Identify the master switches and explain how they operate in sex determination in mammals. Solution In mammalian sex determination, the master switch is the presence or absence of the SRY gene, which is ordi-narily located on the Y chromosome. In the presence of the protein product of this gene, which acts as a DNA-binding protein, certain cells of the gonad (Leydig cells) synthesize androgens, male-inducing steroid hor-mones. These hormones are secreted into the blood-stream and act on target tissues to induce the transcription-factor activity of the androgen receptors. In the absence of androgen-receptor activation, development proceeds along the default pathway leading to female develop-ment. The factors that activate SRY expression in the testis are not understood. Because the master switch here is the actual presence or absence of the SRY gene itself, it is likely that the regulatory molecules that activate SRY are present in the indifferent gonad early in development. 3.
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.