Nos explications ont pour but de faciliter la compréhension de la concordance et surtout la discordance des jumeaux MZ et DZ moyennant les analyses chromosomiques. Dans les dernières années tout un nombre d'études concernant les jumeaux ont été publiées où chaque jumeau accusait le même phénotype et caryotype. Il s'agit de jumeaux MZ avec les syndromes de Down (Mikkelsen et Melchior, 1962), de Klinefelter (Bertrand et al, 1966; Holub et al, 1958; Nowakowski et al, 1963; Vague et al, 1968) et de Turner (Decourt et al. 1964; Frasier et al, 1961; Nance et Uchida, 1964; Turner et Zanartu, 1962); aneuploïdies doubles (Hustinx et al, 1961; Turpin et al, 1964); mosaïques gonosomiques (Mikkelsen et al, 1963) et caryotype XYY (Rainer et al, 1969).

Le problème de la discordance des jumeaux MZ nous préoccupe davantage que celui de la concordance, car en effet la discordance a été l'objet de maintes études. Le résultat n'est pas satisfaisant pour tous les jumeaux étudiés (Nielsen, 1967; v. Verschuer, 1927). À ce sujet, je tiens à rappeler la discussion qui a eu lieu dans l'Institut Mendel, au cours du Premier Symposium International de Génétique Médicale du 6 au 7 septembre 1953, entre Kallmann et v. Verschuer. Parmi les premiers savants il faudrait citer Meirowsky (1926) qui expliquait les causes de la discordance par un partage inégal du plasma lors du clivage du disque embryonnaire.

The incidence of twinning in Western Nigeria is very high, being 45‰ (Nylander, 1969a), as compared with 11-12‰ in Caucasian populations (Registrar General, 1967). A recent study of twinning in Ibadan, in which zygosity of individual twin pairs was determined, has shown that the pattern of placentation and zygosity in this population is also very different from that in Caucasian populations.

As many as 1475 consecutive newborn twin pairs delivered in three hospitals in Ibadan between March 1967 and April 1969 were investigated.

The placentae of the twins were examined macroscopically by the author personally to determine the form (whether single, fused, double or separate) and the membrane relationships (whether monochorionic or dichorionic) as shown in Figs. 1 and 2. The membrane relationship was confirmed by histological examination.

Cord blood samples were collected from each twin pair and studied for (1) ABO, Rh (using antisera, C, c, D, E, e), MNSs and Gonzales blood groups and (2) G6PD electrophoretic pattern. Other blood groups like Fya, k and Jk, used initially, were discontinued later, because the gene frequencies in this population are such that they are uninformative for zygosity determination.

Samples of the placenta of each twin were sent in refrigerated containers to the Galton Laboratory, London University, for placental enzyme studies — phosphoglucomutase and peptidase (Hopkinson and Harris, 1968; Lewis and Harris, 1967).

With the revision of application of terms “endogamous” and “exogamous” the inbreeding levels have been calculated from certain parts of Madhya Pradesh. Higher inbreeding levels in persons with rural background are in conformity with other workers.

De nombreux auteurs estiment, sur la base de ressemblances électroencéphalo graphiques observées au sein d'une même famille ou chez des paires de jumeaux MZ, que l'EEG est déterminé génétiquement (Lennox et al, 1945; Weiss et Rudkowka, 1962; Vogel, 1965; Heuschert, 1965). Beaussart et al (1959) pensent même que cet examen peut servir de critère pour le diagnostic de monozygotie.

Nous avons tenté de vérifier si l'étude de la maturation de l'EEG conduisait aux même conclusions. Nous avons comparé le degré de maturation de l'électrogenèse corticale che: des paires de jumeaux concordants et discordants pour les groupes ABO, MN et Rh. Nous nous sommes adressés à des enfants et des adolescents. On sait que, chez ceux-ci, l'EEG comporte, en plus du rythme α, des ondes θ dont les fréquences sont de 4-7/sec, et des ondes δ de fréquences inférieures à 4/sec. Ces ondes lentes diminuent progressivement avec l'âge poui disparaître chez l'adulte normal. À un âge donné, leur nombre est très variable d'un suje à l'autre. Le degré de maturation de l'EEG peut donc être apprécié de façon simple par l'évaluation de la proportion d'ondes lentes du tracé.

Notre étude a porté, d'une part, sur 24 paires de jumeaux concordants sanguins, âgés en moyenne, de 10.5 ans (6-18 ans) et, d'autre part, sur 34 paires de jumeaux discordants âgés, en moyenne, de 9.3 ans (6-16 ans). Les sujets étaient d'origine rurale et appartenaien à un même groupe socio-économique. Le quotient intellectuel moyen était de 92.8 pour les concordants et de 93.5 pour les discordants.

Dans le laboratoire d'anthropologie de l'Institut Royal des Sciences Naturelles de Belgique, le Prof. Twiesselmann a organisé, en 1961, une enquête gémellaire au cours de laquelle une centaine de paires (jumeaux de même sexe) et de couples (jumeaux de sexes opposés) furent analysées. Seules 63 paires pour lesquelles nous possédons des données biométriques complètes seront envisagées dans cette note.

Neuf sérums sanguins — respectivement deux, deux et cinq pour les systèmes ABO, MN et Rh — permirent de séparer les 63 paires en deux groupes:

1) 30 paires de jumeaux concordants pour les neuf sérums;

2) 33 paires de jumeaux discordants pour au moins un sérum.

Les 33 paires étant certainement DZ, nous nous proposons d'évaluer, en termes de probabilité, le nombre k de paires DZ qui peuvent se cacher parmi les 30 paires de jumeaux concordants, dont la majorité est MZ.

