The O Positive blood type is defined by the specific combination of antigens on your red blood cells. Understanding your blood type allows us to trace the inheritance pattern that led to this combination. To determine the possibilities for your parents’ blood types, we must explore the genetic rules governing both the ABO system and the Rh factor separately.
The Genetics of ABO Blood Types
Blood type inheritance follows Mendelian genetics, determined by a single gene locus with three alleles: A, B, and O. Every person inherits one allele from each parent to determine their blood type. The A and B alleles are codominant, meaning both traits are expressed if present, resulting in the AB blood type.
The O allele is recessive, meaning its trait is only expressed when two copies are inherited, one from each parent. An individual with Type O blood must possess the genotype OO. This means they inherited an O allele from both their biological mother and father.
A Type A individual can have the genotype AA or AO, and a Type B individual can have BB or BO. The dominant A or B allele masks the recessive O allele in these heterozygous combinations. Therefore, parents who are not Type O can still carry the recessive O allele and pass it down to their offspring.
Possible Parental Pairings for Type O
Since you inherited an O allele from each parent, both parents must possess and be able to donate this specific allele. The most straightforward pairing is two Type O parents (OO x OO), as they only have O alleles to pass on. However, many other combinations are possible, provided each parent carries the recessive O allele:
- Two Type A parents (AO x AO).
- Two Type B parents (BO x BO).
- A Type A parent and a Type O parent (AO x OO).
- A Type B parent and a Type O parent (BO x OO).
- A Type A parent and a Type B parent (AO x BO).
How the Rh Factor is Inherited
The “Positive” part of your blood type refers to the presence of the Rh factor, a protein on the surface of red blood cells known as the D antigen. Rh factor inheritance follows a simple dominant and recessive pattern. The presence of the Rh factor (Rh-positive, denoted by the D allele) is dominant over its absence (Rh-negative, denoted by the d allele).
Because you are Rh-positive, you must possess at least one D allele, meaning your genotype is either DD or Dd. This requires that at least one of your parents must be Rh-positive to contribute the dominant D allele.
An Rh-positive child can result from two Rh-positive parents or from one Rh-positive and one Rh-negative parent. The only combination that cannot produce an Rh-positive child is two Rh-negative parents (dd x dd). Since Rh-negative parents only carry the recessive d allele, their offspring would always be Rh-negative.
Definitive Exclusions: Types Your Parents Cannot Be
By combining the rules of ABO and Rh factor inheritance, we can definitively rule out certain blood types for your parents. The first exclusion relates to the ABO system: neither of your parents can have the Type AB blood type. An individual with Type AB blood has the genotype AB and possesses only the A and B alleles, meaning they have no O allele to pass on.
Since you are Type O (genotype OO), it is impossible for a Type AB parent to contribute the required O allele. The second exclusion relates to the Rh factor, established by the dominance of the D allele. It is impossible for both of your parents to be Rh-negative, regardless of their ABO type.
If both parents were Rh-negative (dd genotype), you could only inherit a d allele from each, resulting in an Rh-negative blood type. Since you are O Positive, you must have an Rh-positive parent to provide the dominant D allele. Therefore, any pairing involving a Type AB parent, or where both parents are Rh-negative, is genetically excluded.