The blood type you have is determined by the genes passed down to you from your biological parents. This inheritance follows predictable patterns for two separate systems: the ABO system, which gives you the letter (A, B, AB, or O), and the Rhesus (Rh) system, which determines the sign (positive or negative). Understanding the basic rules of genetic inheritance for both the ABO and Rh factors will explain the possible blood types your parents could have to produce a child with O Positive blood.
The Genetics of ABO Blood Types
The ABO blood group system is controlled by a single gene that has three possible forms, known as alleles: A, B, and O. Since you inherit one allele from each parent, your combination of these two alleles, called your genotype, determines your blood type, or phenotype. The A and B alleles express a relationship called codominance, meaning that if both are present in your genotype, both are fully expressed, resulting in the AB blood type.
The O allele is recessive to both A and B. This means that the O allele is only expressed when two copies of it are inherited, one from each parent. A person with the genotype AO will have Type A blood because the A allele masks the presence of the O allele. Similarly, a person with the genotype BO will have Type B blood.
For an individual to have Type O blood, they must inherit two O alleles, resulting in the genotype OO. The O allele does not code for any antigen on the red blood cell surface. This specific requirement—inheriting an O allele from both parents—is the foundation for determining the possible parental ABO types for an O blood type child.
The Genetics of the Rh Factor
The second part of the blood type, the positive or negative sign, is determined by the Rhesus (Rh) system, specifically the presence or absence of the D antigen on red blood cells. The Rh system is inherited independently of the ABO system. The trait for Rh Positive is dominant, represented by the presence of the D antigen.
A person is Rh Positive if they possess at least one dominant D allele (genotypes DD or Dd). The Rh Negative trait is recessive, meaning a person must inherit two recessive d alleles to be Rh Negative (genotype dd). Since the child in question is O Positive, they must have inherited at least one D allele, meaning their Rh genotype is Dd or DD.
Determining Possible Parental ABO Types
For a child to have Type O blood, their genotype must be OO, which requires that each parent contributes one O allele. This means neither parent can have a genotype that is purely AA or BB. The Type AB blood group (genotype AB) is the only one that absolutely cannot produce a Type O child because it lacks the O allele entirely.
Parents with Type O blood (genotype OO) can only pass on an O allele, so an O/O combination will always produce a Type O child. A Type A parent must be heterozygous (AO) to pass on the O allele, and a Type B parent must be heterozygous (BO) to contribute the necessary O allele to their child.
Any combination of parental blood types—A, B, or O—is possible, provided that both parents carry and pass on the recessive O allele. This includes combinations like Type A (AO) and Type O (OO), Type B (BO) and Type O (OO), or even Type A (AO) and Type B (BO).
Determining Possible Parental Rh Factors
A child with O Positive blood must have inherited at least one dominant D allele to express the Rh Positive trait. This means the parents’ Rh genotypes must include at least one D allele between them. The parents could both be Rh Positive (DD or Dd).
If both parents are Rh Positive with the Dd genotype, they each have a chance to pass on the D allele, resulting in an Rh Positive child. A parent who is Rh Positive (DD or Dd) paired with a parent who is Rh Negative (dd) can also produce an Rh Positive child, as the Rh Positive parent may pass on the dominant D allele. For example, a Dd parent and a dd parent have a 50% chance of producing a Dd (Positive) child.
The only combination that is genetically impossible to produce an Rh Positive child is two Rh Negative parents. Since an Rh Negative person has the genotype dd and can only pass on the recessive d allele, their child would necessarily be Rh Negative (dd). Therefore, for an O Positive child to be born, at least one parent must be Rh Positive or both must carry the dominant D allele.