Genetic screening is used to determine whether a couple is at increased risk of having a baby with a hereditary genetic disorder. Hereditary genetic disorders are disorders of chromosomes or genes that are passed down from generation to generation.
Any couple can request genetic screening, but screening is particularly recommended when
Some genetic disorders are not hereditary (see Overview of Genetic Disorders) and thus cannot be identified by screening the parents.
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)
Babies with birth defects born to the couple or to family members
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.
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:
In the United States, examples of disorders that meet these criteria include sickle cell anemia, the thalassemias, Tay-Sachs disease, and cystic fibrosis. The number of hereditary disorders that can be screened is steadily increasing, and some laboratories offer carrier screening for dozens of disorders (called expanded carrier screening). People should meet with a genetics professional before deciding whether to pursue expanded carrier screening.
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.