Blood types are characteristics found on the surface of red blood cells, which are inherited from both parents. These characteristics are determined by the presence or absence of specific protein markers, known as antigens. The two major systems used to classify human blood are the ABO system and the Rh factor system, which results in the eight common blood types, such as O positive, A negative, or AB positive. Understanding the combination of genes passed down from each parent is the only way to determine the possible blood type of an offspring.
How the ABO Blood Group System Works
The ABO blood group system is governed by a single gene that has three possible variations, called alleles: A, B, and O. Every person inherits one allele from each parent, resulting in a pair of alleles that determines their blood type. The resulting combination of two alleles is known as the genotype, while the observable blood type is the phenotype.
The A and B alleles produce distinct antigens on the red blood cell surface, and they are considered co-dominant, meaning both are fully expressed if present together, resulting in the AB blood type. The O allele is recessive because it does not produce a functional antigen. For a person to have Type O blood, they must inherit the O allele from both parents, resulting in the “OO” genotype.
An individual with Type A blood can have a genotype of either AA or AO, and a person with Type B blood can be BB or BO. The recessive nature of the O allele means that parents with Type A or Type B blood can still carry and pass on the O allele.
How the Positive or Negative Rh Factor is Determined
The positive or negative sign of a blood type is determined by the Rh factor, based on the presence or absence of the D antigen on red blood cells. If the D antigen is present, the person is Rh positive (+); if absent, they are Rh negative (-). The Rh factor is controlled by a separate set of genes from the ABO system, but the inheritance pattern is simpler.
The gene for Rh positive is dominant, while the gene for Rh negative is recessive. A person who is Rh positive can have two dominant genes or one dominant and one recessive gene. Conversely, a person is Rh negative only if they inherit two recessive genes, one from each parent.
Since the O positive blood type includes the positive Rh factor, the child must have at least one dominant Rh positive gene. This dominant gene may have come from one parent or both parents. If both parents are Rh positive, they can still have an Rh negative child, but only if both carry the recessive Rh negative gene.
Parent Combinations that Result in O Positive Offspring
To be O positive, a child must inherit two O alleles (OO genotype) for the ABO system and at least one dominant positive allele for the Rh factor. Since each parent must contribute one O allele, neither parent can have Type AB blood, as the AB phenotype does not carry the recessive O allele to pass on.
A parent can be Type O (genotype OO), Type A (genotype AO), or Type B (genotype BO) and still contribute the required O allele. For instance, two Type A parents can result in an O child if both have the AO genotype. Similarly, a Type A parent (AO) and a Type B parent (BO) can produce an O child, as both can contribute an O allele.
For the positive Rh factor, the child must inherit the dominant positive gene from at least one parent. This is possible if both parents are Rh positive and carry the dominant gene. It is also possible if one parent is Rh positive and the other is Rh negative, provided the Rh positive parent passes on their dominant gene.
The simplest path to an O positive child is when both parents are O positive themselves. Many other combinations are possible, such as a pairing of O positive and A positive, provided both parents carry the recessive O allele and at least one carries the dominant positive Rh factor.