Buy the Book

Print This Topic Email This Topic

Pronunciations amniocentesis chorionic villus sampling cri du chat syndrome fibrosis

Chromosomes are structures within cells that contain a person's genes. Abnormalities in chromosomes are always genetic abnormalities. Some genetic abnormalities affect the genes, but do not alter the structure of the chromosomes (see Genetics: Transcription and Translation). Thus, doctors often discuss chromosomal abnormalities separately from the broader category of genetic abnormalities. Some genetic abnormalities, such as sickle cell disease and cystic fibrosis, are very common.

A person normally has 23 pairs of chromosomes, each containing hundreds of genes. The sex chromosomes are one of these pairs of chromosomes. Normal people have 2 sex chromosomes; 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 can affect any chromosome, including the sex chromosomes. A chromosomal or other genetic abnormality can affect the number of chromosomes, the structure of certain chromosomes, or the composition of chromosomes (for example, genetic material from one chromosome may be attached to another). If the material found in chromosomes is balanced so that the expected amount is found in each cell, no abnormalities occur. If too much (addition) or too little (deletion) genetic material is found within each cell, abnormalities occur. These abnormalities can have profound physical effects.

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 chance of chromosomal abnormalities increases by a barely noticeable degree if the father is older. A marriage between close relatives increases the chance of developing some genetic abnormalities, but usually not chromosomal abnormalities.

Chromosomal abnormalities can cause a wide range of abnormalities or effects, usually birth defects (see Birth Defects: Introduction) or death of the embryo or fetus before birth. Genetic abnormalities can cause birth defects or diseases (for example, sickle cell disease) or have many different effects.

A person's chromosomes can be analyzed with a sample of blood. A fetus can be tested for chromosomal abnormalities before birth, for example, using amniocentesis or chorionic villus sampling (see Genetic Disorders Detection: Chorionic Villus Sampling). If the fetus is found to have a chromosomal abnormality, further tests may be performed to detect specific birth defects. Although chromosomal abnormalities cannot be corrected, some of the defects can sometimes be prevented or treated.

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. In the rare cri du chat syndrome (cat's cry syndrome, 5p minus syndrome), part of chromosome 5 is missing. An infant with this syndrome is usually underweight at birth; has a small head with many abnormal features, including a round face, small jaw, wide nose, widely separated eyes, and 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. Heart defects are common. Mental and physical development are greatly retarded. Despite these abnormalities, many children with cri du chat syndrome survive to adulthood. 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. Obsessive-compulsive behaviors are common. Weight gain is excessive, which can lead to other health problems. Obesity can be severe enough to require gastric bypass surgery.

Last full review/revision February 2003