Answer to Question 1
The three defining symptoms of ASD are impaired social interaction, impaired communication, and restricted, repetitive behaviors. Also sometimes seen are sensory deficits, self-injurious behavior, and savant performance.
Answer to Question 2
The prevalence of ASD is much greater among siblings of a child with ASD. Researchers have also found higher concordance rates among MZ than among DZ twins. In the largest twin study to date, concordance rates were 60 percent for MZ twins and 0 percent for DZ twins. For a broader spectrum of disturbances, concordance rates were 92 percent for MZ pairs and 10 percent for DZ pairs. Other research has suggested that a combination of different genes, or even spontaneous genetic mutations, may be involved.