Human Birth Defects and Genetics

Birth defects are structural abnormalities that develop as a fetus grows in the womb. They often have complex or unknown causes. In particular, genetics can interact with other conditions, including environmental factors, to cause some human birth defects.

Genetic Transmission

Genes determine many aspects of physical development. As a rule, one copy of each gene is inherited from each parent. Several types of inheritance are associated with transmission of genetic information.

Some traits follow dominant inheritance; only one copy of the gene is necessary to experience a condition, and it overrides the other copy. Others follow recessive inheritance; two copies of the gene are needed in order for its effects to manifest, and individuals with a single copy are unaffected. Other genes follow more complex patterns of inheritance. This is likely the way birth defects are affected by genetics; the extent of the effect of genetics and the exact methods of transmission are not fully understood.

Genetic Defects and Disorders

Certain genetic defects cause syndromes, which are clusters of typical symptoms and conditions associated with a particular genetic condition. This type of genetically transmitted syndrome isn't always classified as a genetic "birth defect," although it may include characteristic physical abnormalities present from birth.

For example, Velo-cardio-facial syndrome causes abnormalities of the face (including cleft lip or palate), as well as eye, ear and heart defects. A birth defect, on the other hand, is more likely to affect only one structure (such as the lip or palate in orofacial clefts), and to and to have genetics as a contributing factor, rather than a definitive cause.

Genetic Birth Defect Factors

Human birth defects can have a complex network of causes. Genetics can make a developing baby more susceptible to a birth defect, particularly when it occurs in the presence of certain environmental factors, or conditions such as maternal infection. Genetics can also increase chances of developing birth defects with exposure to materials found to be toxic to fetal development, called teratogens. When genetics and environmental factors interact to cause a birth defect, it is called "multifactorial inheritance."

Though genetics are not the sole cause of most birth defects, you may be more likely to have a child with a birth defect if you have already had one or more affected children. Such is the case for neural tube defects, which affect the development of the brain and spinal cord very early in pregnancy. Though genetics do not cause neural tube defects, they do interact with environmental factors in such a way that they are more likely to occur in families with a history of genetic defects. Thus, it is important to speak to your doctor about extra steps to take to safeguard your pregnancy if you are found to be susceptible to genetic influence.