For over 4,000 years, people have noticed that certain diseases run in families. However, the "why" was unknown until modern science showed how genetic information is transmitted.

Present-day medicine recognizes that genetic diseases are inherited based on the nature of DNA, genes, and chromosomes. Now that the human genome has been completely sequenced, scientists are better able to study how changes in DNA cause human disease. This will ultimately help in diagnosing and treating genetic disorders.

Many diseases that involve a single gene defect can now be diagnosed very early in pregnancy. Prenatal diagnosis looks at fetal cells in the mother's blood, amniotic fluid, or chorionic villi. This may detect problems while the baby is still growing or after birth. In late pregnancy, tests may examine blood from the umbilical cord.

Couples who have a high risk of passing a genetic (hereditary) disorder to a child may consider pre-implantation genetic diagnosis (PGD). The procedure is done in the lab after artificial insemination about 3 - 4 days after fertilization.

Laboratory scientists remove a single cell from each embryo and screen the material for specific genetic disorders. According to the American Society for Reproductive Medicine, PGD can help parents decide which embryos to implant. This decreases the chance of passing a disorder to a child. The technique is controversial and not offered at all centers.

Genetic counseling and prenatal diagnosis provides parents with the knowledge to make intelligent, informed decisions regarding possible pregnancy and its outcome. Some parents choose to become pregnant and have the disease status of the fetus determined early in the pregnancy. The pregnancy is continued if the fetus is disease-free.

If a genetic defect is found in the fetus, parents who decide to continue the pregnancy may be better prepared to care for the infant by educating themselves about the disease in advance. For example, if a baby has a genetic disease that leads to diet problems, both the mother and baby may need to be treated with special diets.

Based on genetic counseling and the potential severity and treatability of the particular disease, parents may elect to adopt, become pregnant with egg or sperm from an unaffected known or unknown donor, prevent pregnancy, or teminate an affected fetus.

Genetic counseling is useful to any adult, child, or newborn who shows unusual symptoms that are known or suspected to be caused by a genetic condition. The counseling will help families choose treatment and care, and early intervention, where possible.

People may also seek genetic counseling if they feel they are at risk for developing a genetic disorder based on their family history. Counseling may or may not lead to the person having tests to determine their "carrier status" for the particular disease.

See also:

• Heredity and disease (genetics)