A person’s blood type is determined by specific genetic rules, meaning a child’s blood type will not always match either parent. Blood type is defined by antigens—molecules present on the surface of red blood cells—which are passed down from both biological parents. Understanding the genetic possibilities involved explains why family members can possess different blood types.
The Components of Blood Type
Blood is classified using two primary systems: the ABO system and the Rhesus (Rh) system. The ABO system categorizes blood into four major types—A, B, AB, and O—based on the antigens found on the red blood cell surface. Type A has the A antigen, Type B has the B antigen, Type AB has both, and Type O has neither.
The Rh system adds a positive or negative sign to these four types, resulting in eight common blood groups (e.g., A+ or O-). This classification is based on the presence or absence of the RhD protein, another antigen. If the RhD protein is present, the blood type is positive; if absent, it is negative. The combination of these two systems defines a person’s complete blood type.
Understanding Alleles and Genetic Rules
Blood type inheritance follows Mendelian genetics, where an individual receives one gene variant, or allele, from each parent. The ABO blood type is governed by three possible alleles: A, B, and O. Since everyone inherits two alleles, their combination determines the final blood type.
The A and B alleles are co-dominant; if both are present, the person expresses both antigens, resulting in Type AB blood. The O allele is recessive, meaning Type O blood is only expressed when an individual inherits two O alleles, one from each parent.
Understanding inheritance requires distinguishing between the phenotype (the outward expression, like Type A) and the genotype (the underlying genetic code, like AO or AA). For instance, a person with Type A blood can have the genotype AA or AO. If two Type A parents both have the AO genotype, they carry a recessive O allele. This makes it possible for their child to inherit an O allele from each parent, resulting in Type O blood.
Predicting Blood Type Outcomes
The rules of dominance and recessiveness lead to predictable blood type outcomes for children.
If one parent has Type O blood (OO) and the other has Type AB blood (AB), the child must receive an O allele and either an A or B allele. This limits the child’s possible blood types to Type A (AO) or Type B (BO), meaning the child cannot have the exact blood type of either parent.
If both parents have Type AB blood, the child’s possibilities include Type A, Type B, or Type AB. Since neither parent possesses the recessive O allele, it is genetically impossible for them to have a child with Type O blood. Conversely, if both parents have Type O blood, their child is guaranteed to be Type O. The greatest range of possibilities occurs when one parent is Type A/O and the other is Type B/O, as their child could inherit any of the four ABO blood types: A, B, AB, or O.
Rh Factor Inheritance and Pregnancy Implications
The Rh factor is inherited separately from the ABO system, following a dominant-recessive pattern where the Rh-positive allele is dominant. An Rh-positive person can have two positive alleles or one positive and one negative allele. A person must inherit two negative alleles to be Rh-negative.
Rh status is important in pregnancy when an Rh-negative mother carries an Rh-positive fetus. If the fetus’s Rh-positive blood enters the mother’s bloodstream, her immune system recognizes the RhD protein as foreign and produces anti-D antibodies.
This process, called Rh sensitization, usually does not affect the first Rh-positive pregnancy but risks subsequent ones. Maternal antibodies can cross the placenta and attack the fetus’s red blood cells, potentially causing hemolytic disease of the newborn. Modern medicine prevents this complication by administering Rh immunoglobulin (RhIg), which temporarily blocks the mother’s immune system from reacting to the Rh-positive blood.
Why Blood Typing is Medically Necessary
Knowing one’s blood type is a fundamental requirement in medicine, primarily for safe blood transfusions. When a person receives blood, the recipient’s immune system must not attack the antigens on the donated red blood cells. Receiving incompatible blood triggers a severe, life-threatening immune reaction.
Blood typing is also important for organ transplants, as compatibility reduces the risk of rejection. Type O- blood, lacking A, B, and Rh antigens, is the universal donor because it can be safely given to nearly anyone in an emergency. Conversely, Type AB+ blood is the universal recipient because a person with this type possesses all major antigens and will not produce antibodies against transfused blood.