Blood type, like eye or hair color, is determined by genes inherited from both biological parents. An individual’s complete blood type, such as O Negative, is a combination of two distinct genetic systems. Understanding the inheritance patterns of these two systems clarifies exactly which blood types are genetically possible for the parents of an O Negative child.
The Two Parts of Your Blood Type
A full blood type designation, like O Negative, is defined by two separate classifications: the ABO group and the Rh factor. The ABO system determines if a person has A, B, AB, or O blood, based on the presence or absence of A and B antigens on the surface of red blood cells. The Rh factor, also known as the Rhesus factor, determines the Positive or Negative part of the blood type and is based on the presence or absence of the Rh(D) antigen. These two genetic systems are inherited independently of one another, as the genes for each are located on different chromosomes.
For a person to be classified as Type O, their red blood cells must lack both the A and B antigens. The “Negative” part of the blood type means the red blood cells also lack the Rh antigen. To express this specific O Negative phenotype, the child must inherit the genetic information for the ‘O’ blood group and the ‘Negative’ Rh factor from both the mother and the father. This foundational requirement sets the parameters for the possible parental blood types.
Determining Parental ABO Possibilities
The ABO blood group system is governed by a gene that has three possible alleles: A, B, and O. The A and B alleles are co-dominant, meaning they are both expressed if present, while the O allele is recessive. Because the O allele is recessive, an individual will only express Type O blood if they inherit an O allele from each parent, resulting in the genotype OO.
This means both parents must carry at least one O allele to pass it on to their child. A parent with Type O blood has the genotype OO and can only pass on the O allele. A parent with Type A blood can be genotype AA or AO, and a Type B parent can be genotype BB or BO. Therefore, any Type A or Type B parent must be heterozygous (AO or BO) to carry and pass on the necessary O allele.
The only ABO blood type that is genetically impossible for a parent of an O child is Type AB. A Type AB parent possesses the genotype AB, meaning they carry the A allele and the B allele, but they do not possess an O allele to pass down. If a child is Type O, their parents could be O + O, A + O, B + O, A + A (if both are AO), B + B (if both are BO), or even A + B (if both are heterozygous, AO and BO). The key requirement is the presence of the recessive O allele in both parents.
Determining Parental Rh Factor Possibilities
The Rh factor is determined by a separate gene, simplified to two alleles: D for Rh Positive and d for Rh Negative. The Rh Positive trait (D) is dominant, while the Rh Negative trait (d) is recessive. Since the Rh Negative trait is recessive, an O Negative child must inherit the ‘d’ allele from both parents, giving them the genotype dd.
An Rh Negative parent has the genotype dd and will only pass on the recessive ‘d’ allele. However, two Rh Positive parents can also have an Rh Negative child because the Rh Positive trait is dominant. For a parent to be Rh Positive and still have an Rh Negative child, they must be heterozygous, carrying one dominant ‘D’ allele and one recessive ‘d’ allele (Dd). The presence of the dominant ‘D’ allele makes them Rh Positive, but the recessive ‘d’ allele can still be passed to the child.
If both parents are Rh Positive but have the Dd genotype, there is a 25% chance their child will inherit the ‘d’ allele from both, resulting in an Rh Negative child. The parents of an O Negative individual could therefore be two Rh Negative parents, one Rh Negative and one Rh Positive (Dd), or two Rh Positive parents (Dd). The only impossibility is for a parent to be homozygous Rh Positive (DD), as they would not carry the recessive ‘d’ allele to pass on.
Summary of All Possible Parent Pairings
Synthesizing the requirements for both the ABO group and the Rh factor yields a comprehensive list of possible parental blood types for an O Negative child. Both parents must carry the O allele (meaning neither can be Type AB) and both must carry the recessive Rh Negative allele (meaning neither can be homozygous Rh Positive, DD). This combination of genetic requirements allows for a wide range of parental blood types.
Possible parental blood types include any pairing of A, B, or O, provided the A and B types are heterozygous, combined with any pairing of Rh Positive (Dd) or Rh Negative (dd). For instance, one parent could be B Positive (genotype BO, Dd) and the other could be A Negative (genotype AO, dd). Other combinations that are possible include two O Positive parents, two A Positive parents, or one B Negative and one O Negative parent. Any parent who is Type AB, or any parent who is homozygous Rh Positive (DD), is genetically excluded.