None of those alternatives sound correct to me. If I had to convert (1) to that sort of pattern then I would say "I have a friend, the son of whom is a doctor", but it seems kind of convoluted, and I prefer the original. "I have a friend, whose son is a doctor" does not look correctly punctuated to me.
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Michelle ChaMy first question here is which sentences are grammatical and the same as sentence 1 in meaning.They are all grammatical and they all have the same meaning.