What Blood Types Can Produce a Child with Type O Blood?

Both parents must have type O blood or carry the recessive gene for type O blood in order to produce a child with type O blood. The inheritance of blood types generally follows Mendel's laws. Typically, the inheritance of the ABO blood type system is determined by a single gene. The ABO gene is located on the long arm of chromosome 9 (9q34) and has three main alleles: i(A), i(B), and i(O). The products of i(A) and i(B) are active enzymes that catalyze the synthesis of A and B antigens, respectively, while the product of the i(O) allele lacks enzymatic activity and cannot catalyze the synthesis of A or B antigens.

Since human chromosomes are diploid, an individual typically possesses two of the three alleles, one from each parent. The combination of these two alleles, known as the genotype, determines the phenotype, or blood type, of the individual. I(A) and I(B) are dominant to i(O), meaning that only individuals with the genotype ii will have type O blood. Individuals with the genotype I(A)I(A) or I(A)i(O) will have type A blood, and those with the genotype I(B)I(B) or I(B)i(O) will have type B blood.

Additionally, I(A) and I(B) are codominant, so individuals with the genotype I(A)I(B) will have both A and B antigens, resulting in AB blood type. In rare cases, the I gene may not be expressed due to suppression by upstream genes, resulting in type O blood. Generally, if both parents have type O blood, their children will also have type O blood. If one parent has type AB blood, the children cannot have type O blood. Parents with type A and type O blood cannot produce children with type B or AB blood, and parents with type B and type O blood cannot produce children with type A or AB blood. If one parent has type O blood, the children cannot have type AB blood.