Blood types are a fascinating aspect of human biology, often sparking curiosity about how these characteristics pass from parents to children. While some inheritance patterns seem straightforward, others, like the possibility of parents with A and B blood types having a child with AB blood, might appear counter-intuitive at first glance. Understanding how blood types are determined involves delving into the specific biological markers present in our blood and the genetic rules that govern their transmission across generations.
What Are Blood Types
Human blood is categorized into different types based on the presence or absence of specific markers, known as antigens, found on the surface of red blood cells. The ABO blood group system is the most important classification, defining four main blood types: A, B, AB, and O. Type A blood has A antigens, and Type B blood has B antigens. Individuals with Type AB blood possess both A and B antigens, while those with Type O blood have neither.
How Blood Types Are Inherited
Blood types are inherited from parents, similar to other genetic traits. The ABO blood group is determined by a single gene, the ABO gene, which has three common forms, or alleles: IA, IB, and i. Every person inherits two alleles for blood type, receiving one from each biological parent.
The IA allele leads to the production of A antigens, while the IB allele results in B antigens. The ‘i’ allele does not produce either A or B antigens. In terms of dominance, both the IA and IB alleles are dominant over the ‘i’ allele. This means if an individual inherits an IA allele and an ‘i’ allele, their blood type will be A. The same applies if they inherit an IB allele and an ‘i’ allele, resulting in Type B blood.
The IA and IB alleles are co-dominant. Co-dominance means that when both IA and IB alleles are inherited, both are fully expressed.
Why AB Blood is Possible
Given the principles of blood type inheritance, particularly co-dominance, it becomes clear how two parents can have a child with AB blood. Consider a scenario where one parent has Type A blood and the other has Type B blood. For the child to have AB blood, they must inherit an IA allele from one parent and an IB allele from the other.
A parent with Type A blood can have either two IA alleles (IAIA genotype) or one IA allele and one ‘i’ allele (IAi genotype). Similarly, a parent with Type B blood can have either two IB alleles (IBIB genotype) or one IB allele and one ‘i’ allele (IBi genotype). If, for example, the Type A parent passes on their IA allele and the Type B parent passes on their IB allele, the child’s genotype will be IAIB.
Because the IA and IB alleles are co-dominant, both the A and B antigens are produced and expressed on the child’s red blood cells. This simultaneous expression of both antigens results in the child having Type AB blood.