Case 1: Man with blood group A married a woman with blood group O. Blood group for man can be expressed by dominant genotype IAIA or IAIO and blood group O for woman is expressed by recessive genotype IOIO. The daughter will have O blood group type as a result of this IOIO inheritance of recessive traits from mother and father.
Case 2: Blood group A in father if has recessive genotype IAI A and mother has dominant genotype IAIO or IOIO. The daughter will have O blood type as a result of inheritance of dominant allele IO from mother and recessive allele IA from father.
Blood group A is expressed in both homozygous and heterozygous condition but Blood group O is expressed in recessive condition.
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