Human blood types are a fundamental biological characteristic. While many are familiar with common blood types, their underlying genetic code, known as genotypes, dictates these observable traits. Genotypes represent the specific combinations of genes inherited from parents, providing the blueprint for an individual’s blood type.
The Genetic Blueprint of Blood Types
Genes are fundamental units of heredity, carrying instructions that determine various traits, including blood type. For blood typing, a specific gene dictates the production of certain proteins on the surface of red blood cells. Different versions of this gene, called alleles, exist, and for the ABO blood group system, there are three primary alleles: A, B, and O.
An individual inherits two alleles for each gene, receiving one from each biological parent. These two inherited alleles together form an individual’s genotype. The interaction between these alleles determines which proteins, or antigens, are present on the red blood cells.
Alleles can exhibit dominance, where one allele’s trait is expressed even when paired with a different allele. In blood typing, the A and B alleles are dominant over the O allele, meaning if an A allele is present, the A antigen will be produced, regardless of whether the other allele is O.
Decoding the ABO Blood Group Genotypes
There are six distinct genotypes that determine an individual’s ABO blood group.
One genotype is AA, where an individual inherits two A alleles, leading to A antigens on red blood cells. Another genotype, AO, involves inheriting one A allele and one O allele. Because the A allele is dominant, individuals with this genotype also display A antigens.
Similarly, the BB genotype means two B alleles are inherited, resulting in B antigens. The BO genotype involves one B allele and one O allele; due to the dominance of the B allele, B antigens are present.
The AB genotype is unique, as it involves inheriting one A allele and one B allele. In this case, both A and B antigens are present on the red blood cells, demonstrating a codominant relationship between A and B alleles.
Finally, the OO genotype occurs when an individual inherits two O alleles. Since the O allele is recessive, no A or B antigens are produced on the red blood cells with this genotype.
From Genotype to Phenotype: Your Observable Blood Type
The relationship between an individual’s genotype and their observable blood type, known as the phenotype, is determined by the dominance and codominance patterns of the alleles. While there are six genotypes, these map to the four commonly recognized ABO blood types.
Individuals with either the AA or AO genotype will have Blood Type A. This is because the A allele is dominant over the O allele, ensuring that A antigens are produced on the red blood cells.
Similarly, both the BB and BO genotypes result in Blood Type B, as the B allele is dominant over the O allele, leading to the presence of B antigens.
The AB genotype uniquely results in Blood Type AB. In this case, both the A and B alleles are expressed simultaneously, meaning both A and B antigens are present on the red blood cells. This phenomenon is known as codominance.
Lastly, only the OO genotype produces Blood Type O. With two recessive O alleles, neither A nor B antigens are produced on the red blood cells.
Inheritance Patterns of Blood Type Genotypes
Blood type genotypes are passed from parents to offspring following predictable patterns of inheritance. Each parent contributes one of their two alleles for the ABO gene to their child.
For instance, if both parents have the OO genotype, they can only pass on O alleles, and all their children will inevitably have the OO genotype and thus Blood Type O. Conversely, parents with different genotypes can produce children with a wider range of blood types, depending on the specific alleles each parent contributes.