The cry of "It's a boy" or "It's a girl" marks the newborn child's first and most basic label of personal identity. But researchers' understanding of sex is undergoing profound and surprising changes due to new insights gained from sociology, biology, and medicine. The differences between females and males, once believed black and white--or pink and blue--now appear like a blurred rainbow of confusion. Researchers are learning, for example, that the Y chromosome has degenerated over the centuries. They have found that, in mice, some genes involved in early stages of sperm production are on the female X chromosome; and they have identified the gene that can produce ambiguous genitalia.

Genetic studies are revealing that men and women are more similar than distinct. So far, of the approximately 31,000 genes in the human genome, men and women differ only in the two sex chromosomes, X and Y, and only a...

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