How do genetics affect mental illness?

One of the major causes of mental illness is genetics. In other words, mental illness can be hereditary, therefore if a family member suffers one, you'll have a predisposition to that illness.

Mental illness can  be caused by an abnormality in some genes. For example, schizophrenia has been related to abnormalities in a sever number of genes. Also, this kind of illnesses can be caused for a wrong interaction between genes .
In a recent research, it has been shown that two of our genes,that codify to cells to regulate the amount of Calcium that goes into our neurons,  also affect other parts of our brain system. That will explain why an abnormality in these genes may cause problems related to emotions, thinking, attention or memory,  increasing the risk of mental illness.

DATA REVEAL EXTENT OF GENETIC OVERLAP
 BETWEEN MAJOR MENTAL DISORDERS

Mental disease can also be developed because of a negative environmental exposure before or after birth. Those environmental threats are an early exposure to drugs, both consumed by the parents or introduced in the child body, or psycho-social conditions that can affect the perceptions the individual may have of the world in the future.

Actually, there are some disease that have shared symptoms, what can possible mean that they have similarities in a biological level. An example of illnesses that may be caused by this similarities are: autism, ADHD (Attention Deficit Hyperactivity Disorder), bipolar disorder, major depression and schizophrenia.