More than 5,000 different genetic disorders have been identified in humans. Some, such as cystic fibrosis and muscular dystrophy, are so serious that people who risk passing them to the next generation may choose not to have children. Others, such as color blindness, are rather benign. In between is a wide range of conditions, the precise causes of which scientists are now beginning to unravel.

WHAT CAUSES GENETIC DISORDERS?
A genetic disorder is caused by a mutation, or change, in a gene. A gene is a sequence of building blocks of deoxyribonucleic acid (DNA) that instructs a cell to manufacture a particular protein. Genes are arranged on 23 pairs of chromosomes located in the cell's nucleus.

HOW DO GENETIC DISORDERS DEVELOP?
The symptoms of some genetic diseases are caused by an abnormal or absent protein, which in turn is caused by a defect in the gene that tells the cells to produce (or not produce) that protein. The genetic mutations that give rise to genetic disorders may be passed from one generation to another or arise spontaneously in the reproductive cells (sperm and egg) that unite at conception to form a new individual. When the mutation occurs spontaneously, the affected child is the first in the family to have the condition.

Genetic diseases are transmitted in three main patterns—dominant, recessive, and sex-linked. Both parents must carry and pass on the same defective gene to transmit a recessive disorder to their children. Only one parent must carry a defective gene for a dominant disorder to be transmitted. Sex-linked disorders are passed from mother to son on the X chromosome, one of the two chromosomes that determines gender. Geneticists estimate that all people have at least seven or more recessive mutant genes, but the chance of two unrelated people having the same mutant gene is very low.

Another type of condition involving heredity occurs when a child receives an abnormal number of chromosomes. Down's syndrome, for example, arises when an embryo's cells contain one extra chromosome. These conditions, however, are not passed through the generations in predictable patterns. Instead, they are the result of accidental errors in chromosome distribution occurring at conception.

WHEN SHOULD I SUSPECT THAT MY CHILD HAS A GENETIC DISORDER?
Possible symptoms of genetic disorders are extremely wide ranging. Rather than worrying about these disorders after children are born, parents should review their family medical histories—on both sides—before having children. Do your aunts, uncles and siblings (and their children) have any particular disorders in common? If so, find out everything you can about the disorder, and mention it to your physician when you start planning your own family.

IS MEDICAL ATTENTION NECESSARY?
Yes. If there is an identified genetic disorder in your family or your partner's, ask your doctor for a referral to a genetic counselor or medical geneticist. In recent years, scientists have developed tests to determine whether individuals carry specific disease genes that run in their families. Such tests can help prospective parents make their own choices about having children. Some common genetic diseases can also be detected prenatally in the fetus.

WHAT TREATMENTS ARE AVAILABLE?
Unfortunately, there is currently no way to correct a defective gene. Improved therapies, however, are giving children with serious genetic disorders longer, more comfortable lives. The outlook may improve even more in the next few decades. Researchers the world over are in the process of conducting a huge study, called the human genome project, in which all of the genes that make a human are undergoing systematic analysis. This project has already spawned numerous diagnostic tests and experimental treatments. Researchers are now working on ways to deliver healthy genes or proteins precisely where they are needed in the bodies of afflicted individuals.

COMMON GENETIC DISORDERS
Sex-linked recessive disorders

• muscular distrophy

• hemophilia

• retinitis pigmentosa (an eye disease that causes tunnel vision and progressive loss of sight)

• color blindness

Recessive disorders

• cystic fibrosis

• sickle-cell anemia

• thalassemia

• Tay-Sachs disease

Dominant disorders

• Huntington's disease (a progressive neurological disease that develops in adulthood)

• polydactyly (extra toes or fingers)

• achondroplasia (formerly called dwarfism)

• adult polycystic kidney disease

Mulitfactorial disorders
(produced by interaction of a genetic predisposition and some environmental trigger)

• diabetes

• high blood pressure

• coronary heart disease

• cleft lip and palate

Excerpt from THE DISNEY ENCYCLOPEDIA OF BABY & CHILD CARE, © 1995 DSH Communications, Inc. This material is based on current medical research and, to the best of the editors' knowledge and understanding, is accurate and valid. However the reader should not use information contained in this material to alter a medically prescribed regimen or as a form of self-treatment, without seeking the advice of a licensed physician.