John and jane are planning a family, but each has a brother who has sickle-cell disease, so they are concerned that their children may develop sickle-cell disease. neither john nor jane nor their respective parents have the disease. they consult a genetic counselor who tells them that

there is no chance that any of their children will have sickle-cell disease
it's possible that they cannot produce a child with sickle-cell disease, but blood tests on them both will be required to make sure
all of their children will have sickle-cell disease
one of four of their children can be expected to have sickle-cell disease