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come pregnant and less likely to undergo amniocentesis. About 5% of cases of trisomy 21 can be traced to nondisjunction in the sperm.

The age factor in Down syndrome may be due to the fact that meiosis in the female is completed after conception. The older a woman is, the longer her oocytes have been arrested on the brink of completing meiosis. During this time, the oocytes may have been exposed to chromosome-damaging chemicals or radiation. Other trisomies are more likely to occur among the offspring of older women, too.

Many of the medical problems that people with Down syndrome suffer are treatable, so life expectancy is now fifty-five years. In 1910, life expectancy was only to age nine! ■

Gene Therapy

Understanding how an absent or malfunctioning gene causes disease can sometimes be applied to prevent or treat the disease.

Gene therapy is a new type of treatment approach that alters, replaces, silences, or augments a gene's function in an attempt to ameliorate or prevent symptoms. Gene therapy operates at the gene level, but treatment of some inherited disorders at the protein level has been standard medical practice for years. For example, a person with hemophilia receives the missing clotting factor, and someone with cystic fibrosis takes cow digestive en zymes. Clinical Application 4.4 describes how a dietary regimen prevents the mental retardation that is associated with an inborn error of metabolism, PKU.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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