Genes are small components of every cell. They determine the appearance and function of our body. Genes are packed together in larger structures called chromosomes. Humans have 23 pairs of chromosomes, or 46 total, in every body cell. The sperm and eggs, however, have 23 chromosomes each. Twenty-two of the chromosomes are matched in pairs. These are known as autosomes and are numbered from #1 to #22. Thus, everyone has two copies of genes carried on the matched chromosomes. The 23rd pair, which are the sex chromosomes (X and Y), are matched in females (XX) but not in males (XY). For information about the genetic causes of birth defects and accessing genetic counseling see the Centers for Disease Control and Prevention (CDC) website for basic facts on the causes of birth defects and the March of Dimes website on genetic counseling.
There are three types of genetic disorders and birth defects:
In single gene disorders, there is a change in the genetic blueprint. These disorders are passed on from parents to children in different ways. The most common ways are:
Autosomal dominant disorders are transmitted from an affected parent to an affected child (e.g., neurofibromatosis). Genes for autosomal recessive disorders are passed from two unaffected carrier parents to an affected child (e.g., cystic fibrosis). In the X-linked dominant pattern, the disease is passed from an affected mother to her sons or daughters. In the last pattern, X-linked recessive, the gene for the disorder is passed from an unaffected or mildly affected carrier mother to an affected son (e.g., hemophilia).
Chromosomal anomalies occur when there is a change in the number or structure of chromosomes. For example, Down syndrome, a common cause of mental retardation, results from the presence of an extra chromosome #21. Chromosomal abnormalities are usually caused by an error in cell division. Many are incompatible with fetal development and are an important cause of miscarriage, stillbirth, and infertility.
Multifactorial disorders are caused by the interaction of several genes with environmental factors. Spina bifida, cleft lip and palate, and clubfeet are examples of birth defects with multifactorial etiology. Schizophrenia and mood disorders are also multifactorial conditions. Some of the so-called life style disorders, such as cardiovascular disease, alcoholism, and lung cancer, are also multifactorial in that a genetic predisposition interacts with environmental factors to cause the disease.
For information about the genetic causes of birth defects and accessing genetic counseling see the Florida Regional Genetic Program information page and the March of Dimes Genetic Counseling information page.
Certain environmental factors, such as exposure to certain drugs and chemicals, infections that occur during the pregnancy, and the overall health of the mother both before and during her pregnancy, may interfere with a baby's development.
Some drugs and chemicals are known to cause structural or functional abnormalities in the embryo or fetus. They are generally referred to as teratogens, and include drugs such as thalidomide, retinoic acid, valproate (a seizure medication), and others. Little is known about the potential effect that many other drugs may have on a particular embryo or fetus. For this reason, no drug can be considered safe during pregnancy. Consequently, a pregnant woman should never take a medication that has not been prescribed for her own benefit or that of her fetus. When prescribing a drug, the physician must weigh the benefits against the possible risks to both the mother and the fetus. He/she must also consider the possibility that a woman may not be aware of her pregnancy or may become pregnant while taking the drug.
Different chemicals may pollute the environment or contaminate the workplace. Mercury, PCB, dioxin, and other herbicides are some examples.
Alcohol is the most commonly used drug in the world. The use of alcohol during pregnancy interferes with normal prenatal development. Children born to women who have consumed alcohol during pregnancy may show different problems. These may includes Fetal Alcohol Syndrome or other physical, neurologic, and/or behavioral abnormalities recognized as Fetal Alcohol Spectrum Disorders (FASDs).
Rubella (German measles), cytomegalovirus, herpes, varicella, and toxoplasma are among the infectious agents that may produce congenital anomalies in infants. Vaccination of non-immune females before pregnancy has markedly decreased the incidence of congenital rubella. The recently introduced varicella (chicken pox) immunization may have the same effect on congenital varicella.
Maternal diseases may also produce abnormalities in embryonic and fetal development. The best recognized ones are diabetes mellitus, phenylketonuria (PKU), myotonic dystrophy, and myasthenia gravis. In addition, maternal high temperatures, whether caused by disease or other factors (e.g. sauna) also may produce birth defects.