"Where is the house key?"
Translation:Ev anahtarı nerede?
That is incorrect Turkish. I am not sure which suffixes you are adding to which words.
First, there is no genitive case here. "Ev anahtarı" is a noun compound with the 3rd person possessive suffix after it (which is how you form noun compounds).
I think what you were trying to do was add the genitive case and the possessive suffix to anahtar, which isn't how this is done.