Disorders that reflect the activity of several genes rather than one are known as multifactorial traits. In most cases the environment, especially during pregnancy, plays an important role in determining the severity of the disease in the child. Several relatively common disorders fall into this category: for example, cleft lip and palate, pyloric stenosis (obstruction of the stomach), and spina bifida (defect of the bony spinal column). In general the recurrence risk for parents who have an affected child is in the range of 3-5%. If one parent is affected, the risk for any pregnancy is also in the 3-5% range.
Mitochondria are cytoplasmic organelles that contain DNA (mtDNA) and are essential for the energy-producing activities of a cell. They are inherited from the mother's egg. Mutations in mtDNA result in a diminished ability of the cell to generate a sufficient amount of energy for its needs and lead to the malfunction or death of the cell. This is especially critical for muscle and nerve cells that have a high energy requirement. Diseases that are caused by mtDNA mutations include Leber hereditary optic neuropathy (loss of central-field vision), myoclonic epilepsy with ragged-red fiber disease (epilepsy, uncoordinated muscle movement), and Kearns-Sayre disease (cerebellar damage, heart failure).