The right answer is 25%
The offsprings are: AO, OO, AB, BO
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Red blood cells with phenotype O can only correspond to a single OO genotype, but to phenotype A can correspond homozygous AA or heterozygous AO subjects. Phenotype B may also correspond BB homozygotes or BO heterozygotes. On the other hand, subjects AB are necessarily heterozygous, having inherited an allele A and a allele B.
