A child with type O blood can only have parents that carry the 'i' allele. Couple 1 (Mother's Type A, Father's Type B) and Couple 2 (Mother's Type O, Father's Type A) might carry the recessive 'i' allele necessary to produce a type O child. Couples 3 and 4 cannot be the child's parents as a type AB individual lacks the 'i' allele.
If a child has type O blood, this means that the child's genotype is ii, possessing neither the A nor the B allele. Parents of the child must each contribute an 'i' allele. Looking at the options, Couple 2 (Mother's Type O and Father's Type A) could be the parents, considering that the father could genetically be AO (carrying a recessive 'i' allele).
Similarly, Couple 1 (Mother's Type A and Father's Type B) could potentially be the parents too if both the mother and father carry the recessive 'i' allele, meaning their genotypes could be AO and BO, respectively. Figure 17.6.4 supports this by showing that parents with AO and BO genotypes can have children with A, AB, B, and O blood types.
However, Couple 3 (Mother's Type AB and Father's Type O) and Couple 4 (Mother's Type B and Father's Type AB) cannot be the parents of a child with type O blood. A type AB mother cannot pass on an 'i' allele to the child because AB genotype only contains A and B alleles. Likewise, a type AB father cannot contribute an 'i' allele either.
Therefore, the correct answer is couple 1 or 2 only.