Blood type is determined by specific protein and sugar molecules, called antigens, displayed on the surface of red blood cells. These molecular markers assign the blood group—A, B, AB, or O—which is dictated by the genetic code inherited from parents. Understanding dominance in the ABO system requires examining the genes that govern this trait. Blood type inheritance involves a complex interplay of multiple alleles, not a simple dominant-recessive relationship, and this mechanism determines blood compatibility for transfusions.
The Three Alleles of the ABO System
The ABO blood group system is controlled by a single gene with three forms, or alleles: A, B, and O. Every individual inherits two alleles, one from each parent, which determine their specific blood type.
The A and B alleles instruct the body to produce enzymes that add specific sugar molecules to the red blood cell surface, creating the A and B antigens, respectively.
The O allele results in an inactive enzyme, meaning it produces no functional antigen. Therefore, a person inheriting two O alleles will have red blood cells displaying neither the A nor the B antigen.
Defining Dominance Codominance and Recessiveness
The relationship between the three ABO alleles involves two distinct patterns: dominance and codominance. The O allele is recessive, meaning its trait—the absence of A or B antigens—is only expressed when two copies are inherited.
Both the A and B alleles exhibit complete dominance over the recessive O allele. For example, an individual inheriting an A allele and an O allele will only express the A antigen (Type A blood). Similarly, inheriting a B allele and an O allele results in Type B blood.
The complexity arises when the A and B alleles are inherited together; in this scenario, they are codominant. Codominance means both alleles are fully and equally expressed in the phenotype. A person with one A allele and one B allele will display both the A and B antigens, leading to the Type AB blood group.
How Blood Type is Inherited
The genetic makeup for blood type is the genotype, while the observable blood type is the phenotype. The three alleles (A, B, O) combine into six possible genotypes, resulting in four phenotypes (A, B, AB, O). For example, Type A blood can result from the AA or AO genotype.
The inheritance of these two alleles explains how blood type can appear to “skip” a generation. For an individual to have Type O blood, they must inherit an O allele from both parents, resulting in the OO genotype. This is the only genotype that produces the Type O phenotype.
A common illustration involves two parents who both have Type A blood but are heterozygous (AO genotype). They both express the A antigen but carry the recessive O allele. When their reproductive cells combine, there is a 25% chance the offspring will inherit the O allele from both parents, resulting in a child with Type O blood.
The Significance of the Rh Factor
The ABO system is only one part of an individual’s complete blood type; the second major component is the Rhesus, or Rh, factor. This factor is governed by a separate set of genes and is entirely independent of the ABO alleles. The Rh factor is another protein that may or may not be present on the surface of red blood cells, determining whether a person is designated as positive or negative.
Unlike the complex codominance and multiple allele pattern of the ABO system, the Rh factor follows a more straightforward Mendelian inheritance pattern involving a simple dominant-recessive relationship. The presence of the Rh protein is the dominant trait, denoted as Rh-positive. The absence of the protein is the recessive trait, denoted as Rh-negative.
A person is Rh-positive if they have at least one Rh-positive allele, while a person is only Rh-negative if they inherit two Rh-negative alleles. This factor is of particular clinical importance in pregnancy, where an Rh-negative mother carrying an Rh-positive fetus may develop antibodies against the fetal red blood cells. Medical professionals routinely screen for this factor during pregnancy to prevent potential complications for the developing fetus.