Research has found that autism clusters in families. The federal Centers for Disease Control and Prevention (www.cdc.gov) has data showing the following diagnosis rates among family members:
- Identical twins, who have the same genetic makeup, have about a 75 percent concordance rate (meaning that both twins have autism).
- Fraternal (nonidentical) twins have a 3-percent concordance rate.
- The risk of autism in normal siblings ranges from 2 to 8 percent.
- Among families that contain diagnoses of autism, research shows a 10- to 40-percent increase in the diagnoses of other developmental disabilities, such as language delays and learning disabilities.
Researchers have concluded that families that carry autism genes also carry other conditions in members who don’t necessarily have autism. The inheritance pattern for autism spectrum disorders is complex and suggests that mutations in a number of different genes (at least 10) may be involved, according to some research. That explains what Temple Grandin, an author and professor who has autism, calls the “highly variable nature” of autism.
Craig Newschaffer at the Johns Hopkins School of Medicine estimates that 60 to 90 percent of all autism cases are genetically based. However, because of the complex nature of autism genetics, scientists don’t have a test parents can order to see if their children are at an increased risk of developing the disorder.
Autism also tends to occur more frequently among individuals who have certain inherited medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU).