In mammals, individuals with two X chromosomes are female, and individuals with an X and a Y chromosome are male
It had long been known that a gene located on the Y chromosome was sufficient to induce the gonads to form testes, which is the main male-determining factor in development, and researchers sought the product of this gene, the so-called testes-determining factor (TDF). For several years, the TDF was incorrectly thought to be a zinc finger protein encoded by a gene called BoY. Which of the following observations would most strongly suggest that BoY might not be the TDF? Explain your answer.
(a) Some XY individuals that develop into females have mutations in a different gene, SRY, but are normal at BoY.
(b) BoY is not expressed in the adult male testes.
(c) Expression of BoY in adult females does not masculinize them.
(d) A few of the genes that are known to be expressed only in the testes have binding sites for the BoY protein in their upstream regulatory sequences, but most do not.
(a) XY individuals that develop as females presumably lack the testes-determining factor (TDF). If BoY is normal in these individuals, it would strongly suggest that BoY is not the TDF. Although expression of TDF is necessary for testes development, this does not mean that it must be expressed in adult males once the gonads have already formed. Similarly, even though TDF expression is sufficient to induce testes formation, once the structures have been formed, TDF may not be able to exert any additional effect; thus, choices (b) and (c) are not considered strong evidence against BoY being TDF. Choice (d) is not compelling evidence against BoY being the TDF, because the TDF will not necessarily bind upstream of all of the genes whose expression it influences; some of the genes it regulates directly probably encode other transcription regulators that bind to regulatory sites different from the TDF site.
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