It seems the jury is still out on what tone deafness actually is and if it can be overcome. Neverless there have ben some insightful postings offering great hope for anyone who 'thinks' they cannot sings. It appears that most feel that singing can be taught and that practice can, and does, makes perfect.
Music in all forms is so rich. There is a vast depth of tunefulness and songs out there. So many singers have made it who, in many a way, have no conventional voice but nonetheless suit what they write/sing. Kris Kristofferson and Leonard Cohen are but two examples. They have fantastic voices, but many would say not in a conventional way. What they do do is sing in tune though. It appears from all that has been written here so far that what we accept from singers is that they should be able to sing in tune. Drifting too far outside of that remit will get censure.
Truly enjoying this thread...