Find a parent's blood type given their children's blood type

This question is a little more complex than it seems because of how blood genotypes work.

Blood exhibits codominance among A and B blood types whereas O is a recessive blood type. An individual with type-O blood has genotype OO. An individual with A or B blood could have two dominant alleles, yielding genotype AA or BB, or they could have one parent with type O blood which would make their genotype AO or BO.

To determine the outcome of a paring between these parents you need to know their exact genotypes. You can then draw the Punnett square to determine the odds for a particular genotype in each offspring.

If one parent is AA and the other is OO, all crosses between these parents will yield genotype AO and all children will have blood type A.

If one parent is AO, blood type A but with one recessive allele and the other is OO, as he must be since O is recessive, the Punnett square looks like this

...OO
A
O

What percentage of the offspring will have blood type O? Find the crosses that yield OO from these parents.

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