Karen and steve each have a sibling with sickle-cell disease. neither karen nor steve nor any of their parents have the disease, and none of them have been tested to see if they have the sickle-cell trait. based on this incomplete information, calculate the probability that if this couple has a child, the child will have sickle-cell disease. express your answer as a fraction using the slash symbol and no spaces (for example, 1/16).