Final answer:
If one parent has blood type A and the other has type B, their children could have any blood type: A, B, AB, or O. This is because both A and B can be heterozygous, carrying a recessive 'i' allele. Thus, an individual with type O blood could be born to such parents if both parents are heterozygous.
Step-by-step explanation:
The ABO blood group system demonstrates codominance and multiple alleles. For a person with blood type A, the possible genotypes could be homozygous (IAIA) or heterozygous (IAi), and for type B, the genotypes could also be homozygous (IBIB) or heterozygous (IBi). If one parent has type A blood and the other has type B, their offspring could potentially have any blood type: A, B, AB, or O, depending on whether each parent carries the recessive i allele.
For example, if the type A parent is heterozygous (IAi) and the type B parent is also heterozygous (IBi), the possible genotypes for the offspring can be IAIA, IAi (both resulting in Type A), IBIB, IBi (both resulting in Type B), IAIB (resulting in Type AB), or ii (resulting in Type O)
In this situation, an individual with blood type O (genotype ii) could indeed be the legitimate child of these parents, as each parent could have contributed one recessive i allele to the offspring. This also means that each blood type (A, B, AB, O) has an equal chance of appearing in the offspring - a clear demonstration of codominance and multiple alleles at play in the ABO blood system.