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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Drosophila Sex Determination: Every Cell for Itself Figure 16-3. The pathway of sex determination and differen-tiation in Drosophila. The X:A ratio is evaluated through the interaction of numerator and denominator monomeric protein subunits that interact to produce an active complex referred to as NUMNUM transcription factor. The level of active numerator transcription factor determines whether Sxl (Sex lethal) is to be permanently turned on or is to remain off. If Sxl is on, then the female sex differentiation pathway is turned on, ultimately causing splicing of a form of the dsx mRNA that produces a transcription factor that represses male-specific genes. If Sxl is off, then the sex differentiation pathway is not activated and the default dsx (doublesex) splicing pattern creates an mRNA encoding a female-repressing transcription factor.

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