In the first weeks after conception the structures that will become the reproductive organs, or gonads, are the same in males and females (see 1 Figure 5.2a). Differentiation begins about 6 weeks after conception. Genetic signals determine whether the mass of undifferentiated sexual tissue develops into male or female gonads (Dragowski et al., 2011; Wilhelm et al., 2007). At this time an SRY gene product (or products) in a male fetus triggers the transformation of embryonic gonads into testes. In the absence of SRY,
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I Figure 5.2 Prenatal development of male and female internal duct systems from (a) undifferentiated (before 6th week) to (b) differentiated.
and perhaps under the influence of the DSS or other femaleness gene, the undifferentiated gonadal tissue develops into ovaries (see I Figure 5.2b; Dragowski et al., 2011).
Once the testes or ovaries develop, these gonads begin releasing their own sex hormones. As we will see next, these hormones become the critical factor in further sex differentiation, and genetic influence ceases.