Causes and Symptoms (Magill’s Medical Guide, Sixth Edition)
Hereditary units called genes determine the majority of the physical and biochemical characteristics of an organism. Genes are composed of a chemical compound called deoxyribonucleic acid (DNA) and are organized into rod-shaped structures called chromosomes that reside in each cell of the body. Each human cell carries forty-six chromosomes organized as twenty-three pairs, each composed of several thousand genes. Twenty-two of the chromosome pairs are homologous pairs; that is, similar genes are located at similar sites on each chromosome. The remaining chromosomes are the sex chromosomes. Human females bear two X chromosomes, and human males possess one X and one Y chromosome.
During the formation of the reproductive cells, the chromosome pairs separate and one copy of each pair is randomly included in the egg or sperm. Each egg will contain twenty-two autosomes (non-sex chromosomes) and one X chromosome. Each sperm will contain twenty-two autosomes and either one X or one Y chromosome. The egg and sperm fuse at fertilization, which restores the proper number of chromosomes, and the genes inherited from the baby’s parents will determine its sex and much of its physical appearance and future health and well-being.
Genetic diseases are inherited as a result of the presence of abnormal genes in the reproductive cells of one or both parents of an affected individual. There are two broad classifications of genetic...
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Diagnosis and Detection (Magill’s Medical Guide, Sixth Edition)
Most, but not all, genetic diseases manifest their symptoms immediately or soon after the birth of an affected child. Rapid recognition of such a medical condition and its accurate diagnosis are essential for the proper treatment and management of the disease by parents and medical personnel. Medical technology has developed swift and accurate diagnostic methods, in many cases allowing testing of the fetus prior to birth. In addition, tests are available that determine the carrier status of an individual for many autosomal recessive and X-linked diseases. These test results are used in conjunction with genetic counseling of individuals and couples who are at risk of transmitting a genetic disease to their offspring. Thus, such individuals can make informed decisions when planning their reproductive futures.
Errors in chromosome number and structure are detected in an individual by analyzing his or her chromosomes. A small piece of skin or a blood sample is taken, the cells in the sample are grown to a sufficient number, and the chromosomes within each cell are stained with special dyes so that they may be viewed with a microscope. A picture of the chromosomes, called a karyotype, is taken, and the patient’s chromosome array is compared with that of a normal individual. Extra or missing chromosomes or alterations in chromosome structure are determined, thus identifying the genetic disease. The analysis of karyotypes...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
The scientific study of human genetics and genetic disease is relatively new, having begun in the early twentieth century. There are many early historical records, however, which recognize that certain traits are hereditarily transmitted. Ancient Greek literature is peppered with references to heredity, and the Jewish book of religious and civil laws, the Talmud, describes in detail the inheritance pattern of hemophilia and its ramifications for circumcision.
The Augustinian monk Gregor Mendel worked out many of the principles of heredity by manipulating the pollen and eggs of pea plants over many generations. His work was conducted from the 1860’s to the 1870’s but was unrecognized by the scientific community until 1900.
At about this time, many disorders were being recognized as genetic diseases. Pedigree analysis, a way to trace inheritance patterns through a family tree, has been used since the mid-nineteenth century to track the incidence of hemophilia in European royal families. This analysis indicates that the disease was transmitted through females (indeed, hemophilia is an X-linked disorder). In the early twentieth century, Archibald Garrod, a British physician, recognized certain biochemical disorders as genetic diseases and proposed accurate mechanisms for their transmission.
In 1953, Francis Crick and James D. Watson discovered the structure of DNA; thus began studies on the molecular...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Alliance of Genetic Support Groups. http://www .geneticalliance.org. Site provides information on an international coalition of individuals, professionals, and genetic support organizations working together to enhance the lives of everyone affected by genetic conditions.
Bellenir, Karen, ed. Genetic Disorders Sourcebook: Basic Consumer Information About Hereditary Diseases and Disorders. 3d ed. Detroit, Mich.: Omnigraphics, 2004. This nontechnical sourcebook offers basic information about lifestyle expectations, disease management techniques, and current research initiatives for the most common types of genetic disorders, including a resource list of three hundred genetic disorders and related topics.
GeneTests. National Center for Biotechnology Information. http://www.genetests.org. Site contains genetics disease database and information relating genetic testing to diagnosis, management, and counseling of individuals and families with inherited disorders.
Gormley, Myra Vanderpool. Family Diseases: Are You at Risk? Baltimore: Genealogical Publishing, 2002. The author, a certified genealogist and syndicated columnist, explores the relationship between family trees and genetic diseases. Written in popular language, this book gives instruction on how to assess a family’s genetic risk, information on the latest scientific breakthroughs, and directions for obtaining further...
(The entire section is 648 words.)