Hereditary diseases such as hemophilia, sickle-cell anemia, Huntington's disease, muscular dystrophy, and Tay-Sachs disease are caused by mutated genes inherited from one or both parents (Genetic Disorders). Certain other diseases, such as diabetes mellitus, hypertension, and some types of cancer, often run in families, which suggests that heredity is at least partially responsible for their development.

Congenital diseases, or birth defects, are disorders that are present at birth. Some are hereditary, others develop while a baby is in its mother's uterus or during the process of delivery. For example, if the mother contracts German measles, or rubella, during the early stages of pregnancy, her child may be born with heart defects, eye cataracts, deafness, or mental retardation. Use of alcohol during pregnancy can cause fetal alcohol syndrome, characterized by mental and physical retardation. Abnormal development of any body part in a fetus may produce a congenital defect; for example, if walls that separate the chambers of the heart fail to form completely, the baby is born with congenital heart disease.