Introduction
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 Section 1, Chapter 2). 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 Section 22, Chapter 256). 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 Section 23, Chapter 265) 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 Section 22, Chapter 256). 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.
 See the sidebar When Part of a Chromosome Is Missing.
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