Blood type is a common inherited trait classified by the ABO system, which relies on the presence or absence of specific markers, called antigens, on the surface of red blood cells. These antigens are determined by the genes an individual inherits, following predictable patterns established by Mendelian inheritance. Understanding how these genes interact is central to predicting the blood type of any offspring.
The Basics of Blood Type Inheritance
Blood type is determined by a single gene located on chromosome 9, which exists in three primary variations, or alleles: A, B, and O. Every person inherits two of these alleles, one from each biological parent, forming their unique genetic combination, known as the genotype. The physical expression of this genotype, which is the actual blood type detected in a lab test, is called the phenotype.
The four main blood type phenotypes are A, B, AB, and O. For example, a person with Type A blood may have a genotype of two A alleles (AA) or one A and one O allele (AO). Similarly, a person with Type B blood can have a genotype of BB or BO.
The Recessive Nature of Type O Blood
The inheritance pattern of the ABO system is governed by relationships of dominance and recessiveness. The A and B alleles are considered co-dominant, meaning that if a person inherits both (genotype AB), both antigens are expressed, resulting in Type AB blood. Both the A and B alleles are dominant over the O allele.
Because the O allele is recessive, its trait—the absence of A and B antigens—is only expressed if no dominant A or B allele is present. Therefore, for a person to have the Type O blood phenotype, they must inherit an O allele from both parents, resulting in the homozygous genotype, written simply as OO. If an individual inherits an A allele and an O allele, the dominant A allele will mask the O allele, and the person will have Type A blood.
Predicting Offspring Blood Types
The rules of genetic inheritance provide a clear answer to whether two parents with Type O blood can produce a child with Type A blood. Since both parents must have the OO genotype to express the Type O phenotype, they can only contribute the O allele to their offspring.
Consequently, when two Type O parents reproduce, the combination of alleles in the offspring must result in the mandatory OO genotype. This genotype can only produce the Type O blood phenotype. Under the standard mechanism of the ABO blood group system, 100% of the offspring from two Type O parents will be Type O. It is genetically impossible for a child from two Type O parents to inherit the necessary A allele.
Explaining Apparent Genetic Mismatches
While the rules of ABO inheritance are fixed, the perception of a genetic mismatch, such as two Type O parents having a Type A child, suggests a non-standard explanation. The most common explanations involve non-genetic factors, such as a case of misattributed paternity or adoption. In these situations, the child’s biological father is not the individual assumed to be the father, which would introduce the necessary A allele into the child’s genotype.
In extremely rare instances, a genetic anomaly called the Bombay phenotype can create a scenario where a person may possess the genes for Type A blood but physically test as Type O. This occurs because the individual inherits a rare combination of genes that prevents the formation of the H antigen, which is the precursor molecule required to build the A and B antigens on the red blood cell surface. Without the H antigen, the A antigen cannot be expressed, and the blood test identifies the person as Type O. This condition is exceptionally uncommon and requires specialized testing to be identified. Errors in laboratory blood typing procedures, while infrequent, can also lead to misidentification.