Blood types classify an individual’s blood based on the presence or absence of specific inherited substances, known as antigens, found on the surface of red blood cells. These classifications are crucial for medical procedures, particularly blood transfusions, to ensure compatibility between donor and recipient. An incompatible transfusion can trigger a severe immune response, potentially leading to life-threatening reactions.
The Building Blocks of Blood Types
The two most significant blood classification systems are the ABO system and the Rh system. The ABO system categorizes blood into four main types: A, B, AB, and O. Type A blood has A antigens, type B has B antigens, type AB has both A and B antigens, and type O has neither A nor B antigens.
The Rh system classifies blood as either Rh-positive or Rh-negative, depending on the presence or absence of the RhD antigen (Rh factor). If the RhD antigen is present, the blood is Rh-positive; if absent, it is Rh-negative. Combining these two systems results in the eight common blood types, such as A positive or O negative.
How Blood Types Are Passed Down
Blood type inheritance follows specific genetic principles, similar to other inherited traits. Each person inherits two copies, or alleles, of the gene responsible for blood type—one from each biological parent. For the ABO system, there are three main alleles: A, B, and O.
Alleles A and B are considered codominant, meaning that if both are inherited, both antigens will be expressed, resulting in AB blood type. Both A and B alleles are dominant over the O allele. This means that an individual inheriting an A allele and an O allele will have type A blood, and similarly for type B. Only when two O alleles are inherited will an individual have type O blood. The specific combination of inherited alleles is called the genotype (e.g., AO, BB), while the observable blood type is the phenotype (e.g., A, B).
Predicting Your ABO Blood Type
Predicting a child’s ABO blood type involves understanding the alleles contributed by each parent. If both parents have type O blood (genotype OO), they can only pass on O alleles, meaning all their children will have type O blood. When one parent has type A blood and the other has type B, the possibilities vary. For instance, if both parents are heterozygous (AO and BO), their child could inherit A, B, AB, or O blood types. If one parent has type A (AO) and the other has type AB, their children could potentially have A, B, or AB blood types.
Understanding Your Rh Factor and Combined Possibilities
The inheritance of the Rh factor follows a simpler dominant/recessive pattern compared to the ABO system. The Rh-positive allele is dominant, while the Rh-negative allele is recessive. An individual will be Rh-positive if they inherit at least one Rh-positive allele. They will only be Rh-negative if they inherit two Rh-negative alleles, one from each parent.
For example, two Rh-positive parents can have an Rh-negative child if both carry the recessive Rh-negative allele. Conversely, if both parents are Rh-negative, all their children will be Rh-negative. A person’s complete blood type combines their ABO type and Rh factor, such as A positive or O negative. Knowing both parents’ full blood types allows for determining the possible blood types their children might inherit, which involves probabilities rather than certainty.