英语时差：动物界的染色体有何不同？（上）

In mammals, the sex of a baby is determined by the combination of chromosomes it inherits from its parents. Nearly every cell in a male body has two different sex chromosomes–one is shaped like an X and one is shaped like a Y. Females also have two sex chromosomes in each cell but they are both X-shaped. When the male sperm and the female egg come together, each one contributes one sex chromosome. The female always contributes an X chromosome while the male can provide either an X or a Y. If the male provides an X then the embryo has two X chromosomes and becomes a female. If the male provides a Y then the embryo has one of each and grows into a male.