Draw a pedigree containing two parents and four children. Both of the parents have AB blood type. The first child is type A, the second child is type AB, and the third child is type B.

The fourth child tests as having blood type O, which is not possible given the parental genotypes. Look at Figure 4.4 and read the description of the molecular process that generates ABO blood group antigens. What other mutation could account for this observation?