Family history screening

If a certain disease runs in your family, you may wish to be tested to determine whether you carry the genetic alteration that causes it. For instance, if you have a sibling with an autosomal recessive condition, there's a 50 percent

Unaffected carrier

Unaffected carrier

Unaffected carrier

Unaffected carrier

Recessive gene (altered)

Normal Unaffected Unaffected Affected

Autosomal recessive disorders can result when both parents carry a single altered gene for the same disease. If both parents pass the altered gene to the child, the child is affected by the disease.

I I Dominant gene (normal)

Recessive gene (altered)

Normal Unaffected Unaffected Affected chance that you're a carrier. There's also a 50 percent chance that you're not. If you're not a carrier, you can't pass the disease on to your children.

Sometimes, when testing for a familial disorder, several people in your family may be tested, including those with the condition. The tests can be complicated. It's best to discuss the testing with a knowledgeable health care provider, such as a genetic counselor.

Directed genetic carrier screening can be done in families where a member has received a diagnosis of a certain condition. This is an effective technique for detecting autosomal recessive disorders. Directed genetic carrier screening tests can also look for:

• X-linked disorders. This type of disorder results from an altered gene residing on the X chromosome. A woman who carries two X chromosomes may be a carrier of an X-linked disorder but not be affected by it because her normal X chromosome provides the needed function for that gene. She has a 50 percent chance of passing the altered chromosome on to her child. If the child who receives the altered gene is a boy, who has only one X chromosome, the effects of the altered gene will be seen, meaning the boy

Normal father

Unaffected carrier mother

Carrier daughter

X-linked recessive disorders result from an altered gene carried on one of the mother's X chromosomes.

X X chromosome

X chromosome (recessive) Y chromosome

X X Normal female X Y Normal male X y^ Unaffected carrier female Affected male

Normal daughter

Normal son

Carrier daughter

Affected son

What's a genetic counselor?

A genetic counselor's role is to help you make decisions if you're concerned about the risks of an inherited disease. The genetic counselor can help you:

• Understand if your potential future child is at risk of birth defects or hereditary disorders

• Decide whether to be screened

• Interpret any related test results

• Understand how a specific disease may affect your child

• Make decisions about test results that are consistent with your values and beliefs

Certified genetic counselors (C.G.C.s) have completed special training in human genetics and counseling. Medical doctor (M.D.) geneticists are also specially trained in genetics and counseling. In addition, other doctors or nurses may have training on the subject.

To find a genetic counselor in your area, talk with your health care provider. You can also contact your local March of Dimes office, the National Society of Genetic Counselors (www.nsgc.org) or the American College of Medical Genetics (www.acmg.net).

will have signs and symptoms of the disease. If the child who received the altered gene is a girl, she, like her mother, will be a carrier of the disorder.

X-linked disorders include Duchenne's muscular dystrophy, fragile X syndrome and hemophilia, among others.

• Chromosome rearrangements. In some families, babies are born with birth defects caused by the presence of too much or too little genetic material. This can happen if a parent carries a chromosome rearrangement. These rearrangements may also lead to the occurrence of more miscarriages than expected in a family. If you have a family member with a chromosome disorder or birth defects or if a number of miscarriages have occurred, talk to your health care provider about whether chromosome studies are indicated.

• Mitochondrial disorders. Some disorders are caused by errors in a separate set of genes in each cell. These separate genes are in each mitochondria, the energy-producing organs of the cell. Mitochondrial disorders can have many different signs and symptoms, such as low blood sugar, muscle problems, seizures and other conditions. If you have a family member who's been diagnosed with a mitochondrial disorder, discuss this with your health care provider or a geneticist to decide whether testing might be right for you.

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