One key determinant of sex determination is the SRY gene itself. This gene is located on the Y chromosome, and the protein it encodes is a DNA binding protein that regulates other genes. The normal action of the SRY protein is to induce transcription of a second DNA binding protein, SOX9. The SOX9 gene is located on the X chromosome. The SOX9 protein acts to induce transcription of other genes that lead to the development of testes. Mutations that disrupt SRY or SOX9 protein function thus block the formation of testes. XY individuals with these types of mutations develop as sterile females.
To investigate the possibility that the two female athletes have defects in their SRY or SOX9 genes, you decide to perform a gel-shift assay. Because SRY and SOX9 function as DNA binding proteins, it is possible to detect when they bind to a target DNA fragment. DNA fragments bound to SRY or SOX9 protein will migrate more slowly in an electrophoretic gel, since the DNA-protein complex has a higher molecular weight than the target DNA fragment alone. Binding of the protein to the target DNA thus “shifts” the DNA band higher up on the gel.
You clone the SRY and SOX9 sequences from both athletes and use them to artificially produce SRY and SOX9 proteins in vitro. You then compare the DNA binding ability of the athletes' proteins to proteins derived from a healthy male control. You obtain the following results.
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