"Somatic Cell Genetics" has been established on the basis of the three main events. 1) Somatic cell culture in vitro (Carrel, 1912). 2) Cell fusion by HVJ (Sendai virus) (Okada et al., 1957). 3) Identification of individual chromosomes (Casperson et al. , 1969). Experimental laboratory work about human genetics was opened by the progress in somatic cell genetics. i) On the basis of decreasing in human chromosomes from (human × rodent) somatic cell hybrids in culture (Weiss & Green, 1967), a study on genetic map of human cells was opened and about 80 genetic markers have been mapped on individual human chromosomes. Linkage of genes could also be analyzed in those hybrids. ii) Genetic classification of human congenital diseases could be done by complementation test utilizing cell fusion (Bootsma et al., 1972; Robins et al., 1975). Takebe et al. classified about 30 Japanese family of Xeroderma Pigmentosum. iii) Direct analysis of defective functions corresponding to congenital diseases became to be done by a new technique to insert macromolecules into viable cells utilizing cell fusion reaction with HVJ, which was opened by our group.

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It is known that Down's syndrome is associated with a number of different types of chromosome abnormality. Before discussing the etiology of this syndrome, therefore, it is very important to analyse chromosome conditions of each patient and to accumulate relevant data as many as possible. For this purpose, we have examined 2,000 cases of Down's syndrome since 1961. They were selected irrespective of maternal age. Of the 2,000 cases, 1,881 were found to be of the standard trisomic type, including 4 cases with 48, XXY, +21, and one case with 48, XYY, +21. There were 93 cases with translocaions; 49 of them showed t(Dq21q) type and 44 had t(21qGq) type. 26 cases were mosaics, most of them being mixture of normal cells and trisomic cells. The mean maternal age at the patient's birth showed 30.34±4.76 years for the standard trisomic type and 29.23±5.36 years for the mosaics, apparently higher than in the control (27.50 ±4.29 years). In the sporadic cases of translocations the mean maternal age was increasing for the t (21q21q), while it was decreased for the (Dq21q). In the present study, we have recorded data of five families with two trisomic Down's syndrome in a sibship. They were found among 817 families in which first born chiliden had Down's syndrome. The recurrence risk in parents who had the first born trisomic Down's syndrome may be estimated as about 1/100. There was no evidence due to the increased risk of nondisjunction in relation to maternal age factor in the present five cases.

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[in Japanese]

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