Of the various aspects of Mongolism none is more intriguing than the problem of its etiology, and among the methods of investigation which contributed to the elucidation of this problem is the study of twins in which one or both are Mongoloid. Considering the comparatively high incidence of Mongolism generally, which is about 1 to 2 in every 1000 births,1 it is rather surprising to note that cases of Mongolism in twins are very rare. Moreover, the first cases to be reported in the literature were done so merely as medical curiosities, without realization of the bearing that they may have upon the question of etiology. They were, therefore, reported not only infrequently but also inadequately. It was as late as 1923 that Halbertsma2 pointed out, for the first time, the value of such cases for the elucidation of certain aspects of the etiology of Mongolism.
FRIEDMAN A. Mongolism in TwinsIts Bearing upon the Question of the Etiology of Mongolism. AMA Am J Dis Child. 1955;90(1):43–50. doi:10.1001/archpedi.1955.04030010045008
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