Blood types classify human blood based on the presence or absence of specific surface markers, known as antigens, on red blood cells. These antigens, designated A and B, act as identifiers for the immune system. The four main blood types—A, B, AB, and O—are inherited from parents following predictable genetic patterns. Understanding these inheritance rules helps determine the possible blood types of a child.
Understanding the ABO Alleles
The ABO blood type is determined by a single gene located on chromosome 9. Every person inherits two copies of this gene, known as alleles (one from each biological parent), which come in three primary forms: A, B, and O.
The relationship between these three alleles dictates the resulting blood type. The A and B alleles are co-dominant; if both are present, both antigens are expressed, resulting in Type AB blood. Conversely, the O allele is recessive, meaning its trait is only expressed when two copies are inherited.
This dominance pattern results in specific genetic combinations, or genotypes, for each blood type, or phenotype. A person with Type A blood has a genotype of either AA or AO, and a person with Type B blood has a genotype of BB or BO. Type AB blood is represented by the AB genotype, while Type O blood can only result from the genotype OO. The O allele produces a nonfunctional enzyme, which means it fails to modify the red blood cell surface, leaving the cell without A or B antigens.
The Predictive Outcome of Two Type O Parents
The question of whether two Type O individuals can have a Type A child is answered directly by genetic rules. A person who expresses the Type O phenotype must carry the OO genotype. This is because the O allele is recessive, and the presence of any dominant A or B allele would result in Type A, B, or AB blood.
A Type O parent possesses only O alleles and must contribute one O allele to their offspring. When two Type O parents reproduce, the only possible combination of alleles they can pass to their child is O from the first parent and O from the second parent.
The resulting child’s genotype must therefore be OO, which results in a Type O blood phenotype. Type A blood requires the inheritance of at least one A allele (AA or AO genotype). Therefore, it is genetically impossible for two true Type O parents to produce a Type A child under standard Mendelian inheritance.
Explaining Apparent Deviations from Inheritance Rules
In the rare instance where a Type O couple appears to have a non-Type O child, the discrepancy is usually explained by factors outside of standard genetics. The most common explanation for an unexpected blood type is non-paternity, meaning the presumed biological father is not the actual father. This is a frequent cause of confusion in paternity cases.
Testing Errors
Another possible cause for a reported deviation is an error during the blood typing procedure. Although modern laboratory techniques are highly accurate, mistakes can occur in sample handling or testing, leading to an incorrect assignment of blood type for one of the parents or the child.
The Bombay Phenotype
In extremely rare cases, a genetic anomaly known as the Bombay Phenotype can mask a person’s true ABO type. This occurs when an individual inherits a rare recessive genotype (hh) at a separate gene locus, preventing the expression of the H antigen. The H antigen is the precursor molecule upon which the A and B antigens are built. Without the H antigen, red blood cells will test as Type O, even if the person carries A or B alleles, effectively hiding their true genetic potential. This phenomenon is exceptionally rare, found in about one in a million people in European populations.