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 Overview of Chromosome and Gene Disorders Chromosomes are structures within cells that contain a person's genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or... read more 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 Overview of Genetic Disorders Genetic disorders are disorders caused by abnormalities in one or more genes or chromosomes. Some genetic disorders are hereditary and others are spontaneous. (See also Overview of Chromosome... read more ) 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)
Miscarriages Miscarriage A miscarriage is the loss of a fetus due to natural causes before 20 weeks of pregnancy. Miscarriages may occur because of a problem in the fetus (such as a genetic disorder or birth defect)... read more , stillborn babies Stillbirth Stillbirth is death of a fetus after 20 weeks of pregnancy. Pregnancy complications are problems that occur only during pregnancy. They may affect the woman, the fetus, or both and may occur... read more , or babies who have died soon after birth in the couple or in family members
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 Recessive x-linked disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more ). 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 Abortion Induced abortion is the intentional ending of a pregnancy by surgery or drugs. A pregnancy may be ended by surgically removing the contents of the uterus or by taking certain drugs. Complications... read more or elective sterilization Permanent Contraception Permanent contraception involves making a person incapable of reproduction. Disrupting the tubes that carry sperm or the egg ends the ability to reproduce. This form of contraception should... read more ) are available and acceptable to the parents.
In the United States, examples of disorders that meet these criteria include sickle cell anemia Sickle Cell Disease Sickle cell disease is an inherited genetic abnormality of hemoglobin (the oxygen-carrying protein found in red blood cells) characterized by sickle (crescent)-shaped red blood cells and chronic... read more , the thalassemias Thalassemias Thalassemias are a group of inherited disorders resulting from an imbalance in the production of one of the four chains of amino acids that make up hemoglobin (the oxygen-carrying protein found... read more , Tay-Sachs disease Tay-Sachs Disease and Sandhoff Disease Tay-Sachs disease and Sandhoff disease are types of lysosomal storage disorder called sphingolipidoses and are caused by a buildup of gangliosides in the tissues in the brain. These diseases... read more , and cystic fibrosis Cystic Fibrosis (CF) Cystic fibrosis is a hereditary disease that causes certain glands to produce abnormally thick secretions, resulting in tissue and organ damage, especially in the lungs and the digestive tract... read more . 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 Prenatal Diagnostic Testing Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more . 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.