Human blood type is determined by codominant alleles. There are three different alleles:
[tex]\begin{gathered} I^A \\ I^B \\ i \end{gathered}[/tex]
The I^A and I^B alleles are codominant, and the allele i is recessive.
The possible phenotypes are type A, type B, type AB, and type O
Type A and B can be homozygous or heterozygous, type AB is the result of the interaction between the codominant alleles, and type O individuals are homozygous recessive:
[tex]\begin{gathered} typeA\colon I^AI^AorI^Ai \\ typeB\colon I^BI^BorI^Bi \\ typeAB\colon I^AI^B \\ typeO\colon ii \end{gathered}[/tex]
If John's mom has type B blood, then she can be either homozygous or heterozygous:
[tex]I^{B^{}}I^BorI^Bi[/tex]
The correct option is the third one.
