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Overview of Chromosomal Disorders

by Nina N. Powell-Hamilton, MD, FAAP, FACMG

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 Genes and Chromosomes 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 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 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 ( Procedures : Amniocentesis) or chorionic villus sampling (see Procedures : 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).

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