followed by concordant agrammatism.

A survey of a number of twin pairs reveals, however, discordance in speech development, in stammering and in language proficiency: agrammatism is, therefore, e genotypical trait.

Although the heritability of general intelligence has been the subject of much study and can be considered to be well established, the heritability of specific factors in intellectual functioning has not as yet been the object of such intense scientific inquiry.

The recent work of Nichols (1965), utilizing a large sample of twins from the National Merit Scholarship testing program and employing a battery of tests measuring achievement in five different academic areas, is an excellent pioneering study. Nichols used a composite score derived from the five subtests, presuming this to be a measure of general ability, and he found that MZ twins were much more alike than DZ twins with respect to the composite score; about 70% of the variance in the composite could be attributed to heredity. The novel result of his investigation is his discovery that the specific subject areas also have significant heritability components. When the influence of the composite general ability upon subtests was removed statistically, the residual subtest scores also showed considerable heritability.

A limitation of Nichols' findings is that the subtests do not offer any potential theoretical link to basic factors in mental functioning. Such measures, however, may be provided by the fundamental factor analytic explorations of Guilford and his associates during the past twenty years. Since our own interest has been in the area of creative thinking and esthetic judgment, we have restricted ourselves in the present investigation to the use of tests in those areas whose factorial classification has been studied previously.

The hypothesis of seasonality of twin births was investigated in two important maternity hospitals in the State of Sāo Paulo, Brazil. The study included 1,386 twin births that occurred among 154,699 deliveries from 1984 to 1993. No evidence of seasonality has been detected either for the twin birth rate considered as a whole or for dizygotic twinning rate. The distributions of these rates fitted well sinusoidal regression curves but the cyclic trend did not correspond to any specific season.

An old problem of gemellary research lies in the determining cause of gemellogenesis. Two new observations of the family order, in agreement with other similar results, are further evidence in favour of the opinion that in the origin of twins a certain part can be played by some hereditary property of the paternal side.

Gemellary research is the best method to establish the respective importance of heredity and environement in the developement of man. A new problem of gemellary research has arisen owing to the fact that MZ twins can differ in some illness, without the common causal analysis heredity-environement being able to give a satisfactory explanation. For example in the case of cancer it is demonstrated that the “autonomous” genesis of an illness must be taken into account i. e. without an appropriate hereditary cause or without an esogenous cause visible for all. Such an “autonomous” genesis of an illness has become undestandable only on the basis of microphysical knowledge.

The hereditary component of the chronological development of bones and teeth has been studied, in 40 twin pairs aged 5-7 years, through dental age (defined on account of the mineralization of the permanent dentition's dental buds) and bone age (denned on account of the presence and form of the hand ossification nuclei).

The statistical analysis shows a correlation coefficient of 0.95 in MZ and 0.84 in DZ twins for dental age; and of 0.94 in MZ and 0.81 in DZ twins for bone age.

The following are therefore the estimates of the hereditary component (based on Holzinger's formula) for the two parameters studied:

We evaluated the relative influence of genetic and environmental factors on leukocytes, lymphocytes, neutrophils, monocytes, erythrocytes, reticulocytes, hemoglobin, platelets, T lymphocyte subsets, and NK marker cells in monozygotic (MZ) and dizygotic (DZ) twins. Peripheral blood components were measured in 154 healthy twin pairs (118 MZ pairs and 36 DZ pairs) and lymphocyte subsets in 48 healthy twin pairs (38 MZ pairs and 10 DZ pairs).

Platelets and erythrocytes showed higher intraclass correlations in the MZ pairs than in the DZ pairs in the males. However, in the females, the intraclass correlations for these components were similar between the MZ and DZ pairs. Neutrophils and monocytes showed low intraclass correlations in the MZ pairs. The absolute number of CD4+ cells and the CD4+/CD8+ cell ratio showed high intraclass correlations regardless of age in the MZ pairs in both males and females. The intraclass correlation for the absolute number of CD3+ cells was low in pairs aged ≥ 60 years in the MZ pairs. The absolute numbers of CD16+ cells and CD56+ cells showed high intraclass correlations regardless of age in the MZ pairs.

The maternal risk factors that correlate with small-for-dateness among twins have been analyzed using a sample of 659 twin pairs and a matched sample of singletons. Non-marital status, job involvement, and the previous delivery of a low-birth weight (<2,500g) infant present a negative interaction with twinning, as low gestational age-adjusted birth weight does not correlate significantly with these risk factors among twin gestations, while it does among singleton gestations. On the other hand, the effects of parity, habitual maternal weight, smoking during pregnancy, and twinning are additive on gestational age-adjusted birth weight. Indeed, the decrease in adjusted birth weight associated with these risk factors is of the same magnitude among twins and singletons and is statistically significant in both cases. These findings suggest that exposure of twin pregnancies to these latter risk factors, and particularly to smoking during pregnancy, can lead to the delivery of newborns with extremely low birth weights.