Blood Type Inheritance Calculator

Frequently Asked Questions

• How is a child’s blood type inherited?+
  In the textbook model, blood type is set by two separate genes. For ABO, each parent passes one of their two alleles (A, B or O) and the pair the child inherits sets their group; A and B are codominant and O is recessive. For RhD, a child is Rh-positive if they inherit at least one D allele. This calculator combines both to show every type the model allows, then checks each type it excludes against documented variants.
• Can two type-O parents have a child who is not type O?+
  Not in the textbook model: an O parent carries two O alleles, so two O parents can only pass on O. The documented route around it is the Bombay phenotype. A Bombay person lacks the H antigen, so “ABH antigens are absent from their red cells, regardless of their ABO blood group genotype” (Shahshahani et al., 2013). They type as O on red-cell testing yet still carry their ABO genes, so our model lets an O × O pair have an A or B child through them; an Indian case report describes a Bombay mother first typed as O (Shastry et al., 2013). It is rare: an earlier study cited by Joshi and Vasantha (2012) put it at 1 in 7,600 in Mumbai.
• Can an AB parent have a type-O child?+
  Not in the textbook model, where an AB parent passes A or B and never O. Two documented variants change that. cis-AB is a single allele that carries A and B together, so a parent who carries it with an O allele types as AB but can pass on O; Chun et al. (2019) give “the birth of an O child from an AB mother” as their example, and describe cis-AB as “relatively common in the Korean, Japanese, and Chinese populations”. The Bombay phenotype is the other route. The result panel names these routes whenever the model excludes a type.
• How does Rh factor inheritance work?+
  In the textbook model RhD-positive is dominant: a person is RhD-negative with two d alleles, so two RhD-negative parents have RhD-negative children, while one RhD-positive parent leaves both open. Routine typing does not always see the gene, though. Across 16 studies of Han Chinese compiled by Yin and Flegel (2021), a pooled 23.3% of people typed RhD-negative had DEL, a low amount of D antigen that “types as RhD-negative in routine serology”. No source we checked documents an inheritance exception for Rh, so the tool lists none.
• Does this tell me the probability of each type?+
  No. It shows which types the textbook model allows, not how likely each is, because a visible blood group does not reveal the genotype: a type-A parent could be AA or AO, and the odds depend on which. The set shown is what standard ABO and Rh inheritance allows; rare variants such as cis-AB and the Bombay phenotype can produce results outside it, and the panel lists them where they apply.
• What if a family’s recorded blood types do not fit the grid?+
  The grid is the textbook model, so a recorded type outside it is first a question about the typing. Documented variants produce children outside the textbook list, and recorded groups can be wrong: Chun et al. (2019) describe cis-AB/A cases that “would have been typed as typical A” without careful family studies, and a device that misidentified cis-AB samples “as typical AB”; Shastry et al. (2013) describe a Bombay mother “wrongly typed as O positive”. The panel names the documented routes for the pair you choose.
• What happens when a blood group does not fit?+
  Laboratory grouping checks a result two ways. Malaysia’s National Blood Centre guideline requires red-cell (forward) grouping to match serum (reverse) grouping and says that where they do not, “an investigation of this discrepancy shall be carried out”. Its reverse grouping includes an O-cell test “incorporated to detect Bombay blood group”, and it treats a finger-prick group as “a preliminary result” only (Transfusion Practice Guidelines, 4th ed., 2016, sections 6.1 and 6.2).
• Is O-negative really the universal donor?+
  O-negative donors are “often called ‘universal donors’”, in the words of NHS Blood and Transplant. The documented exception is the Bombay phenotype: these patients “can receive only autologous blood or blood from another Bombay blood group”, and “transfusing blood group O red cells to them can cause a fatal hemolytic transfusion reaction” (Shahshahani et al., 2013). This calculator gives no transfusion guidance.
• Is this calculator medical advice?+
  No. It is an educational genetics tool. It computes the textbook ABO and Rh model and names documented ABO exceptions where the model excludes a type; it does not say how likely those are for any family, and it should never be used for medical, transfusion or legal decisions. Those belong with laboratory blood grouping.
• Is the Blood Type Inheritance Calculator free?+
  Completely free, with no account or limit. It works offline once the page has loaded and collects no data — your selections never leave your device.