Most birth defects are caused by genetic or environmental factors or a combination of the two (multifactorial birth defects). In most cases, however, the cause is unknown.
Genetic or inherited causes include: Chromosomal defects - caused by too few or too many chromosomes, or problems in the structure of the chromosomes. Example includes Down syndrome (and extra copy of chromosome 21 and sex chromosome abnormalities (missing or extra copies of the sex chromosomes X or Y).
Single gene defects – a mutation in one gene causes the defect.
Dominant inheritance – A person can inherit a genetic disease when one parent (who may or may not have the disease) passes along a single faulty gene. Examples include achondroplasia and Marfan syndrome Recessive inheritance – occurs when both parents who do not have the disease pass along the gene for the disease to the child.
Examples are cystic fibrosis and Tay Sachs. Multifactorial birth defects are caused by a combination of genes and environmental exposures. In other words, a person can inherit a gene that increases sensitivity to an environmental trigger. Examples include cleft lip or palate, certain heart defects, and neural tube defects.
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