Sexual differentiation is a sequential process beginning with the establishment of the genetic sex at fertilization of the oocyte by a sperm. In the fertilized oocyte the testis determining gene drives the process of sexual differentiation into the male direction. In normal development the SRY gene that codes for the testis determining factor is located on the short arm of the Y chromosome. In males the indifferent gonad will turn into a testis, in females the gonad becomes an ovary. In the mammal, the heterogametic sex (XY) is male and the homogametic sex (XX) is female. The male phenotype is formed as a consequence of endocrine secretions of the testis. Female characteristics will develop in the presence of an ovary or if no gonads are present. The presence or absence of the testis determining gene and the presence of XY or XX chromosomes are known as genetic sex and chromosomal sex, respectively (Table 1). Subsequently, the type of gonad depicts the gonadal sex, the male or female anatomic characteristics form the phenotypic sex. Ambiguous differentiation of sexual characteristics occurs if one of the steps is disturbed or lacking.

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Ter Meulen Fund, Royal Netherlands Academy of Arts and Sciences (Amsterdam),Foundation for Urological Research (Rotterdam),Stichting Urologie 1973 (Haarlem),Merck, Sharp & Dohme B.V. (Haarlem), Abbott B.V. (Amstelveen)
F.H. Schröder (Fritz)
Erasmus University Rotterdam
Erasmus MC: University Medical Center Rotterdam

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