General rules about sex determination and differentiation, elucidated in recent years, are presented. It Is also suggested that the genetic information carried by two sex chromosomes XX or XY, or by the helerochromatic chromosome Y or heterochromatic X, is basically important for the proper development and function of tissues of mesodermal origin, and that the somatic and gonadal abnormalities of Turner's syndrome originate from defects in tissues of mesodermal originndash;ie, bone, connective tissue and somatic cells of the gonads. The preponderance of dysgenetic testes in individuals with XX/XY as well as XO/XY mosaicism is explained by the fact that in the presence of the Y chromosome the testis has a better chance developing than the ovary.
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