Human ABO blood type is controlled by the I locus and has three possible alleles. The alleles which code for type A blood (IA) and type B blood (IB) are codominant, while the allele for type O blood (i) is recessive. In a cross between a person with a genotype of IAi and a person with genotype IAIB, what is the probability of seeing three offspring with type A blood?

1/8 (12.5 %)

Explanation:

Cross: IAi X  IAIB

F1: 1/4 IAIA 1/4 IAIB 1/4 IAi 1/4 IBi

Phenotypes:

- IAIA and IAi >> type A blood

- IBi >> type B blood

- IAIB >> type AB blood

Phenotypic frequency type A blood: 1/2  (i.e., 1/4 IAIA +  1/4 IAi) >>

Probability to have 3 offspring with type A blood: 1/2 x 1/2 x 1/2 = 1/8

The human ABO blood group is a system consisting of there different alleles: A, B, and O. This system is used in genetics as an example of codominance because both A and B alleles are expressed in heterozygous individuals (i.e., IAIB), whereas only the allele A is expressed in individuals that have A and O alleles (since O allele is recessive to the A and B alleles).

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