Disorders where genetics play an important role, so-called genetic diseases, can be classified as single gene defects, chromosomal disorders, or multifactorial.
Schizophrenia has complex genetic heritability. It is also genetically heterogeneous. To the extent that genes are associated with symptom constellations in schizophrenia, they do so by affecting the development and function of neural systems that mediate the expression of such diverse behavioral, cognitive and perceptual phenomena.
Our brain is made of circuits. One of the key thinking circuits links a region that deals with reward (striatum) and a region that manages thoughts and actions (prefrontal cortex). There is a protein in the striatum that acts as switch between these two. The cortex is the one that processes the streams of information from the different brain parts.
Now, the "switch" protein is produced as result of a gene. This gene has more variants. One of these variants optimizes connections between cortex and striatum so it makes people better at cognitive tests and more efficient in filtering information..so it makes people smarter!
There is also another aspect not so beneficial! Children from families with a history of schizophrenia commonly have this gene. Normality it works great but if environmental events or genes interfere, the cortex can't handle the streams of information and backfires predisposition to schizophrenia.
It is such thin threshold between such different realities!