Schizophrenia is known to have genetic factors that can increase its likelihood, but is also linked to environmental factors like cannabis use and stress. The group without the environmental factors has a lower incidence of schizophrenia, so it's generally believed that the onset of schizophrenia is triggered by them.

Male homosexuality is believed to have a small genetic component but is also linked to birth order. One theory is that in utero testosterone from older brothers can affect the sexuality of a younger brother.

I disagree that it makes no sense for environmental factors to lead to the onset of autism, on the basis that environmental factors can make someone gay or schizophrenic. That is to say, they can influence a personality and a mind in complicated ways that don't seem intuitive.