Each person normally has 23 pairs of chromosomes, or 46 in all. We inherit one chromosome per pair from our mother and one from our father.

Transcription

1 AP Psychology 2.2 Behavioral Genetics Article Chromosomal Abnormalities About 1 in 150 babies is born with a chromosomal abnormality (1, 2). These are caused by errors in the number or structure of chromosomes. There are many different chromosomal abnormalities. Many children with a chromosomal abnormality have mental and/or physical birth defects. Understanding what chromosomes are may make it easier to understand the wide range of problems chromosomal abnormalities can cause. What are chromosomes? Chromosomes are tiny string-like structures in cells of the body that contain the genes. Humans have about 20,000 to 25,000 genes that determine traits like eye and hair color. They also direct the growth and development of every part of the body (3). Each person normally has 23 pairs of chromosomes, or 46 in all. We 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 missing a piece or are rearranged. These errors in the number or structure of chromosomes can cause a wide variety of birth defects ranging from mild to severe. Some chromosomal abnormalities result in miscarriage or stillbirth. What causes chromosomal abnormalities? Chromosomal abnormalities usually result from an error that occurred when an egg or sperm cell was developing. It is not known why these errors occur. 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 cells have only 23 unpaired chromosomes. When an egg and sperm cell join together they form a fertilized egg with 46 chromosomes. But sometimes something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in 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. Children with Down syndrome have varying degrees of mental retardation, characteristic facial features and, often, heart defects and other problems.the risk of Down syndrome and other trisomies increases with maternal age.

2 In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the pregnant woman has a miscarriage. This often happens very early in pregnancy, before a woman may realize she's pregnant. Up to 75 percent of first trimester miscarriages are caused by chromosomal abnormalities in the embryo (4). Other errors also can occur, usually before fertilization. These errors can alter the structure of one or more chromosomes. Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be deleted, duplicated, inverted, misplaced or exchanged with part of another chromosome. These structural rearrangements sometimes have no effect, if all of the chromosome is there but just rearranged, or it may result in pregnancy loss or birth defects. Errors in cell division also can occur soon after fertilization. This can result in mosaicism, a condition in which an individual has cells with different genetic makeups. For example, individuals with the mosaic form of Turner syndrome are missing an X chromosome in some, but not all, of their cells. Some individuals with chromosomal mosaicism may be mildly affected, but the severity of the condition depends largely on the percentage of abnormal cells. How are chromosomal abnormalities diagnosed? Chromosomal abnormalities can be diagnosed after birth using a blood test, or before birth using prenatal tests (amniocentesis or chorionic villus sampling). Cells obtained from these tests are grown in the laboratory, and then their chromosomes are examined under a microscope. The lab makes a picture (karyotype) of all the person's chromosomes, arranged in order from largest to smallest. The karyotype shows the number, size and shape of the chromosomes and helps experts identify any abnormalities. What are the most common chromosomal abnormalities? Down syndrome is one of the most common chromosomal abnormalities, affecting about 1 in 800 babies (4, 5). The risk of Down syndrome and other trisomies increases with the mother's age. The risk of having a live-born baby with Down syndrome (4) is about: 1 in 1,250 for a woman at age 25 1 in 1,000 at age 30 1 in 400 at age 35 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 the fact sheet on Down syndrome.) Babies also can be born with an extra copy of chromosome 13 or 18. These trisomies are usually more severe than Down syndrome, but fortunately less common. About 1 in 10,000 babies is born with trisomy 13 (also called Patau syndrome), and about 1 in 6,000 with trisomy 18 (also called Edwards syndrome) (3, 6). Babies with trisomies 13 or 18 generally have severe mental retardation and many physical birth defects. Most affected babies die before their first birthday.

