Speculation: If you went back in time and murdered your grandfather your hair would change colour. Or you would become taller. Or shorter. Or fatter, or thinner, or black, or white, or have different shaped ears, nose, etc. Or whatever... Because of the necessity of carrying the genes of a different grandfather. So you'd try the experiment again. Kill another grandfather. And another. And another. And another. Until eventually you'd be arrested by the time police. "Got another one sargeant, one of these damn serial grandfather killers!"