Pour chacune des 30 paires de concordants, Mme Vrydagh a calculé la probabilité de monozygotie en utilisant la méthode de Maynard-Smith et Penrose (1954) qui est basée sur le théorème de Bayes de la probabilité des causes. Pour nos 30 paires de concordants, ces probabilités vont de 83 à 92%; elles auraient été plus élevées si plus de sérums sanguins avaient été utilisés.

Although published reports of cancer in twins are not numerous, there are case reports of concordance in a variety of tumors in twins. Cancer of the breast is most frequent; concordance has also been noted in uterus, gonads, eyes, stomach and rectum. In general, these reports tend to support the theory that genetic factors operate either in the concordance of cancer or in the site of the specific tumors. The significance of concordance remains in doubt, however. Reported study groups are not comparable because the materials and methods are disparate. In many reviews the methods of zygosity determination are questionable. The fact that not all cases of cancer in twins are reported introduces a constant source of bias. The following review illustrates this well.

In 1940, Madge T. Macklin reviewed the publications of tumors in MZ and DZ twins and concluded that tumors affected MZ twins far more than both members of a DZ pair. She observed concordance in tumor type, site, and age of onset, as more frequent in MZ than DZ twins. She later (1947) reexamined the same material and some additional twin pairs and concluded that MZ twins have identical tumors far more frequently than DZ twins. She emphasized that rare types of tumors were genetically determined.

In 1948, Busk et al reported on a series of 185 twin study pairs from the Danish Cancer Registry; they concluded that (1) there was a tendency toward a higher incidence of cancer in partners of MZ than of DZ cancerous twins; (2) these deviations from expected values were not considered statistically significant; and (3) there was a tendency for tumors in MZ twins to affect corresponding organs in both partners, this being not the case in DZ twins.

Studies on cancer in man and pure strain animals have shown that the malignant neoplasm, both in animals and in human beings, is not a single, genetically homogeneous disease: it consists of a lot of forms differing in localization, histological structure and etiopathogenesis. This circumstance is not always taken into account by the investigators in cancer research by the twin method, and especially in the determination of within-pair concordance/discordance, which is often a source of misunderstandings and faulty conclusions.

In order to establish a more objective criterion for estimating the real twin concordance in the studies of such diseases as cancer, we have worked out a quantitative index called Concordance Index:

where: CI = Concordance Index %; a = number of twin pairs, MZ or DZ, which are fully concordant for the principal features; b = number of partially concordant twin pairs; n = total number of twin pairs, MZ or DZ.

The accuracy of twin zygosity determination acquires a particular importance in cases of low penetrance of the disease. In such cases, even a minor inaccuracy in the zygosity diagnosis may considerably influence the twins' CI, and always in the same direction: any mistake in the determination of twin zygosity always leads to an artificial diminution of the role of hereditary factors. Therefore, we consider that the questionnaire method of twin zygosity determination, as well as any other insufficiently objective method, should not be used in the genetic studies on cancer.

We undertook an extensive review of world-wide clinical reports of leukemia in twins. Our survey was a careful one; exclusions are inevitable, but, if so, are inadvertent, except for undocumented “personal communication” cases. These have sometimes been included in large reviews of either leukemia or leukemia in twins. Also excluded are cases mentioned only tangentially in reports of cancer in large population groups, if no clinical details were given. In some instances, we were able to adapt available clinical information to our tables.

Leukemia appears to occur at four periods in the lives of twins: (1) the perinatal-congenital period; (2) early childhood; (3) late childhood; and (4) adulthood. Those cases occurring around the time of birth and the first year of life are combined under the title: Perinatal-Congenital. In Tab. I, Early Childhood extends from about 2 to 7 years, and Late Childhood from 7 through 12 years. All other cases have been considered as Adult. Two cases occurring in teenagers have been placed in this group in accordance with clinical practice. We have compiled 62 cases of leukemia in twins from clinical reports from the world publications in the following categories: Perinatal-Congenital, 15; Early Childhood, 26; Late Childhood, 9; Adulthood, 12. Comparative clinical details of all these cases will be published in a subsequent report.

Except for those instances about which writers admitted that the diagnosis of zygosity was in question, most reported clinical cases occurred in MZ twins. There were 39 cases of leukemia in MZ twins, 13 cases in DZ twins and 10 cases in whom zygosity was in doubt.

Important works on the question of hereditary influence on the occurrence of cancer with application of twin series are those by v. Verschuer and Kober (cf. Spranger and v. Verschuer, 1964), Jarvik and Falek (1961) and Harvald and Hauge (1963). Their findings are by and large consistent with most earlier reports (Schull et al, 1959) and indicate that hereditary factors do not seem to influence the development of cancer as such, but possibly play a role in the localization of the tumor. In view of the fact that (except for Harvald and Hauge's investigation) the twin series studied have been comparatively small and yielded statistically unreliable numbers of cancer of specific site, it seems to be of value to extend the data by reporting findings from the Swedish Twin Registry, comprising about 11000 unselected twin pairs.

The present paper reports mortality and morbidity in cancer of different site as well as concordance rates for MZ and DZ twins. In addition, the influence of smoking habits on cancer development has been analyzed, as this environmental factor is known to be associated with certain forms of cancer, particularly lung cancer (Smoking and Health, 1964).

Cancer data have been obtained from two sources, the Swedish Cancer Registry and the Registry of Causes of Death.

The Swedish Twin Registry in the present report comprises 10 945 twin pairs, and covers, in principle, all same-sexed twins born in Sweden in 1886-1925, and still living in the country as unbroken pairs at the time of compilation (1961). A detailed report of the compilation procedures and demographic data is given by Cederlöf (1966).

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: