Multiple Allele Crosses - ABO Blood Groups

Crosses involving the ABO blood group gene use the same method as any monohybrid cross. Remember: I^A and I^B are codominant, and both are dominant to I^O.Cross 1: heterozygous group A × heterozygous group B
Parents: I^AI^O × I^BI^O
Gametes: I^A or I^O, and I^B or I^O

	IB	IO
IA	IAIB (AB)	IAIO (A)
IO	IBIO (B)	IOIO (O)

Ratio 1 AB : 1 A : 1 B : 1 O. This one cross can produce all four blood groups, and a child can have a group different from both parents.Cross 2: group AB × group O
I^AI^B × I^OI^O gives I^AI^O and I^BI^O in equal numbers: 1 group A : 1 group B. None of the children can be group AB or group O.Working out genotypes from a family
A group A father and a group B mother have a group O child. The child must be I^OI^O, so it received I^O from each parent. The father must be I^AI^O and the mother I^BI^O.Worked example
For the parents in Cross 1, what is the probability that two children are both group O?0.25 × 0.25 = 0.0625Each fertilisation is independent, so the probabilities are multiplied.