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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Sex Determination in Mammals: Coordinated Control by the Endocrine System Genetics In Process 16-2: The mutational analysis of mammalian sex determination A molecular map of the distal part of the short arm of the human Y chromosome. The gene for testis-determining factor (TDF) was localized to a 34-kb region on the basis of the following logic: to the left of this region, the DNA sequences are the same on the X and Y chromosomes. (These sequences to the left are called the pseudoautosomal region.) Pieces of the Y chromosome starting at the telomere and ending at position 34 (kb) of the map are sufficient to cause the sex-reversal syndrome. Within this region, much of the DNA is repeated many times throughout the genome, but eight blocks of DNA unique to the Y chromosome were identified (the black regions). Cross-hybridization tests showed that only one of these (pY53.3) is present in bovine and mouse Y chromosomes. This fragment encodes SRY. In some humans with sex reversal, point mutations in SRY have been found. (From A. H. Sinclair et al., Nature 346, 1990, 240.)

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