Birth Defects
Birth Defects, also called congenital disorders, any abnormalities of structure or function that are present at birth. The process of fetal development can be disrupted by a variety of external factors such as exposure to radiation, heat, chemicals, infectious agents, or maternal disease. An external agent that alters fetal development is called a teratogen (Greek teratos, “monster”; genes, “born”). Developmental malformations can also be the result of abnormalities in the genetic makeup of the fetus, or they can be caused by a combination of genetic and environmental influences. Some genetic disorders have symptoms that are manifest at birth. These disorders may result from mutations in a single gene or from more general chromosomal abnormalities. Many diseases and conditions are inherited in a recessive manner: Neither parent may have the defect even though they both carry the causative gene. When both parents have a dominant gene A and a recessive gene a, their offspring may inheri