How come that a child of two non autistic people may have autism?

first,special traits are made with complex amount of genes,not only one.Also, these genes can be categorized into two groups;one of them is the recessive gene type ,which does not show its ability and is mostly dominated when combined with the second gene type,dominant.All of these traits are made with the combination of these two gene types.the recessive gene type is only efficient when combined with another recessive gene.however, dominant is active whenever it is in combination.Such diseases like autism are mostly inherited from their ancestors,but the person with autism may have hybrid combination, leading to a child who doesnt have autism.