The Key to Masculinity; Men: The Scientific Truth; Scientific American Presents; by Lahn, Jegalian; 6 Page(s)

What a difference a Y can make. In humans, the tiny Y chromosome-one of the smallest of the 46 chromosomes that carry all our genes-essentially dictates a person's sex. Genes on the Y chromosome trigger the development of male features-everything from the formation of the testes and the ability to produce sperm to the emergence of secondary sexual characteristics, such as facial hair and a deep voice. Without a Y chromosome and its resident maleforming genes, an embryo will develop as a female. Thus, at the most fundamental level, the problem of understanding the biological differences between the sexes can be reduced to a problem of understanding the Y chromosome and how it encodes maleness.

Under a microscope, the Y chromosome appears as a small rod about one third the size of the X-the other sex chromosome, present in both males and females. (Males, who normally have one X and one Y chromosome, are designated XY; females normally have two Xs, hence the XX designation.) Although it has been known for decades that it takes a Y to make a male, until about 10 years ago scientists believed that the human Y chromosome contained few genes aside from those that determined maleness. This view stemmed, at least in part, from the relative abundance of "junk" DNA-repetitive DNA sequences that do not encode any proteins-on the Y chromosome.