Due to personal experiences, I do believe mental illness is genetic. Even though there may be no scientific evidence to support this, you can see it from watching certain families members who have a mental illness. There are many parts of a biological make up, and that involves looks along with personality factors.
Yes, mental illness can be genetic. There have been cases where schizophrenia was diagnosed in a parent and later in one of their children. In families with several children you usually have at least one of the children turn out to be almost a clone of one of the parents – in looks, mannerisms, speech patterns, etc. If these likenesses are genetic, then it seems right that mental illness could also be genetic and passed on to a child.
For some mental issues, researchers have found links between a persons genetic makeup and illness. A recent example of this is in the field of Alzheimer's data where neurologists have found links to genetic factors that allow someone's brain to atrophy early on in life. They have also found gene markers that can explain why some people can inherit things like schizophrenia and bipolar disorder.
Genes could contribute and in fact definitely contribute since technically without any genes you wouldn't have formed in the womb in the first place. But mental illness is not itself genetic. Mental illness just means you are observed by a psychiatrist to fit a list of symptoms. They don't even use brain scans in diagnostics, so you might not even have the typical brain chemistry that would go along with it. In fact you could have the typical brain chemistry and none of the problems. One psychiatrist had a brain scan that looked an awful lot like psychopathic serial killers but was totally healthy though his family commented that some of his personality traits were similar, but that's just personality traits and what that suggests is that at the very least the chemical imbalance theory is exaggerated. More likely than the brain chemistry causing the illness the brain chemistry causes a certain personality disposition that is then more likely to develop certain mental illnesses over others in the wrong environment(and environment can include choice too remember everyday when you make choices about what to do and what and who to interact with that's guiding the sort of environment you are exposed to). This helpless image of mental illness is often portrayed in the media. It's not beyond people's control in most cases, it's more like a person has a set of bad habits that are difficult to restrain and they may not want to but if they want to and are dedicated they can.
Some mental illnesses may run in families while others may not. So much is misunderstood about mental illnesses that it may be hard to determine. Alcoholism is one such illness that may be passed from one generation to the next. Yet other forms of mental illness may occur due to a brain injury, traumatic event or a chemical imbalance.
There are probably some genetic factors to mental illness, but there are also many other factors. Environment plays a big role- people in stressful situations are going to develop mental illness more frequently. Of course, there are other physical factors, too; someone who suffered a brain injury is more likely to develop a mental illness. Genetics is far from the only, and, I think, far from the most important factor determining whether someone is going to get mental illness.