Work Step by Step
From the pedigree information, the parents must be carriers for some of their children to have the disease. Let H=normal hemoglobin allele and h=sickle cell allele. Thus, the parental cross is Hh x Hh. Since Steve and Karen are not affected, hh, there is a 2/3 chance that either Steve or Karen is Hh. If that is true for both, the only way in this setup that they could have an hh child, the chance of a child being hh is 1/4. Multiply and simplify to get the answer: 2/3 x 2/3 x 1/4=4/36=1/9.