3 The X and Y chromosomes are referred to as sex chromosomes. About 1 in 500 babies has missing or extra sex chromosomes (2). Generally, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Sex chromosome abnormalities may cause infertility, growth abnormalities, and in some cases, behavioral and learning problems. However, most affected individuals live fairly normal lives. Turner syndrome is a sex chromosome abnormality that affects about 1 in 2,500 girls (3, 4). Girls with Turner syndrome are missing all or part of one X chromosome. They usually are infertile and do not undergo normal pubertal changes unless they are treated with sex hormones. Affected girls are short, though treatment with growth hormones can help increase height. Some have other health problems, including heart and kidney defects. Girls with Turner syndrome generally have normal intelligence, though some have difficulties with mathematics and spatial concepts (7). About 1 in 1,000 females has an extra X chromosome, referred to as triple X (3). Affected girls tend to be tall. They usually have no physical birth defects, undergo normal puberty and are fertile. Affected girls usually have normal intelligence, though many have learning disabilities. Because these girls are healthy and have a normal appearance, their parents often don't know they have a chromosomal abnormality. Some parents may learn that their daughter has this abnormality if they have prenatal testing (with amniocentesis or chorionic villus sampling). Klinefelter syndrome is a sex-chromosome abnormality that affects about 1 in 500 to 1,000 boys (3, 4) 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 usually have normal intelligence, though many have learning disabilities. As adults, they produce lower-than-normal amounts of the male hormone testosterone (and often are treated with this hormone) and are infertile. About 1 in 1,000 males is born with one or more extra Y chromosomes (3). Affected males are sometimes taller than average, have normal sexual development and are fertile. Most have normal intelligence, though some have learning disabilities, and behavioral and speech/language problems. As with triple X females, many affected males and their families don't know they have a chromosomal abnormality unless it is diagnosed with prenatal testing. Are there other, less common, chromosomal abnormalities? New techniques for analyzing chromosomes have made it possible to identify tiny chromosomal 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 chromosomal abnormalities include: Deletion: A small section is missing. Microdeletion: A minute amount of material (sometimes only a single gene) is missing. Translocation: A section of a chromosome is attached to another chromosome. Inversion: A section of chromosome is snipped out and reinserted upside down. Duplication: A section of a chromosome is duplicated, so there is extra genetic material. Ring chromosome: Material is deleted at both ends of a chromosome, and the new ends join together to form a ring. Some of these abnormalities are so rare that only one or a few children are known to be affected. In such cases, it may be impossible for a doctor to predict a child's long-term health and development.

4 Some abnormalities (such as some translocations and inversions) may not affect a person's health if no genetic material is missing or duplicated. Some rare disorders can be caused by small chromosomal deletions. Some examples are: Prader-Willi syndrome (deletion on chromosome 15): Affected children usually have mental retardation or learning disabilities, behavioral problems and short stature. They also may develop extreme obesity. Cri-du-chat (cat cry) syndrome (deletion on chromosome 5): Affected children have a cat-like highpitched cry during infancy, mental retardation and physical abnormalities. Wolf-Hirschhorn syndrome (deletion on chromosome 4): This disorder is characterized by severe mental retardation, heart defects, poor muscle tone, seizures, high blood pressure and other problems. 22q11 deletion syndrome (deletion on chromosome 22): Deletions in a specific region of chromosome 22 cause a variety of problems that can include heart defects, cleft lip/palate, immune system abnormalities, characteristic facial features and learning disabilities. Certain combinations of these features are sometimes called DiGeorge or velocardiofacial syndrome. Individuals with this disorder have a 50 percent chance of passing the chromosomal abnormality on to their offspring with each pregnancy. With the exception of individuals with 22q11 deletion syndrome, individuals with these disorders generally do not reproduce. Are all children with the same chromosomal abnormality alike? No. Each child with a chromosomal abnormality should be evaluated as an individual. Even people with apparently identical chromosomal abnormalities can differ substantially from each other. How a person is affected depends greatly, but not wholly, on the exact genetic material involved. Each chromosome contains hundreds to thousands of genes, and each gene influences different characteristics or body functions. New techniques of analyzing chromosomes sometimes can pinpoint exactly where missing or extra genetic material comes from. If doctors know what genes are contained in that section and their function, they sometimes can give parents a better prediction of a child's future development. Will the brothers and sisters of a child with a chromosomal abnormality 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 the birth defect will recur in another pregnancy. Fortunately, in most cases, parents of a baby with a chromosomal abnormality learn that the risk is low. For example, parents of a baby with Down syndrome usually have about a 1 percent risk of having another affected baby, if the mother is under age 35 (1, 2). Mothers over age 35 are believed to have a risk similar to other women their age. However, there are exceptions. For example, a small number of parents of children with Down syndrome or various other chromosomal abnormalities have a chromosomal rearrangement (balanced translocation) that does not affect their own health. However, these rearrangements can be harmful when passed on to their offspring. Sometimes a genetic counselor recommends blood tests to determine whether parents carry such a rearrangement in order to give the parents the most accurate

5 picture of their risk in another pregnancy. Testing also may occasionally show that a parent of a child with 22q11 deletion syndrome has a mild, previously undiagnosed form of the disorder that can be passed along to future offspring.