Interviewee: Matt Ridley. The Y chromosome carries a relatively tiny number of genes. However, it carries an important region, known as SRY, that determines the male sex. In the seventh week of development, this gene triggers a flurry of biological activity that causes a human embryo to develop into a male. In the absence of a Y chromosome, the embryo will continue to develop as a female. (DNAi Location: Genome > Tour > Genome spots > Chromosome Y: Sex-determining factor > A chromosome Y story)

Y chromosomes are found only in men. A man inherits one from his father. Most mammals are born female, unless they have a Y chromosome, which turns them into males. There's a gene on this Y chromosome, which is responsible for this masculinization process. It's called SRY, and it sets off a cascade of events in the masculine brain that changes it. So if you want to know why your father or brother or son doesn't stop to ask for directions, blame the SRY gene on the Y chromosome.

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