Birth Defects

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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 inherit one of four different combinations: AA, Aa, aA, or aa. If the recessive gene a is defective, the statistical probability is that one in four of the offspring will bear the defective trait. In other congenital disorders the presence of only one copy of the recessive gene is sufficient to cause the condition. See also Genetics.

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