With hereditary amyloidosis what is the percentage of individuals who exhibit clinical symptoms?

Hereditary amyloidosis (familial amyloidosis): this inherited disorder often affects the nerves, heart and kidneys. It most commonly happens when a protein made by your liver is abnormal. This protein is called transthyretin (TTR). People of African descent may be more likely to carry the gene that causes this kind of amyloidosis. I could not find a reliable answer for your second question.