Genetic Engineering

Human Disorders and Gene Therapy

Genetic disorders are the harmful effects on an individual caused by inherited genetic diseases or mutations. Usually genetic disorders are recessive, so they are only expressed in a small percentage of the population, but a much larger percentage are carriers. When expressed in the homozygous recessive individual, they often code for the wrong protein or amino acid sequence. There are many genetic disorders; however, two are common in today's population: hemophilia A and sickle-cell anemia.

Hemophilia A is a recessive sex-linked genetic disorder that is exhibited by approximately 1 in every 10,000 Caucasian males. Multiple genes code for the multistep process of blood clotting. Mutation in any one of them creates hemophilia A, the inability to form blood clots. Individuals with this disease must avoid all cuts and bruises, both internal and external. In severe cases, the individual may lose massive amounts of blood.

Sickle-cell anemia is a recessive genetic disorder that affects 1 in every 500 African Americans. A mutation of an allele causes the allele to code for a sickle-shaped hemoglobin molecule. The defective hemoglobin molecules do not transport as much oxygen as the hemoglobin in normal red blood cells because they tend to rupture. They also sometimes wedge in a blood vessel, blocking the flow of blood cells. Tissues and organs downstream from the obstruction may suffer serious damage. Interestingly, sickle cells are a survival advantage in certain areas because they are a defense against malaria and may protect some people from the disease.

Although most genetic disorders cannot be treated because of technology limitations, certain ones such as phenylketonuria (PKU) can be treated if discovered in time. For instance, a baby with PKU is maintained on a low-phenylalanine diet to prevent mental retardation caused by its buildup.

Most humans inherit genetic disorders because of the improper functioning of a particular gene sequence. In theory, replacing the defective gene with a healthy one should solve the problem, which is the essence of gene therapy. Although in its infancy as a treatment for disorders such as hemophilia and sickle-cell anemia, patients have received genetically engineered cells as an experimental treatment for missing genes. At this time the data are incomplete regarding the results. Currently, researchers are attempting to engineer cells, usually from bone marrow, to enhance the abilities of immune cells to fight off cancer and resist infection by HIV. This approach may lead to an effective treatment for nonhereditary diseases.

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Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

To order this book direct from the publisher, visit the Penguin USA website or call 1-800-253-6476. You can also purchase this book at and Barnes & Noble.


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