They are two
different things. The
genes are a set of
instructions, but nowhere is it
guaranteed that those instructions are going to be
correctly followed.
Nature is an
imperfect process, and rather
complex, with plenty of
opportunity for
mistakes in following the
genetic code.
With humans, there's about nine months of pregnancy. Nine months where the body and brain are being developed. Nine months where hormones that go out of whack, where drugs, both prescription and recreational can affect development, where physical harm can occur. None of these can be predicted in the genes.
So, just because something has a biological cause, it doesn't mean you can check the genes and find it.