What is a Blood Type?
There are four main groups of blood: A, B, AB, and O. Each group contains different antigens (carbohydrates or proteins) on the membrane of red blood cells. Depending on the presence or absence of these antigens — as well as specific antibodies in the blood plasma — it is possible to determine which blood group your blood belongs to.
The blood type is determined genetically — each person inherits it from their parents. As it is controlled by a single gene, each person has two alleles describing their blood group:
- Blood type A: genotype AA or AO
- Blood type B: genotype BB or BO
- Blood type AB: genotype AB
- Blood type O: genotype OO
Alleles A and B are dominant, and allele O is recessive. Only people with OO alleles can have blood type O.
The Rh Factor
Our calculator also considers the Rh factor, which divides blood into two additional groups: Rh+ and Rh−. The Rh factor describes the presence or absence of the Rh(D) antigen on the surface of red blood cells. The negative allele is recessive: two Rh− parents will always have Rh− children.
Note for Rh− mothers: Rh-negative mothers carrying an Rh-positive child may need to receive immunoglobulins (Rh immunoglobulin / RhoGAM) to prevent hemolytic disease in the fetus and newborn. Always consult your healthcare provider.
What Blood Type Will My Baby Have?
Your child receives one allele from each parent. The calculator enumerates all possible combinations, taking into account that when a parent's genotype is uncertain (e.g., type A can be AA or AO), probabilities are averaged across both possibilities:
- Type A parent contributes an A allele with 75% probability and an O allele with 25% probability.
- Type B parent contributes a B allele with 75% probability and an O allele with 25% probability.
- Type AB parent contributes an A or B allele each with 50% probability.
- Type O parent always contributes an O allele (100%).
The same logic applies to the Rh factor: an Rh+ parent contributes Rh+ with 75% probability and Rh− with 25%; an Rh− parent always contributes Rh−.
Blood Transfusion Compatibility
Not all blood types are compatible for transfusion. Incompatible blood can trigger a life-threatening immune reaction. The most important rules:
- O− is the universal donor — O-negative blood can be given to patients of any blood type in emergencies.
- AB+ is the universal recipient — people with AB-positive blood can receive from any donor.
- In general, donors and recipients must share the same ABO group and Rh factor, or the donor must have a compatible (less antigenic) blood type.
What Are the Rarest Blood Types?
Blood type frequency varies by population. Approximate global averages:
- O+ — ~37.4% (most common)
- A+ — ~35.7%
- B+ — ~8.5%
- O− — ~6.6%
- A− — ~6.3%
- AB+ — ~3.4%
- B− — ~1.5%
- AB− — ~0.6% (rarest)
Can a Child Have a Different Blood Type Than Both Parents?
Yes — it is entirely possible. For example, if both parents are type A (but each carries an O allele, i.e., genotype AO), they can have a child with type O blood (OO genotype). This is not unusual and does not imply non-paternity.
Frequently Asked Questions
How do I find out my blood type?
The most reliable way is through a blood test, which can be done at a hospital, clinic, or blood donation centre. Some countries include blood type on identity documents or military dog tags.
Does blood type affect health?
Research suggests certain blood types may correlate with slightly different risks for some conditions (e.g., cardiovascular disease, certain infections), but blood type alone is not a strong predictor of health outcomes.
Is this calculator a substitute for medical advice?
No. This tool provides educational estimates based on genetics. For medical decisions — especially relating to blood transfusions or pregnancy — always consult a qualified healthcare professional.