Schizophrenia is something like 50 percent concordant in identical twins, if your twin has schizophrenia, you're 50 times as likely to be schizophrenic as the general population. One parent, 6 percent, two parents 25 percent.

If you consider mental illness in general, the concordance with two parents gets stronger. The study written up here puts probability of some psychiatric diagnosis at around 67 percent, the probability of a "related" disorder (schizoaffective, paranoid etc.) at 40 percent.

https://www.schizophreniaforum.org/...ring-skyrockets

The show please like me once quoted a statistic on bipolar, I can't find the source, but I've seen it. How's that for a reference? Anyways, the quote was that 80 percent of people who were bipolar had at least one parent who was bipolar. But if a bipolar person had children, only 10 percent would be bipolar. Things are likely to be skewed by the fact that if your parent is in the system, you're more likely to be in the system--I'm third generation undiagnosed schizo-affective-ish.

Even at that first cell division, identical twins aren't identical, unless there was perfect symmetry in that division. Who knows? One twin could get somewhat crummier mitochondria, on average. Or something.