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Trisomies 13 and 18: These trisomies usually are more severe than Down syndrome, but fortunately less common. About 1 in 16,000 babies is born with trisomy 13 (also called Patau syndrome), and about 1 in 5,000 with trisomy 18 (also called Edwards syndrome) (4). Babies with trisomies 13 or 18 generally have severe intellectual disabilities and many physical birth defects. Most affected babies die before their first birthday.
The last pair of chromosomes are the sex chromosomes, called X and Y. 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. Common sex chromosome abnormalities include:
Turner syndrome: This abnormality affects about 1 in 2,500 girls (4). Girls with Turner syndrome have one X chromosome and are missing all or part of the other X chromosome. They usually are infertile and do not undergo normal puberty changes unless they are treated with sex hormones. Affected girls are short, though treatment with growth hormone 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 learning difficulties, particularly with mathematics and spatial concepts (5).
Triple X: About 1 in 1,000 females has an extra X chromosome (4). Affected girls tend to be tall. They usually have no physical birth defects, experience normal puberty and are fertile. Affected girls usually have normal intelligence, though many have learning problems. 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 CVS).
Klinefelter syndrome: This abnormality affects about 1 in 500 to 1,000 boys (4). Boys with Klinefelter syndrome have two, or occasionally more, X chromosomes along with their Y chromosome. Affected boys usually have normal intelligence, though many have learning problems. As adults, they produce lower-than-normal amounts of the male hormone testosterone (and often are treated with this hormone) and are infertile.
XYY: About 1 in 1,000 males is born with one or more extra Y chromosomes (4). Affected males are sometimes taller than average, have normal sexual development and are fertile. Most have normal intelligence, though some have learning, 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:
Some chromosomal 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 health care provider to predict a child’s long-term health and development. Some abnormalities (such as some translocations and inversions) may not affect a person’s health if no genetic material is missing or duplicated.
Some uncommon disorders can be caused by small chromosomal deletions. Examples are:
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 providers 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 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.
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 called a balanced translocation. It does not affect their own health, but the rearrangement 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 parents the most accurate picture of their risk in another pregnancy. Testing 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.
Where can families affected by chromosomal abnormalities turn for help?
An affected baby’s health care provider should be able to discuss specific medical problems in detail or refer a family to appropriate specialists. Parents also may find it helpful to contact a support group 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:
References
December 2009
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