Blood type classifies blood based on inherited antigens present on the surface of red blood cells. The presence or absence of these antigens determines an individual’s blood group. A person’s complete blood type is determined by genetic information passed down from their biological parents. Understanding the rules of genetic inheritance allows for the prediction of all possible blood types an offspring may inherit, involving two distinct systems: the ABO group and the Rh factor.
The Genetic Rules of ABO Blood Types
The ABO blood group system uses three distinct genetic variations, known as alleles: A, B, and O. Every person inherits two alleles, one from each parent, which form the individual’s genotype (genetic code). The resulting blood type, or phenotype, is the physical expression of this combination.
The A and B alleles are dominant over the O allele. If a person inherits an A or B allele along with an O allele, the blood type will be A or B, respectively. The O blood type only appears if a person inherits two O alleles, resulting in the OO genotype.
A unique relationship called codominance exists between the A and B alleles. If an individual inherits both A and B alleles, both are fully expressed, resulting in the Type AB blood phenotype. These three alleles combine in six possible genotypes (AA, AO, BB, BO, AB, OO) to produce the four common blood types (A, B, AB, O).
Calculating Potential Offspring Blood Types
Determining a child’s possible blood type involves identifying the two alleles each parent can pass down and considering all four resulting combinations. This calculation starts by knowing each parent’s two-allele genotype, which can often be narrowed down from their known blood type. For instance, a person with Type O blood must have the OO genotype, while Type A blood could be either AA or AO.
Consider a scenario where one parent has Type A blood with the AO genotype, and the other has Type B blood with the BO genotype. The first parent can pass on A or O, and the second can pass on B or O. The four potential combinations for their offspring are AB (Type AB), AO (Type A), BO (Type B), and OO (Type O). In this specific pairing, the child has an equal chance of inheriting any of the four ABO blood types.
The calculation also reveals certain impossibilities. For example, two parents who both have Type O blood (OO genotype) can only pass on an O allele, making it genetically impossible for their child to have Type A, B, or AB blood. Similarly, a parent with Type AB blood (AB genotype) cannot pass on an O allele, meaning they can never have a child with Type O blood, regardless of the other parent’s type.
Incorporating the Rh Factor (Positive or Negative)
The Rh factor is inherited separately from the ABO system. This factor is determined by the presence or absence of the Rh(D) protein (antigen) on the surface of red blood cells. If the antigen is present, the blood is Rh-positive (+); if it is absent, the blood is Rh-negative (-).
The inheritance of the Rh factor follows a classic dominant and recessive pattern. The allele for Rh-positive is dominant, and the allele for Rh-negative is recessive. This means that only one Rh-positive allele is needed for a person to have Rh-positive blood.
An individual is Rh-negative only if they inherit two recessive Rh-negative alleles, one from each parent. Because the positive allele is dominant, two parents who are both Rh-positive can still have an Rh-negative child. This happens if both Rh-positive parents carry one hidden Rh-negative allele and both pass that recessive allele to their child.