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Genetic Screening

by Jeffrey S. Dungan, MD

  • Screening involves assessing the couple’s family history and, if needed, analysis of blood or tissue samples.

Genetic screening is used to determine whether a couple is at increased risk of having a baby with a hereditary genetic disorder. Any couple can request genetic screening, but screening is particularly recommended when

  • One or both partners know they have a genetic abnormality.

  • Family members have a genetic abnormality.

  • Partners belong to a high-risk ethnic group.

Genetic screening involves assessing the couple’s family history and sometimes having genetic tests (using a sample of blood or of cells from the inside of the cheek).

Family History Assessment

To determine whether a couple has an increased risk of having a baby with a genetic disorder, doctors ask the couple about the following:

  • Disorders that family members have had

  • The cause of death in family members

  • The health of all living first-degree relatives (parents, siblings, and children) and second-degree relatives (aunts, uncles, and grandparents)

  • Miscarriages, stillborn babies, or babies who have died soon after birth

  • Babies with birth defects

  • Intermarriages among relatives (which increases the risk of having the same abnormal gene)

  • Ethnic background (certain groups are at higher risk of certain disorders)

Information about three generations is usually needed. If the family history is complicated, information about more distant relatives may be needed. Sometimes doctors review the medical records of relatives who may have had a genetic disorder.

Carrier Screening

Carriers are people who have an abnormal gene for a disorder but who do not have any symptoms or visible evidence of the disorder.

In carriers, the abnormal gene is usually recessive—that is, two copies of the gene are needed to develop the disorder (see Recessive disorders). Such carriers have one normal gene and one abnormal gene for the disorder.

Only women can carry an X-linked (sex-linked) recessive gene. Women have two X chromosomes. Thus, on the other X chromosome, the corresponding gene may be normal and protect women from developing the disorder. Because men have only one X chromosome, all men who have an abnormal X-linked recessive gene have the disorder that the abnormal gene causes.

Carrier screening involves testing people who do not have symptoms but are at higher risk of carrying a recessive gene for a particular disorder. Risk is higher when one or both partners have a family history of certain disorders or have characteristics (such as ethnic background or racial or geographic group) that increase the risk of having certain disorders. However, screening is done only if the following criteria are also met:

  • The disorder is very debilitating or lethal.

  • A reliable screening test is available.

  • The fetus can be treated, or reproductive options (such as abortion or elective sterilization) are available and acceptable to the parents.

In the United States, examples of disorders that meet these criteria include sickle cell anemia, the thalassemias, Tay-Sachs disease, and cystic fibrosis.

Carrier screening usually consists of analyzing the DNA from a blood sample. But sometimes a sample of cells from the inside of the cheek is analyzed. People provide the sample by swishing a special fluid in their mouth, then spitting it into a specimen container, or by rubbing a cotton swab inside their cheek.

Ideally, carrier screening is done before a woman becomes pregnant. If it is done afterward and if it indicates that both partners have a recessive gene for the same disorder, they may decide to have prenatal diagnostic testing. That is, the fetus may be tested for the disorder before birth. If the fetus has the disorder, treatment of the fetus may be possible, or termination of the pregnancy may be considered.

Who Should Consider Genetic Screening?

Group

Disorder*

Screening Tests

All

Cystic fibrosis

DNA analysis of a sample of blood or of cells from the inside of the cheek

Ashkenazi Jews

Canavan disease

DNA analysis of a sample of blood or of cells from the inside of the cheek

Familial dysautonomia (hereditary dysfunction of the autonomic nervous system)

DNA analysis of a sample of blood or of cells from the inside of the cheek

Tay-Sachs disease

Blood tests to measure the enzyme that is deficient in this disorder (hexosaminidase A)

Possibly DNA analysis

Blacks

Sickle cell anemia

Blood tests to check for abnormal hemoglobin

Cajuns

Tay-Sachs disease

Blood tests to measure the enzyme that is deficient in this disorder (hexosaminidase A)

Possibly DNA analysis

Mediterranean, Southeast Asian, Asian Indian, and Middle Eastern people

Beta-thalassemia

Blood tests to measure the average size of red blood cells (mean corpuscular volume)

Southeast Asians, Cambodians, Chinese, Filipinos, Laotians, and Vietnamese

Alpha-thalassemia

Blood tests to measure the average size of red blood cells

If average size is small, blood tests to check for abnormal hemoglobin

*This list is not comprehensive. Screening for other disorders can be requested.

Most (90%) Jews are Ashkenazi. Thus, Jews who do not know whether they are Ashkenazi should be screened. Some experts also recommend screening for other disorders, such as Gaucher disease, Niemann-Pick disease type A, and Fanconi anemia (syndrome) group C.

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