## Problem 12

Karen and Steve each have a sibling with sickle-cell
disease. Neither Karen, Steve, nor any of their parents has the
disease, and none of them has been tested to reveal sickle-cell
trait. Based on this incomplete information, calculate the
probability that if this couple should have another child, the child
will have sickle-cell anemia.
In order for Karen and Steve to
have siblings with sickle cell anemia their parents must be carriers
(Nn). We also know that John and Carol are not homozygous recessive
(nn) because they do not have the disease. Therefore the chance
that Karen is a carrier is 2/3 (NN, Nn, nN) and the chance that Steve
is a carrier is also 2/3. If they have a child and both Karen and
Steve are carriers then the child has one chance in 4 of having
sickle cell anemia. Since each event is independent of one another
the overall probability of the child having sickle cell anemia
is:

2/3 x 2/3 x 1/4 =
1/9.