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Chromosomal Abnormalities About 1 in 200 babies is born with a chromosomal abnormality. Down syndrome, in which a baby is born with an extra chromosome 21, is among the most common chromosomal abnormalities, and the one whose effects are familiar to most people. Children with Down syndrome have varying degrees of mental retardation, characteristic facial features and, often, heart defects and other problems. However, there are many other chromosomal abnormalities besides Down syndrome. Some are less severe than Down syndrome, and others are more severe, or even fatal before birth. Many, but not all, children with chromosomal abnormalities have mental retardation, learning disabilities, or behavioral problems. Understanding what chromosomes are may help you understand the wide range of problems chromosomal abnormalities can cause. What are chromosomes? Chromosomes are tiny string-like structures in cells of the body. They contain the estimated 20,000 to 25,000 human gene pairs that determine traits like eye and hair color, as well as direct the growth and development of every part of our physical and biochemical systems. Each person normally has 23 pairs of chromosomes, or 46 in all. We normally inherit one chromosome per pair from our mother and one from our father. Sometimes, however, a baby can be born with too many or too few chromosomes, or with one or more chromosomes that are broken or rearranged. Errors in the number or structure of chromosomes cause a wide variety of birth defects ranging from mild to severe. What causes chromosomal abnormalities? Chromosomal abnormalities generally result from an error that occurred when an egg or sperm cell was developing. It is not known why these errors occur. (The March of Dimes supports grantees who are trying to find out the causes of these errors, in order to help prevent chromosomal abnormalities.) However, as far as we know, nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in his or her child. Sperm and egg cells are different from other cells in the body. These reproductive cells have only 23 unpaired chromosomes. When an egg and sperm cell unite -- and pregnancy begins -- they form a fertilized egg with 46 chromosomes. But sometimes something goes wrong before pregnancy begins. In the process of cell division, an error occurs that leaves an egg or sperm cell with too many or too few chromosomes. When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. A common type of chromosomal abnormality is called a trisomy. This means that an individual has three copies, instead of two, of a specific chromosome. Down syndrome is an example of a trisomy. Individuals with Down syndrome generally have three copies of chromosome 21. In most cases, an embryo with the wrong number of chromosomes will not survive. In such cases, the pregnant woman has a miscarriage, often without knowing it. Up to 70 percent of first trimester miscarriages are caused by chromosomal abnormalities. Other accidents also can occur, usually before pregnancy begins, that can alter the structure of one or more chromosomes. While such individuals may have the normal number of chromosomes, small pieces of a chromosome (or chromosomes) may be deleted, duplicated, inverted, misplaced, or exchanged with part of another chromosome. These structural rearrangements may also result in pregnancy loss and birth defects. What are the most common chromosomal abnormalities? Down syndrome is among the most common of these disorders, affecting about 1 in 800 to 1000 live-born babies. The risk of this and other trisomies increases with the mother’s age. The risk of having a live-born baby with Down syndrome is about 1 in 1250 for a woman at age 25, 1 in 1000 at 30, 1 in 400 at 35, and 1 in 100 at age 40. The outlook for children with Down syndrome is far brighter than it once was. Most have mental retardation in the mild to moderate range. With early intervention and special education, many learn to read and write and participate in diverse childhood activities. (For more information, see Down syndrome fact sheet.) Babies also can be born with extra copies of chromosomes 13 or 18. These trisomies are usually much more severe than Down syndrome, but fortunately less common, each affecting about 1 in 5000 babies. Babies with trisomies 13 or 18 generally have severe mental retardation and many physical birth defects. Sadly, most affected babies die before their first birthday. One of the 23 pairs of human chromosomes is called the sex chromosomes. Among the most common chromosomal abnormalities are those that involve missing or extra sex chromosomes (referred to as X and Y). Normally, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Abnormalities involving the X or Y can affect sexual development and may cause infertility, growth abnormalities, and in some cases, behavioral and learning problems. However, most affected individuals live essentially normal lives. Turner syndrome is a sex chromosome abnormality that affects about 1 in 2500 girls. Girls with Turner syndrome have only one X chromosome, instead of the normal two. They usually are sterile, and do not undergo normal pubertal changes unless they are treated with sex hormones. Affected girls are short, though treatment with growth and sex hormones can help increase height. Some have other