The blood group antigens are controlled by an autosomal gene designated I (isohaemaglutinin) which has 3 alleles IA, IB and IO.
Allele IA is dominant over the allele IO and produces type A blood. IOIO produces type O blood. An A type individual can have the genotype IAIA or IAIO. Similarly allele IB produces blood group B, and is dominant over IO. Based on this information, we can deduce that the father is IAIO and mother is IBIO.
Only if the parents have this combination of genes, will children with O blood group be formed. This will happen when IO of father combines with IO of mother. When IO of father combines with IB of mother, the child will have blood group B. Also when IO of mother combines with IA of mother, the child will have blood group A.
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