Twin psychology has been studied to a very limited extent. Only few remarkable peculiarities of the twins' development are known for sure, such as delayed intellectual development, language retardation with frequent cryptophasia, difficulties and fragility of self consciousness, reduced sociability. These and other peculiarities may result from a number of different factors: biological, peculiar parental attitude, and especially the twin situation itself, the psychology of the twin as an individual being a function of the psychology of the twin pair. Twin peculiarities have for a long time been ignored. On the one hand, psychology ignored the couple reality (it was a “one-body” psychology), and on the other, the twin method classically postulated that twins are not atypical and may therefore be used as a test for the general population. As a matter of fact, twin psychology opens a new way to science. The twin situation may serve as a paradigm for the general study of the couple effect, namely, for the objective analysis of personality as a consequence of the relations between self and others. For certain traits of personality, it is already known that MZ twins brought-up together are significantly less similar than MZ twins brought-up apart. In fact, couple effects may mask or considerably reduce genetic factors.

A total of 123 twin pregnancies were studied, 68 by records analysis only, and 55 by serial haematological assessment in detail. The incidence of low haemoglobin levels is found to be greater than in singleton pregnancy; evidence of iron and folic acid deficiency is common in sternal marrow aspirates but not in peripheral blood films. Routine haematinic prophylaxis in twin pregnancy is not advocated.

The historiography of genetics is not very extensive. As a tribute to Gregor Mendel the Genesis of modern Genetics is briefly described in its full range, from the Paleolithic Age to Lysenko. Since the earliest times, the knowledge of heredity had been growing in proportion with the development of ideas on reproduction and continuity of the species. For various forms of life, the basic problems of interest remained essentially the same during all historical periods; only the emphasis shifted from the gross to the more detailed, and the answers oscillated between the theories of preformation and epigenesis. Against this historical background, Mendel and the Mendelian laws stand out as the basic foundation of genetics.

The vital growing ideas on generation and evolution of plants, animals, and man are briefly reviewed and exposed as they had occurred before and after Mendel. The historical sketch leads us to ancient Assyria, India, the biblical times, and the classical Antiquity, to the early Greek philosophers and physicians whose works are sampled to illustrate the ancient beliefs concerning the role of male and female semen in generation, and concerning the proportional share of parents in the formation of body and mind of the offspring. The medieval knowledge on heredity is at its highest in the writings of Albertus Magnus whose work represents the refinement of scholastic science.

In these earlier times people did not know the exact functions of the organs of generation, or the true nature of sex. After the invention of the microscope in the 16th, and with a more liberal spirit of research in the 17th centuries, the sexual life and the male and female germ cells of many bisexual beings, including man, were gradually discovered during the 200-year period from 1677 to 1877. This happened with all sorts of speculation about heredity and about the origin and evolution of life. Meanwhile, many practical and theoretical hybridizers saw the various peculiarities in the filial generations, and many observed normal and abnormal characters transmitted from parents to offspring. Yet, none could formulate these observations into a mathematical regularity of inheritance until the reports of Gregor Mendel in 1865 on the result of his plant-crossings.

His discovery was buried, however, until 1900 when other biologists came to the same results, and revived and accepted his rules as the laws of heredity. From then on, modern genetics advanced rapidly and branched into many activities, everywhere fully supporting the views of Mendel. Thus, Mendel is truly the father of genetics, except for communist Russians whose political theories demand the denial of Mendelian heredity and the adherence to older discarded theories.

Genetics has the destiny to solve many practical problems in the life of nations, and to investigate a number of important yet unknown factors, in order to utilize the gene theory of heredity, and the “nuclear energy”, for a wider and brighter service of humanity.

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.

A study of the finger and palm prints and of the chromosomal patterns of 122 Down's syndrome patients, among which 9 had a D/G or a G/G translocation and 4 a mosaic, could not detect significant differences between dermatoglyphics of regular trisomic mongoloids and of the translocated ones. Mosaic cases had few Down's syndrome dermatoglyphic peculiarities, in relation with the number of the trisomic cells in the chromosomal set.

A large-scale adoption study is underway at the University of Texas where IQ test scores are available for over 1200 women who gave up their children for adoption immediately following delivery. The adopted children and their adoptive families are now being located and tested and, if current trends continue, complete data should eventually be available on over 400 families. About half of these families will contain two or more adopted children or both adopted and natural children. At present, data are available for 89 families. The results are consistent with twin data and support a genetic hypothesis. The biological mother-adopted child correlation is 0.37 whereas the adoptive mother-adopted child and adoptive father-adopted child correlations are only 0.22 and 0.04 respectively. There are 59 pairs of unrelated children reared together in this sample and the IQ correlation for these children is only 0.13.

Twins are regularly reported to invent languages of their own, unintelligible to others. These languages are known as autonomous languages, cryptophasia or idioglossia. Despite current belief, this is not a rare phenomenon. Autonomous languages exist in about 40% of all twins, but often disappear soon. In this study, nine autonomous languages are compared: the circumstances in which they emerge, how these languages relate to the parents' language (the model language) and how they are structured. The prototypical situation is one in which two or more close siblings (not necessarily twins) grow up closely together during the language acquisition period. If an adult model language is frequently absent, the children use each other as a model and acquire the language imperfectly. The language may stabilise at that level. If a model is completely absent, the children probably do not create a language. In all cases known, the language consists of onomatopoeic expressions, some invented words, but for the greatest part of words from the adult language adopted to the constrained phonological possibilities of young children. These words being hardly recognizable, the language may turn out to be completely unintelligible to speakers of the model languages, but they resemble each other in that they lack morphology and that word order is based on pragmatic principles such as saliency and the semantic scope of words. Neither the structure of the languages nor its emergence can be explained by other than situational factors.

OBJECTIVE. The purpose of this study was to evaluate the relationship between the yolk sacs separated or not separated by septum and chorionicity twin pregnancies scanned early in the first trimester, and the relation between size and morphologic features of the yolk sac and the outcome of twin pregnancies.

RESULTS. In all 38 sets of twins two yolk sacs were identified. During the first trimester of a dichorionic twin pregnancy, the yolk sacs were always separated by a septum and not separated (“Eight” sign) in monochorionic twin pregnancy. In five cases, one of yolk sac was abnormally large (> 8mm) and had thin wall. Four of the five mothers spontaneously aborted during the next 2-3 weeks. In one case of monochorionic twin ectopic pregnancy two yolk sacs were seen normally.

CONCLUSION. The sonographic identification of yolk sacs in multiple pregnancies allows an early and efficient recognition of presence and chorionicity of twin pregnancy, both in intra – and extrauterine. Identification of abnormal yolk sac or yolk sacs suggests death of one or all embryos.

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.