How is blood type inherited?
Blood type is inherited in a Mendelian pattern governed by the ABO gene located on chromosome 9. This gene has three primary alleles: IA, IB, and i. The IA and IB alleles are codominant, while the i allele is recessive. An individual inherits one allele from each parent, resulting in six possible genotypes—IAIA, IAi, IBIB, IBi, IAIB, and ii—which correspond to the four major ABO phenotypes: type A, type B, type AB, and type O, respectively. For example, a person with genotype IAi expresses blood type A because the IA allele encodes functional A-transferase enzyme that adds N-acetylgalactosamine to the H antigen, whereas the i allele produces no functional enzyme. Similarly, type AB individuals express both A and B antigens due to coexpression of A- and B-transferases, while type O individuals (genotype ii) express only the unmodified H antigen.
In addition to the ABO system, the Rh factor—particularly the RHD gene on chromosome 1—is independently inherited. The presence or absence of the RhD antigen determines whether a person is Rh-positive or Rh-negative. The Rh-positive trait is dominant; thus, individuals with at least one functional RHD allele (genotype RHD/RHD or RHD/rhd) are Rh-positive, while those homozygous for the nonfunctional or deleted rhd allele (rhd/rhd) are Rh-negative. Because ABO and Rh systems segregate independently, a person’s full blood type (e.g., A-positive or O-negative) reflects the combined inheritance of both loci.
Understanding blood type inheritance is clinically essential—not only for safe transfusion and transplantation but also for predicting and managing hemolytic disease of the fetus and newborn (HDFN), particularly in Rh-incompatible pregnancies. Genetic counseling may be indicated when parental blood types suggest an increased risk of ABO or Rh incompatibility.