In some cases, a single gene is responsible for causing an inherited disorder or birth defect. Other inherited disorders and birth defects are "multifactorial," meaning they are caused by a combination of genetic and environmental factors.
- One gender is often affected more than the other, depending on the specific trait or disorder.
- Multifactorial traits recur in families because they are partly caused by genes.
- Once a child with an ONTD has been born in the family, the chance for an ONTD to occur again is increased to four percent.