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. For each of these pairs, one chromosome comes from the mother and the other chromosome comes from the father. However, an abnormality can affect any chromosome, including the sex chromosomes.
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.
Down syndrome , now known as trisomy 21, is the most common and best known chromosomal abnormality. It is a trisomy condition, where there is an extra, third chromosome rather than the standard two.
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.
About half of all children with Down syndrome have heart defects and many develop thyroid disease . They are also prone to hearing and vision problems.
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 and develop early-onset dementia that is similar to Alzheimer's disease . Heart abnormalities are often treatable with drugs or surgery. Heart disease and leukemia account for most deaths among children with Down syndrome.
Fragile X Syndrome
Fragile X syndrome is the next most commonly diagnosed genetic cause of intellectual disability after Down syndrome. The symptoms of fragile X syndrome are caused by abnormalities in DNA on the X chromosome. Usually, the condition is passed down to affected children by their mothers.
Many children with the syndrome have normal intelligence. The severity of symptoms, including intellectual disability, is worse in boys than in girls with the disorder. Fragile X results in delayed development, large ears, a prominent chin and forehead, and, in boys, large testes, which is most apparent after puberty. The joints may be abnormally flexible, and heart disease may occur. Features of autism may develop. Women may experience menopause in their mid 30s.
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.
Klinefelter syndrome is a disorder in which male infants are born with an extra X chromosome (XXY). While most boys with Klinefelter syndrome have normal or slightly decreased intelligence, many have speech and reading disabilities and difficulties with planning. Most also have problems with language skills. Early problems with language may lead to problems with social interactions that affect behavior. Although physical characteristics can vary greatly, most boys with the condition are tall with long arms but otherwise normal in appearance.
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 diabetes mellitus , chronic lung disease, varicose veins , hypothyroidism , and breast cancer more often than other men.
The syndrome usually is not suspected until puberty when most of the symptoms develop. Chromosomal analysis confirms the diagnosis.
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 obesity is common.
Other disorders often develop, such as:
- Heart defects
- Kidney and eye defects
- Bleeding from abnormal blood vessels in the intestines
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. Intellectual disability is uncommon.
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 Turner syndrome . Both boys and girls can be affected with this condition, which is caused by a gene mutation on chromosome 12.
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 syndrome is the most common chromosomal abnormality in females. It occurs when the female inherits three X chromosomes instead of two. Because females with this disorder either have mild or no symptoms at all, it is believed that there are a lot of cases that go undiagnosed. Those with with symptoms of triple X syndrome tend to have slightly lower intelligence and problems with verbal skills. They also may have:
- Tall stature
- Low muscle tone
- Kidney problems
Sometimes the syndrome causes infertility . However, 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 intellectual disability and physical abnormalities.
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, attention deficit disorder , and minor behavioral disorders.
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.
- Reviewer: Kari Kassir, MD
- Review Date: 05/2014 -
- Update Date: 05/17/2014 -