Your blood type is a characteristic inherited directly from your parents. Blood type is a classification system based on the presence or absence of specific protein and sugar structures, known as antigens, found on the surface of red blood cells. A person’s complete blood type is defined by two major systems: the ABO system and the Rh system, which determines the positive or negative factor. Your genetic code contains the instructions for making these antigens, and you receive half of this code from each biological parent. The specific combination of inherited genes determines your final blood type, which is why it is often used in genetic and forensic science.
How the ABO Alleles Determine Blood Type
The ABO system is controlled by a single gene that has three variations, or alleles: A, B, and O. Humans inherit one allele from each parent, meaning every person carries a combination of two of these three possible alleles. The A allele instructs the body to produce the A antigen on the red blood cell surface, while the B allele produces the B antigen. The O allele does not produce any functional antigen.
The interaction of these three alleles is governed by dominance and codominance. Both the A and B alleles are dominant over the O allele. If you inherit an A allele and an O allele, your blood type will be A because the A antigen is produced. Similarly, inheriting a B allele and an O allele results in Type B blood. The only way to have Type O blood is to inherit an O allele from both parents.
When the A allele and the B allele are inherited together, they exhibit codominance. Neither allele is dominant over the other, and the body produces both the A antigen and the B antigen simultaneously. This results in the Type AB blood group.
Predicting Possible Blood Types for Offspring
Predicting a child’s blood type requires knowing the genetic makeup, or genotype, of both parents, not just their outwardly expressed blood type. For instance, Type A blood can result from the genotype AA or AO, and Type B blood can be BB or BO. Type AB blood always requires the genotype AB, and Type O blood is always OO.
When parents pass on their genes, each contributes one of their two alleles to the child. This explains how two parents with Type A blood can have a child with Type O blood. This is possible if both parents carry the recessive O allele (AO genotypes) and both pass the O allele to the child, resulting in an OO genotype.
The largest range of possibilities occurs when one parent has Type A blood (AO) and the other has Type B blood (BO). In this scenario, the child has an equal possibility of inheriting Type A, Type B, Type AB, or Type O blood. This demonstrates that a child’s blood type is a predictable result based on the alleles each parent carries.
Inheritance of the Positive or Negative Factor
The positive or negative sign attached to an ABO blood type refers to the Rh system, the second major blood classification. This factor is determined by the presence or absence of the RhD antigen, a protein on the surface of the red blood cells. If the protein is present, the blood is Rh-positive; if it is absent, the blood is Rh-negative.
The inheritance of the Rh factor follows a standard dominant/recessive pattern. The allele for Rh-positive is dominant, meaning inheriting one copy of the positive allele is sufficient to make a person Rh-positive. The Rh-negative trait is recessive, requiring a child to inherit the negative allele from both parents.
This pattern means that two Rh-positive parents can still have an Rh-negative child, provided both parents carry one recessive negative allele. Conversely, if both parents are Rh-negative, they can only pass on the negative allele, guaranteeing their child will also be Rh-negative. Knowing the Rh factor is important during pregnancy, as an Rh-negative mother may require monitoring if the developing fetus is Rh-positive.