Two genes lock in a tur-of-war to determine whether a mammal embryo will become male or female , a new study suggests. One of the genes, called SRY, has long been known as the master switch that makes an embryo become male. The New Work suggests that a second gene, DAXI , tries to block its effect. Dr. Michael Weiss, who studies the genetics of sex determination at the University of Chicago, called the study an important step toward understanding how genes work together to produce either a male or a female.