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them are not necessarily destined to develop the disease, but they are at higher risk of developing the disorder than are people who do not have the genes. One of the tests being offered is for women who are at risk of developing breast cancer or ovarian cancer because they have a family history of either disease. This test looks for two genes for breast cancer and ovarian cancer, called BRCA1 and BRCA2. Another test detects the susceptibility genes for hereditary non-polyposis colon cancer, the most common form of colon cancer (see page 775), and for familial adenomatous polyposis (see page 774), which can lead to colon cancer. A test is available for some inherited forms of melanoma (see page 1069), the deadliest type of skin cancer.

As scientists link more and more genes to diseases, more screening tests will be made available to determine a person’s risk of developing a specific disease. If a particular chronic illness runs in your family, your doctor is likely to offer you the option of a genetic test for the disorder and will discuss with you the benefits and risks of having the test. For a disease that can be prevented, such as type 2 diabetes (see page 894) or heart disease (see page 558), knowing that you are at risk of developing the disease may motivate you to take steps to reduce your risks.

Chromosome Abnormalities

Chromosome abnormalities result from defects in the number or structure of chromosomes, the structures that contain genes. These abnormalities can occur when an extra chromosome is present, if chromosomes unite in an abnormal way, or if a chromosome is missing or broken. The majority of chromosome abnormalities arise during the formation of the mother’s egg or the father’s sperm, during fertilization, or during cell division after fertilization; they are usually not inherited and, for this reason, they seldom occur twice in a family.

Chromosome abnormalities are some of the most common and severe types of genetic disorders. An irregularity in the number or structure of chromosomes can interfere in various ways with the development and functioning of a fetus. Only those fetuses with less serious chromosome abnormalities survive until birth; severely defective embryos result in miscarriage. Babies with chromosome abnormalities who do survive can have varying degrees of mental and physical impairment or may not have any noticeable effects.

Down Syndrome

Down syndrome is a common genetic disorder caused by a chromosome abnormality (see above) that usually originates during cell division when an egg or sperm is formed. Eggs and sperm normally have one chromosome from every pair. However, if a pair of chromosomes does not separate during cell division, an egg or sperm will have an extra chromosome. At fertilization, this extra chromosome is transferred to the fertilized egg, and the fetus will have the extra chromosome in every cell. Down syndrome results from an extra copy of chromosome 21.

A person with Down syndrome has mental impairment and characteristic physical traits. The average life expectancy of people with Down syndrome is 15 to 20 years less than that of unaffected people; most do not live beyond age 55, although some people with Down syndrome live into their 70s and 80s. People with Down syndrome are at increased risk of developing Alzheimer’s disease (see page 688) later in life.

Women who have children after age 35 are at higher risk of having a baby with Down syndrome than are younger women. If you are planning to become pregnant and think you may be at risk of having a child with Down syndrome, genetic counseling (see page 952) can help you understand these risks.

An extra chromosome

Normally, we all have 23 pairs of chromosomes—22 pairs of autosomes (which are the same in both males and females) and one pair of sex chromosomes (XX for females and XY for males). A child with Down syndrome is born with an extra copy of chromosome 21.

Symptoms

The symptoms of Down syndrome can vary from mild to severe. Infants with Down syndrome have poor muscle tone, and their