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.

Steve and Karen had a 1/4 probability of having the disease (XX), x 1/2 probability of being a carrier (Xx), and x 1/4 probability of not being a carrier (XX).

We know that Steve and Karen are either Xx or XX since they don't have the disease. That means that there is a 2/3 probability that each of them is a carrier (Xx).

If child have disease, then their parents must be carriers, so 2/3 x 2/3 probability that their parents were carriers = 4/9.

Out of the 4 chances they are carriers out of 9, 1 of their offspring will be XX (non-carriers), 2 will be Xa (carriers) and 1 xx (disease).

It means that the probability that if these parents have a child, then the probability of child will have sickle-cell disease is 1/9.

P=1/9