The 46 chromosomes (22 pairs plus two sex chromosomes) in human body cells contain about 20,000 to 25,000 pairs of genes, and several thousand disorders are directly the result of a defect in a gene or a chromosomal abnormality. Such genetic defects or chromosomal abnormalities vary enormously in their effects, from unnoticeably minor to severe physical and/or mental problems.
This section covers a few examples of chromosome and gene disorders that affect many body systems and become apparent during childhood. Some of these conditions are caused by an extra or absent chromosome or an abnormality in a chromosome. For example, Down’s syndrome is due to an extra chromosome, and Turner’s syndrome is due to an absent chromosome. Other conditions in this section are caused by defective genes. These include cystic fibrosis, muscular dystrophy, and neurofibromatosis.
Genetic disorders that mainly affect one part of the body are covered in the relevant sections of the guide. The principles of chromosome and gene disorders, including how and why they occur and patterns of inheritance, are discussed elsewhere (see Genetic disorders).
For information on the structure and function of chromosomes and genes, see Genes and Inheritance.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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