Diseases & Conditions


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Chromosomal and Genetic Abnormalities: Introduction


Chromosomes are structures within cells that contain a person's genes. A gene is a segment of DNA and contains the code for a specific protein that functions in one or more types of cells in the body (see Genetics: Introduction for a discussion about genetics).

Every normal human cell, except for sperm and egg cells, has 23 pairs of chromosomes for a total of 46 chromosomes. Sperm and egg cells have only one of each pair of chromosomes for a total of 23. Each chromosome contains hundreds to thousands of genes.

The sex chromosomes are one of the 23 pairs of chromosomes. Normal people have 2 sex chromosomes, and each is either an X or a Y chromosome. Normal females have two X chromosomes (XX), and normal males have one X and one Y chromosome (XY).

Chromosomal Abnormalities: Chromosomal abnormalities can affect any chromosome, including the sex chromosomes. Chromosomal abnormalities include having extra chromosomes (addition or duplication), missing chromosomes (deletion), or parts of one chromosome misplaced onto another. Some chromosomal abnormalities cause the death of the embryo or fetus before birth. Other abnormalities cause problems such as mental retardation, short stature, seizures, heart problems, or a cleft palate.

The older a pregnant woman is, the greater the chance that her fetus will have a chromosomal abnormality (see Genetic Disorders Detection: Chromosomal Abnormalities ). The same is not true of a man. As a man gets older, the chance of conceiving a baby with a chromosomal abnormality is only slightly increased.

Genetic Abnormalities: Small changes (mutations) may occur in a specific gene. These changes do not affect the structure of the chromosomes. Some mutations in a gene cause few or no problems. Other mutations cause serious disorders such as sickle cell anemia, cystic fibrosis, and muscular dystrophy. Increasingly, medical scientists are finding specific genetic causes of children's diseases.

It remains unclear how most genetic abnormalities occur. Some genetic abnormalities appear spontaneously. Some probably are caused by toxic substances, such as Agent Orange, or radiation. A union between close relatives increases the chance of a child's having a genetic disorder because both parents are more likely to have the same abnormal genes. Having two copies of an abnormal gene can lead to serious diseases or conditions, such as Tay-Sachs disease.

Testing for Chromosomal and Genetic Abnormalities: A person's chromosomes and genes can be evaluated by analyzing a sample of blood. In addition, doctors can use amniocentesis or chorionic villus sampling (see Genetic Disorders Detection: Chorionic Villus Sampling ) to detect certain chromosomal or genetic abnormalities in a fetus. If the fetus has an abnormality, further tests may be performed to detect specific birth defects.

Prevention: Although chromosomal abnormalities cannot be corrected, some birth defects can sometimes be prevented (eg, taking folate [folic acid] to prevent neural tube defects or screening parents for carrier status of certain genetic abnormalities).

When Part of a Chromosome Is Missing

A number of syndromes can occur in infants who are missing part of a chromosome. These syndromes are called chromosome deletion syndromes . They tend to cause severe birth defects and markedly retarded mental and physical development.

In the rare cri du chat syndrome (cat's cry syndrome), part of chromosome 5 is missing. An infant with this syndrome has a low birth weight; has a small head with many abnormal features, including a round face, small jaw, wide nose, widely separated eyes, crossed eyes (strabismus), and abnormally shaped ears set low in the head; and has a high-pitched cry that sounds like a kitten crying. Often the infant seems limp. The high-pitched cry occurs immediately after birth, lasts several weeks, and then disappears. Webbed fingers and toes (syndactyly) and heart defects are common. Mental and physical development is greatly retarded. Many children with cri du chat syndrome survive to adulthood but have substantial disabilities.

In Prader-Willi syndrome , another chromosomal deletion syndrome, mental retardation is common. Many symptoms vary according to the child's age. Newborns with the defect feel limp, feed poorly, and gain weight slowly. Eventually these symptoms resolve. Then, between the ages of 1 and 6, appetite increases, often becoming insatiable. The hands and feet remain small. Obsessive-compulsive behaviors are common. The function of the reproductive organs is abnormally decreased, which retards growth and sexual development. Weight gain is excessive, which can lead to other health problems. Obesity can be severe enough to justify gastric bypass surgery.

Source: The Merck Manual Home Edition