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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Drosophila Sex Determination: Every Cell for Itself Figure 16-5. Alternative splicing of tra and dsx transcripts. (a) Two forms of tra mRNA are produced. One form is present in both sexes and, because of a stop codon (UAG) in exon 2, does not encode a functional protein. The other form is female specific and encodes the active TRA polypeptide. (b) Different dsx mRNAs are produced in both sexes. In males, exon 4 is not included in two related male-specific mRNAs. Both male-specific mRNAs make related polypeptides, DSX-M, that act to repress transcription of female-specific genes. In females, exons 5 and 6 are not included in the dsx mRNA. The DSX-F polypeptide that is produced acts to repress tran-scription of male-specific genes. (Modified from M. McKeown, Current Opinion in Genetics and Development 2, 1992, 301.)

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