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A child with type O blood has two O alleles because the O allele is recessive to both the A and B alleles. This means that both parents must carry one copy of the O allele because each parent must contribute an O allele to the child. The mother must be heterozygous, meaning she has the genotype AO. The father can be type O, meaning he has two O alleles and can contribute one O allele to his child. He can also have genotype AO or BO; he could then be either type A blood or type B blood, where the O allele is masked, but he can transmit it to his child.