Most of our cells have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (see fig. 4.15, p. 146). A sex chromosome can either be a large X chromosome or a small Y chromosome. Every egg contains an X chromosome, but half of the sperm carry an X while the other half carry a Y. If an egg is fertilized with an X-bearing sperm, it produces an XX zygote that is destined to become a female. If it is fertilized with a Y-bearing sperm, it produces an XY zygote destined to become a male. Thus the sex of a child is determined at conception (fertilization), and not by the mother's egg but by the sperm that fertilizes it (fig. 27.1).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.