The ABO blood group system, which determines a person’s blood type, is a classic example of complex inheritance that goes beyond simple Mendelian genetics. Blood type is defined by the presence or absence of specific carbohydrate structures, called antigens, on the surface of red blood cells. These antigens are controlled by a single gene involving multiple alleles and two distinct patterns of dominance. A person inherits one allele from each parent, but the population has more than two available alleles.
The Alleles Governing Blood Type
The ABO blood group is controlled by the \(ABO\) gene on chromosome 9. This gene instructs the creation of a glycosyltransferase enzyme, which modifies the H antigen, a carbohydrate precursor molecule, on the surface of red blood cells.
This single gene exists in three forms, or alleles, within the human population, classifying the system as having multiple alleles. These three forms are designated \(I^A\), \(I^B\), and \(i\) (sometimes written as \(I^O\)). The \(I^A\) allele instructs the enzyme to add N-acetylgalactosamine, creating the A antigen.
The \(I^B\) allele codes for a different enzyme that adds D-galactose, resulting in the B antigen. The third allele, \(i\), is non-functional, meaning it codes for an inactive enzyme that does not modify the H antigen. The combination of these three alleles accounts for all four blood types.
Co-dominance and Simple Dominance in ABO Genetics
The inheritance of the ABO blood type demonstrates two different dominance patterns simultaneously: co-dominance and simple dominance. These rules dictate how the three alleles interact to determine the final blood type.
Co-dominance exists between the \(I^A\) and \(I^B\) alleles. When both are inherited, neither masks the other; both are fully expressed. This means the red blood cells display both the A and B antigens, resulting in the AB blood type.
Simple dominance is seen between the \(I^A\) or \(I^B\) alleles and the recessive \(i\) allele. Both \(I^A\) and \(I^B\) are completely dominant over \(i\). If an individual inherits \(I^A\) along with \(i\), the \(I^A\) is expressed, resulting in Type A blood.
The \(I^B\) allele similarly produces Type B blood when paired with \(i\). The \(i\) allele is recessive because the non-functional enzyme it codes for is only relevant when no dominant allele is present. Type O blood requires inheriting two copies of the recessive \(i\) allele.
Translating Genetic Combinations to Observable Blood Types
The interplay of multiple alleles and dominance patterns results in six possible genotypes that translate into four observable phenotypes. The six genotypes are \(I^A I^A\), \(I^A i\), \(I^B I^B\), \(I^B i\), \(I^A I^B\), and \(i i\).
The genotypes \(I^A I^A\) and \(I^A i\) both result in the Type A phenotype, due to the complete dominance of \(I^A\) over \(i\). Similarly, \(I^B I^B\) and \(I^B i\) translate to the Type B phenotype.
The \(I^A I^B\) genotype demonstrates co-dominance, producing the Type AB phenotype where both A and B antigens are present. The final genotype, \(i i\), results in the Type O phenotype, as the lack of a dominant allele means no functional enzyme is produced.
This system explains why two heterozygous parents with Type A blood (\(I^A i\)) might still have a child with Type O blood. Each parent can contribute the recessive \(i\) allele, resulting in the \(i i\) genotype. Conversely, if two Type O parents (both \(i i\)) have a child, the offspring can only inherit two \(i\) alleles, making Type O blood the only genetic possibility.