American Medical Association Family Medical Guide - American Medical Association [660]
Genetic Counseling
The rapid increase in knowledge about genes, how they function, and how they cause disease is providing powerful new information about preventing, diagnosing, and treating genetic disorders. This information is also helping people evaluate their risk of inheriting a genetic disorder or of transmitting a disease-causing gene to a child. Of every 100 children born, two or three have a serious mental or physical disability that is genetic in origin or results from damage to, or faulty development of, the fetus during pregnancy.
A child may be born with a genetically influenced disorder from a number of different causes. For example, he or she may have inherited a copy of a defective gene from each parent and have a recessive genetic disorder such as cystic fibrosis (see page 958) or sickle cell disease (see page 613). A child may have an abnormal gene produced by a mutation in the egg or sperm of one of the parents. The occurrence of these spontaneous genetic mutations explains how parents who are completely healthy and don’t carry a disease gene can have children with a genetic disorder such as muscular dystrophy (see page 972), hemophilia (see page 618), or neurofibromatosis (see page 968). A mistake during cell division before or just after fertilization can result in a chromosome abnormality such as Down syndrome (see page 955) or Turner syndrome (see page 957).
It is now possible, with many genetic disorders, to have a test to determine whether you are a carrier of a defective disease gene. Most people who suspect they may carry a defective gene wait until they are planning to have children before they are tested for the gene. However, if one child in a family has, for example, a recessive disorder such as cystic fibrosis, his or her relatives can be tested to see if they carry the cystic fibrosis gene in their cells.
If one or both parents are found to be carriers of a disease gene, genetic counselors can calculate the risk for each of their children of inheriting it and having the disease, depending on whether the abnormal disease gene is dominant or recessive. For some genetic disorders, an accurate risk calculation can be made.
Even for some congenital (present at birth) disorders of unknown origin, such as heart defects, genetic counselors are often able to estimate the risk that a subsequent child will be born with the condition. As with genetic disorders, tests may be performed during pregnancy to determine whether the fetus has a developmental defect. Many of these defects, including most congenital heart defects, can be corrected early in life. For other genetic disorders, however, it is more difficult to calculate the inheritance risk precisely.
If you suspect that you may carry a gene for an inherited disorder, perhaps because a particular illness appears to run in your family, talk to a genetic counselor, who can help you evaluate your risk of passing the disorder on to a child. Also, if you have a stillborn child, ask your doctor if your child might have had a genetic disorder. If a genetic disorder is a possibility, your doctor will refer you to a genetic counselor, who can help you understand your risks of having another affected child.
A genetic counselor will ask you for detailed information about the health of your and your partner’s parents, brothers, sisters, cousins, and other close relatives. It will help if you gather this information beforehand and bring it with you in writing when you see the counselor. You will be asked to give the exact cause of death of relatives, and the age at which they died, especially those who died during infancy. The counselor will also consider your ethnic and racial background and your age, which can be risk factors for some disorders.
Many genetic disorders, including cystic fibrosis and sickle cell disease, can be diagnosed during pregnancy. Chorionic villus sampling, or CVS (see page 511), is a prenatal diagnostic test that can be performed early in pregnancy—between the 10th and 12th weeks. In this procedure, a sample