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Chromosomal Abnormalities

Genetic Counseling

Currently, no cures exist for any of the syndromes caused by chromosomal abnormalities. For many of these conditions, the age of the mother carries an increased risk for giving birth to a child with a chromosomal abnormality. The risk for Down syndrome, for instance, jumps from 1 in 1,000 when the mother is age 15-30 to 1 in 400 at age 35, increasing risk with increasing maternal age. One theory postulates that this is due to the build-up of toxins over time within the ovaries, damaging the egg cells that are present in females since early childhood. By the time they are ovulated after age 35, the chances of fertilization of a damaged egg are greater.

People at high risk for these abnormalities may opt to know whether the fetus they have conceived has one of these abnormalities. Amniocentesis is a procedure in which some of the amniotic fluid that surrounds and cushions the fetus in the uterus is sampled with a needle placed in the uterus. The amniotic fluid contains some of the fetus's skin cells, which can be tested for chromosomally-based conditions. Another test, called chorionic villi sampling, involves taking a piece of tissue from a part of the placenta. If a chromosomal defect is found, the parents can be advised of the existence of the abnormality. Some parents opt to abort the pregnancy; others can prepare before birth for a child with special needs.

In addition to amniocentesis and chorionic villi sampling, researchers are working on devising easier tests to detect certain abnormalities. A new test for Down syndrome, for instance, measures levels of certain hormones in the mother's blood. Abnormal levels of these hormones indicate an increased risk that the fetus has Down syndrome. These enzyme tests are safer and less expensive than the sampling tests and may be able to diagnose chromosomally-based conditions in more women. Most recently, scientists have devised a procedure called in situ hybridization which uses molecular tags to locate defective portions of chromosomes collected from amniocentesis. The process uses fluorescent molecules that seek out and adhere to specific faulty portions of chromosomes. Chromosomes having these faulty regions then "glow" (or fluoresce) under special lighting in regions where the tags bind to, or hybridize with, the chromosome. The procedure makes identification of defects easier and more reliable.

Resources

Books

Harper, Peter S. Practical Genetic Counseling. Boston: Butterworth-Heineman, 1993.

Nussbaum, Robert L., Roderick R. McInnes, Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, David L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

Periodicals

Bos, A.P., et al. "Avoidance of Emergency Surgery in Newborn Infants with Trisomy 18." The Lancet 339, no. 8798 (April 11, 1992): 913-6.

D'Alton, Mary E., et al. "Prenatal Diagnosis." New England Journal of Medicine 328, no. 2 (January 14, 1995): 114-21,

Day, Stephen. "Why Genes have a Gender." New Scientist 138, no.1874 (May 22, 1993): 34-39.

Hoffman, Michelle. "Unraveling the Genetics of Fragile X Syndrome." Science 252, no. 5010, (May 24, 1991): 1070.

Money, John. "Specific Neurological Impairments associated with Turner and Kline-Felter Syndromes: A Review." Social Biology 40, no. 1-2, (Spring-Summer 1993): 147-152.

Nicolaides, K.H., et al. "Ultrasonographically Detectable Markers of Fetal Chromosomal Abnormalities." The Lancet 340, no. 8821, (September 19, 1992): 704-8.

Solomon, Ellen, et al. "Chromosome Aberrations and Cancer." Science 254, no. 5035, (November 22, 1991): 1153-61.

Antonio Farina

Brenda Wilmoth Lerner

Kathleen Scogna

KEY TERMS

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Amniocentesis: —A method of detecting genetic abnormalities in a fetus; in this procedure, amniotic fluid is sampled through a needle placed in the uterus; fetal cells in the amniotic fluid are then analyzed for genetic defects.
Aneuploidy: —An abnormal number of chromosomes.
Angelman's syndrome: —A syndrome caused by a deletion in chromosome 5 inherited from the mother.
Chorionic villi sampling: —A procedure in which hair-like projections from the chorion, a fetal structure present early in pregnancy, are suctioned off with a catheter inserted into the uterus. These fetal cells are studied for the presence of certain genetic defects.
Chromosomes: —he structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.
Cri du chat syndrome: —A syndrome caused by a deletion in chromosome 5; characterized by a strange cry that sounds like the mewing of a cat.
Deletion: —Deletion of a segment of DNA from a chromosome.
Deoxyribonucleic acid (DNA): —The genetic material in a cell.
Diploid: —Means "double number;" describes the normal number of chromosomes for all cells of the human body, except for the sex cells.
Down syndrome: —A syndrome caused by trisomy 13; characterized by distinct facial characteristics, mental retardation, and several physical disorders, including heart defects.
Duplication: —A type of chromosomal defect in which a broken segment of a chromosome attaches to the chromosome pair.
Edwards' syndrome: —A syndrome caused by trisomy 18; characterized by multi-system defects; is usually lethal by age 1.
Fragile X syndrome: —A condition in which an extra X chromosome hangs from the X chromosome by a "thread" of genetic material.
Gene: —A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.
Haploid: —Nucleus or cell containing one copy of each chromosome; the number of chromosomes in a sex cell.
Inversion: —A type of chromosomal defect in which a broken segment of a chromosome attaches to the same chromosome, but in reverse position.
Klinefelter's syndrome: —A syndrome that occurs in XXY males; characterized by sterility, small testes, and female sex characteristics.
Meiosis: —Cell division that results in four haploid sex cells.
Metafemale: —An XXX female.
Mitosis: —Cell division that results in two diploid cells.
Monosomy: —A form of aneuploidy in which a person receives only one chromosome of a particular chromosome pair, not the normal two.
Patau's syndrome: —A syndrome caused by trisomy 13; characterized by a hare lip, cleft palate, and many other physical defects; usually lethal by age 1.
Polyploidy: —A condition in which a cell receives more than two complete sets of chromosomes.
Prader-Willi syndrome: —A syndrome caused by a deletion in chromosome 5 inherited from the father.
Tetraploidy: —A form of polyploidy; four sets of chromosomes.
Translocation: —A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.
Triploidy: —A form of aneuploidy; three sets of chromosomes.
Trisomy: —A form of aneuploidy in which a person receives an extra chromosome of a particular chromosome pair, not the normal two.
Turner syndrome: —A syndrome that occurs in X0 females; characterized by sterility, short stature, small testes, and immature sex organs.
Zygote: —The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

Additional topics

Science EncyclopediaScience & Philosophy: Chimaeras to ClusterChromosomal Abnormalities - Normal Number And Structure Of Human Chromosomes, Normal Cell Division, Alterations In Chromosome Structure, Genetic Counseling - Alterations in chromosome number