An individual can be a carrier for a particular disease yet not have the disease because the disease is recessive. That means to have the disease you need to carry both of the recessive genes of the disease (rr) and to get both the recessive genes your parents need to each be a carrier of the recessive gene, but that doesnt mean they need to have the disease. They just need to carry the gene that causes the disease which is why the disease has to be recessive. So your parents need to both be say Rr. The big R is the dominant gene that does not carry the disease. The little r is the recessive gene that carries the disease. The big R in this case dominates the little r which in a way masks the little r, which is why you dont have the disease but since you carry the little r you carry the disease gene which can be passed down if the other person also carries the little r. So in simple terms a person can carry the disease yet not have it as long as they carry just the recessive gene for the disease which is masked by a normal dominant gene. Sorry if that is confusing! It is hard to explain but to get a better idea of what I mean look you PKU which is a recessive disease where a person can be a carrier but not have the disease! As proof my mom carries the PKU gene and so does my dad but they dont have the disease! Yet my sister does! Good Luck and I hope that helped a little!