Male Sensitivity Written in the Genes In human development, certain genes act as master switches, ensuring that we are born with similar attributes (one head, two lungs, 10 fingers) in nearly all circumstances. Such genes tend to be highly reliable and resistant to environmental factors. But the gene responsible for activating male development is surprisingly unstable, leaving the pathway to male sexuality fraught with inconsistency, a study finds. The SRY gene on the Y chromosome sets off the growth of male sex organs in human embryos (all of which start out essentially female). To study the gene, researchers at Case Western Reserve University looked at families in which daughters inherited a Y chromosome, a rare occurrence in which SRY fails to fire, leaving a genetically male embryo to develop as a sterile female. They found that SRY is highly vulnerable to environmental factors, allowing the slightest interruption to significantly alter male sexual development. That leads to a wide divergence of testosterone-related male attributes (among them muscle mass, aggression and genitalia development) from one man to another, according to the study, which was published in The Proceedings of the National Academy of Sciences. The variability probably serves an evolutionary purpose, said one of the authors, Dr. Michael Weiss. "This tenuous switch is what underlies the variability of testosterone secretion in utero," he said, producing men with a wide range of gender styles and capabilities that can help ensure a community's survival.