Multiple Allele Crosses - ABO Blood Groups

ABO blood groups are controlled by one gene with three alleles: I^A and I^B are codominant, and both are dominant to I^O. The genotypes and phenotypes are:• Group A: I^AI^A or I^AI^O
• Group B: I^BI^B or I^BI^O
• Group AB: I^AI^B
• Group O: I^OI^OWorked example: a heterozygous group A father and a heterozygous group B mother have children.Parental genotypes: I^AI^O × I^BI^O
Gametes: I^A or I^O (father); I^B or I^O (mother)

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

The expected ratio is 1 AB : 1 A : 1 B : 1 O, so each child has a 0.25 probability of each blood group. Two parents can have a child with a blood group different from both of them.Working backwards. If a group A parent has a group O child, the parent must be I^AI^O, because the child needs an I^O allele from each parent. A group AB parent cannot have a group O child, because they must pass on I^A or I^B.Always state the parents' genotypes, the gametes, the Punnett square and the phenotype of each offspring genotype.