Chromosomal abnormalities are a group of conditions that are the result of a problem with one of the 23 pairs of chromosomes. Chromosomes are the structures within the body’s cells that contain genes. Normally, people have 23 pairs of chromosomes, but an abnormality can affect any chromosome, including the sex chromosomes.
Usually, people have two sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
A chromosomal abnormality can affect the number of chromosomes, the structure of certain chromosomes, or the composition of chromosomes. If the material found in chromosomes is balanced so that the expected amount is found in each cell, no abnormalities occur. If that balance is upset, chromosomal abnormalities occur and can cause a wide range of abnormalities, usually birth defects or death of the embryo or fetus before birth.
With Down syndrome, physical and mental development is delayed. Infants with Down syndrome tend to be quiet and passive with somewhat limp muscles. Most children with Down syndrome have a lower than average IQ. Children with Down syndrome tend to have a small head, a broad, flat face with slanting eyes and a short nose. The tongue is large and their ears are small and set low in the head. The hands are short and broad, with a single crease across the palm.
Children with Down syndrome often have heart defects, and many develop
Down syndrome can be diagnosed before birth. Your physician may be able to recognize some of the distinctive physical characteristics of an infant with Down syndrome during an ultrasound. The diagnosis is confirmed by testing the infant's chromosomes for trisomy 21 or other disorders of the 21st chromosome. After the diagnosis is made, doctors use further tests to detect abnormalities associated with Down syndrome.
Most children with Down syndrome survive to adulthood. Life expectancy for a child with Down syndrome is about 45-55 years old. IQ levels and educational/vocational achievement vary greatly. Many, but not all, have progressively worsening mental functioning. Heart abnormalities are often treatable with drugs or surgery. Heart disease and
Fragile X Syndrome
The presence of abnormal DNA on the fragile X chromosome can be detected by tests before or after birth. The greater the number of abnormal repetitions of DNA found, the more likely the child will have symptoms.
Puberty usually occurs at the normal time, but the testes remain small. At puberty, growth of facial hair is minimal, and the breasts may enlarge somewhat. Men and boys with the syndrome are usually infertile. Men with Klinefelter syndrome develop
The syndrome usually isn’t suspected until puberty when most of the symptoms develop. Chromosomal analysis confirms the diagnosis.
Long QT Syndrome
Long QT syndrome is the result of an abnormality of the heart's electrical system, which may cause loss of consciousness or sudden death. It affects as many as one in 7,000 people. In the United States, it may cause sudden death in 3,000 to 4,000 children and young adults each year. In children, this disorder is usually due to a genetic abnormality. A person with the disorder may have family members who died suddenly and inexplicably. In most adults, long QT syndrome is caused by use of a drug or a disorder.
People who have long QT syndrome are predisposed to developing an unusually fast heart rate, which often occurs during physical activity or emotional excitement. When the heart rate is too fast, the brain may not receive enough blood. The result is loss of consciousness. Some people with long QT syndrome are also born deaf, but about one third of people have no symptoms. The condition can cause sudden death at a young age.
Noonan syndrome can be inherited or can develop unpredictably in children whose parents have normal genes. Although children with the syndrome have normal chromosomal structure, they have many characteristics typical of
Symptoms may include webbing of the neck, low-set ears, droopy eyelids, short stature, shortened ring fingers, a high-arched palate, heart and blood vessel abnormalities, and impaired intelligence. Most affected people are short. Boys may have underdeveloped or undescended testes. In girls, the ovaries may be underactive or stop working. Puberty may be delayed, and infertility is possible.
Triple X (trisomy X) syndrome is a rare disorder in which female infants are born with three X chromosomes. Girls with triple X syndrome tend to have slightly lower intelligence and particular problems with verbal skills. Sometimes the syndrome causes infertility, although some women with triple X syndrome have given birth to physically normal children with normal chromosomes.
Extremely rare cases of infants with four or even five X chromosomes have been identified. The more X chromosomes the girl has, the greater the chance of
In Turner syndrome, girls are born with one of the two X chromosomes partially or completely missing.
Many newborns with Turner syndrome have swelling on the backs of their hands and tops of their feet. Swelling or loose folds of skin are often evident on the back of the neck. Many other abnormalities often develop, including a webbed neck, a low hairline at the back of the neck, a broad chest with wide-spaced nipples, and poorly developed nails.
As a girl with Turner syndrome gets older, she has no menstrual periods and the breasts, vagina, and labia remain childlike rather than undergoing the changes of puberty. The ovaries usually do not contain developing eggs. A girl or woman with Turner syndrome is almost always short, and
Other disorders, such as heart defects, kidney and eye defects, and diabetes mellitus, often develop. Occasionally, abnormal blood vessels in the intestine cause bleeding.
Many girls with Turner syndrome have difficulty in assessing visual and spatial relationships and have problems with planning and attention. They tend to score poorly on certain performance tests and in mathematics, even if they achieve average or above-average scores on verbal intelligence tests. Mental retardation is uncommon.
XYY syndrome is a disorder in which a male infant is born with an extra Y chromosome.
Boys with XYY syndrome tend to be tall and have difficulties with language. Their IQ tends to be slightly lower than that of their family members. Many experience learning disabilities,
These are just a few of the conditions that result from chromosomal abnormalities. Talk to your healthcare provider if you are concerned about unusual symptoms in your child.
Chromosomal abnormalities. March of Dimes website. Available at: http://www.marchofdimes.com/professionals/14332_1209.asp . Accessed September 5, 2005.
Chromosomal and genetic abnormalities. Merck Manual website. Available at: http://www.merck.com/mmhe/sec23/ch266/ch266a.html . Accessed September 7, 2005.
Last reviewed April 2009 by
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