Iam black when i am clean, i am white when i am dirty. what am i?

Awoman comes to your genetic counseling center because she knows that huntington disease occurs in members of her family. her paternal grandfather was afflicted, but so far her father shows no symptoms. her two great-great grandmothers on her father's side were healthy well into their 90s, and one of her great-great grandfathers died of unknown causes at 45. testing for huntington disease is extremely expensive, but she is concerned that she may fall victim to this disease and wants to plan her life accordingly. after examining her pedigree you advise her to