Regarding genetic factors in schizophrenia, which of the following statements is false?

Regarding genetic factors in schizophrenia, which of the following statements is false?
 
  a. When one identical twin has schizophrenia, the other twin has a 45 to 50 chance of having the disorder. b. About 13 of people with a schizophrenic parent develop the disorder themselves.
  c. The closer the genetic relationship a person shares with a schizophrenic individual, the greater the likelihood that the person will have or develop schizophrenia.
  d. Investigators have identified a single gene that is responsible for creating a genetic predisposition for schizophrenia.
  e. Heredity plays an important role in schizophrenia, but biological and environmental factors also play important roles.