health problems, including heart defects. Girls with Turner syndrome have normal intelligence, though some have difficulties with mathematics and spatial concepts. About 1 in 1000 to 2000 females has an extra X chromosome, referred to as triple X. These girls, who tend to be tall, have no consistent pattern of physical abnormalities, undergo normal puberty, and appear to be fertile. Intelligence is normal, though learning disabilities are fairly common. Because these girls are healthy and have a normal appearance, parents are most likely to know their daughter has this chromosomal abnormality only if they’ve undergone prenatal testing (with amniocentesis or CVS). Klinefelter syndrome is a sex chromosome abnormality that affects about 1 in 600 to 800 boys. Boys with Klinefelter syndrome have two, or occasionally more, X chromosomes along with their Y chromosome (males normally have one X and one Y chromosome). Affected boys tend to be tall with normal intelligence, though learning disabilities are common. As a group, they have more problems with judgment and impulse control than XY males. As adults, they produce lower than normal amounts of the male hormone testosterone (and often are treated with this hormone) and are infertile. Are there other, less common abnormalities? New techniques for analyzing chromosomes have made it possible to identify tiny abnormalities that may not be visible even under a high-powered microscope. As a result, more parents are learning that their child has a chromosomal abnormality. Some of these uncommon abnormalities include: deletions (small missing sections); microdeletions (a minute amount of missing material that may include only a single gene); translocations (a section of a chromosome is attached to another chromosome); and inversions (a section of chromosome is snipped out and reinserted upside down). Some of these abnormalities are so rare that they have been known to affect only one or a few children. In such cases, it may be impossible for a doctor to predict a child’s long-term health and development. Some rare disorders are caused by minor chromosomal changes. For example, a tiny deletion on chromosome 15 can cause Prader-Willi syndrome (characterized by mental retardation, extreme obesity and other problems), while another on chromosome 5 causes cri-du-chat (cat cry) syndrome (characterized by a cat-like, high-pitched cry in infancy, mental retardation and physical abnormalities). Small deletions from a specific section of chromosome 22 also cause the heart defects, cleft palate and other problems seen in DiGeorge and velocardiofacial syndromes. Are all children with the same chromosomal abnormality alike? Whether a child has a rare or common chromosomal abnormality, it is important that he or she be evaluated as an individual. Even people with apparently identical chromosomal abnormalities can differ from each other substantially. How a person is affected depends substantially but not wholly on the exact genetic material involved -- each chromosome contains hundreds to thousands of genes, and each gene influences a different characteristic or body function. New techniques of analyzing chromosomes sometimes can pinpoint exactly where missing or extra genetic material comes from. If doctors know what genes may be contained in that section, and their function, they some-times can give parents a better prediction of a child’s future development. Will an affected child's brothers and sisters have the same problem? Parents who have had a baby with a chromosomal abnormality should consult a genetic counselor. These health professionals help families understand what is known about the causes of a birth defect, and the chances that it will recur in another pregnancy. Fortunately, in most cases, parents of a baby with a chromosomal abnormality will learn that the risk is low. For example, parents of a baby with Down syndrome usually have a 1 percent risk of having another affected baby, plus whatever additional risk a mother has, based on her age. However, there are exceptions. For example, a small number of parents of children with Down syndrome have a chromosomal rearrangement (translocation or inversion) that does not affect their own health, but can be harmful when passed on to their offspring. In certain cases, a genetic counselor may recommend blood tests to determine whether parents carry such a rearrangement, so he or she can give the parents the most accurate picture of their risk in another pregnancy. Where can families affected by chromosomal abnormalities turn for help? An affected baby’s doctor should be able to discuss specific medical problems in detail, or refer a family to the appropriate specialists. Parents also may find it helpful to contact a support group made up of families of children affected by the same or similar chromosomal abnormalities. A genetic counselor is a good source of referrals to appropriate support groups. Some of them include:
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| © 2008 March of Dimes Birth Defects Foundation. All rights reserved. The March of Dimes is a not-for-profit organization recognized as tax-exempt under Internal Revenue Code section 501(c)(3). Our mission is to improve the health of babies by preventing birth defects and infant mortality. | ||
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