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: Chromosomes and Genes 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 affect the number or structure of chromosomes and may be visible with a microscope in a test called karyotype analysis.

Numerical abnormalities occur when a person has one or more extra copies of a chromosome (one extra is trisomy, and two extra is tetrasomy) or is missing a chromosome (monosomy).

Structural abnormalities occur when part of a chromosome is abnormal. Sometimes part or all of a chromosome incorrectly joins with another chromosome (called translocation). Sometimes parts of chromosomes are missing (called deletion―see Sidebar 1: Chromosomal and Genetic Abnormalities: When Part of a Chromosome Is Missing) or have been duplicated.

Some chromosomal abnormalities cause the death of the embryo or fetus before birth. Other abnormalities cause problems such as intellectual disability, 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 and thus cannot be seen on karyotype analysis. More specific genetic testing is required. Some mutations in a gene cause few or only mild 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. Most genetic abnormalities appear spontaneously. Some substances or agents in the environment are capable of damaging and causing mutations in genes, and these mutations may be passed from parent to child. These substances are called mutagens. Mutagens, such as radiation and Agent Orange, can cause some cancers and birth defects. Having two copies of an abnormal gene can lead to serious diseases or conditions, such as cystic fibrosis or 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 cells from amniocentesis (see Genetic Disorders Detection: 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 done to detect specific birth defects.

Prevention: Although chromosomal abnormalities cannot be corrected, some birth defects can sometimes be prevented (for example, 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 parts of chromosomes. These syndromes are called chromosome deletion syndromes. They tend to cause severe birth defects and markedly retarded mental and physical development. In cri du chat syndrome (cat's cry syndrome), a rare syndrome, part of chromosome 5 is missing. An infant with this syndrome may have 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 often has a high-pitched cry that sounds like a kitten crying. Often the infant seems limp. The high-pitched cry is often noted 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 involving chromosome 15, intellectual disability 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. In Wolf-Hirschhorn syndrome, part of chromosome 4 is missing. Children with this syndrome have profound intellectual disability. Children may also have epilepsy, a broad or beaked nose, scalp defects, drooping upper eyelids (ptosis) and gaps or fissures (colobomas) of the iris, cleft palate, and delayed bone development. Boys may have undescended testes (cryptorchidism) and a misplaced opening of the urethra (hypospadias). Many affected children die during infancy. Children who survive into their 20s have severe disabilities.

Last full review/revision April 2013 by Nina N. Powell-Hamilton, MD