That article speaks of depression as a disease. What are the scientific and verifiable tests one can conduct to see if one has contracted this disease? Is there a blood test for depression?
Just because a condition can't be diagnosed via traditional methods such as blood testing does not make said condition any less legitimate. Mental disorders arise from oftentimes subtle aberrations in brain structure and chemistry for which there is no known test for. The best we have are the inferences made through psychiatric evaluation. However, brain MRIs of people with depression show a marked difference from those without it, lending credence to the hypothesis that depression's origins lie, at least in part, in biology.
That's what the translator says he wrote.
It is highly doubtful there is any error with the translation.
Regardless, jumping from a letter that describes his state at the time to asserting that he had a genetic disease that made him depressed is ludicrous.
One or two mentions of depression? Likely part of the normal mood cycle all humans go through. But as the number of instances increases, a pattern begins to emerge -- a pattern that is consistent with the symptoms of major depressive disorder.
Furthermore, Nietzsche had an image to maintain, especially in light of his writings on the overman. It is likely he sequestered his true emotional state in order to maintain this image. Thus it is not unreasonable to assume his depression was likely more severe than he let on.
The article also states directly that clinical depression is not heritable: "So no one simply “inherits” depression from their mother or father. Each person inherits a unique combination of genes from their mother and father, and certain combinations can predispose to a particular illness."
That claim is much less certain than yours.
There is no single 'depression gene'. Scientists believe there are a multitude of genes whose interrelationships give rise to a proclivity
towards depression. While environmental factor undoubtedly do play a role, one should not discount the 'nature' side of the equation.
In science, claims are assumed to be void until proven true. If you want to assert the existence of depression as a disease and that anti-depressants are safe, you need to back that up.
I hope you realize that cuts both ways. In any case, seeing as you were the first to make a claim on this subject, you can be the first to provide evidence.
It's like a Christian who says to atheists: "You can't prove God doesn't exist, so that means my belief is equally valid."
How are a Christians beliefs any less valid than an atheist's?