Blood Type Calculator — ABO & Rh Factor Inheritance

Blood Type	ABO Group	Rh Factor	Probability
AB+	A & B antigens	Positive (Rh+)	42.2%
AB-	A & B antigens	Negative (Rh−)	14.1%
A+	A antigen	Positive (Rh+)	14.1%
B+	B antigen	Positive (Rh+)	14.1%
A-	A antigen	Negative (Rh−)	4.7%
B-	B antigen	Negative (Rh−)	4.7%
O+	No antigens	Positive (Rh+)	4.7%
O-	No antigens	Negative (Rh−)	1.6%

ABO blood group inheritance diagram showing genotype combinations for blood types A, B, AB, and O — Figure 1. The four ABO blood groups arise from three alleles at the ABO gene locus on chromosome 9. I^A and I^B are codominant, each encoding a distinct glycosyltransferase enzyme that adds different sugar residues to the H antigen on red blood cells. The i allele produces no functional enzyme.

ABO Blood Group Genetics — Alleles, Antigens, and Codominance

The ABO blood group system is one of the most clinically significant genetic traits in humans. Karl Landsteiner discovered it in 1901, winning the Nobel Prize in Physiology or Medicine in 1930. The system is controlled by the ABO gene on chromosome 9q34, which encodes a glycosyltransferase enzyme that modifies the H antigen on red blood cell surfaces.

Three functional alleles exist at this locus. The I^A allele encodes A-N-acetylgalactosaminyltransferase, which adds N-acetylgalactosamine to the H antigen, producing the A antigen. The I^B allele encodes A-galactosyltransferase, which adds D-galactose to produce the B antigen. The i allele encodes a non-functional enzyme that leaves the H antigen unmodified — producing neither A nor B antigens, giving blood type O.

I^A and I^B are codominant — both enzymes function independently in the same cell, so a person with genotype I^AI^Bexpresses both A and B antigens simultaneously, producing blood type AB. This is a textbook example of codominance in genetics. The i allele is recessive to both I^A and I^B.

Blood Type	Possible Genotypes	Antigens on Red Cells	Antibodies in Plasma	Frequency (European)
A	I^A I^A or I^A i	A antigen	Anti-B	~42%
B	I^B I^B or I^B i	B antigen	Anti-A	~10%
AB	I^A I^B	A and B antigens	Neither (universal recipient)	~4%
O	ii	None (H antigen only)	Anti-A and Anti-B	~44%

Rh Factor Inheritance and Rh Incompatibility in Pregnancy

The Rh blood group system is the second most clinically important blood group after ABO. It comprises over 50 antigens, but the D antigen is by far the most significant. The presence of the D antigen on red blood cells defines an individual as Rh-positive (Rh+); its absence defines Rh-negative (Rh−).

Rh factor is controlled by the RHD gene on chromosome 1p36. The D allele (Rh+) is dominant over the d allele (Rh−). Rh+ individuals are either homozygous (DD) or heterozygous (Dd). Rh− individuals are homozygous recessive (dd). Because Rh+ heterozygotes (Dd) are phenotypically identical to homozygotes (DD), two Rh+ parents can produce Rh− children — approximately 25% of offspring if both parents are Dd.

Rh Incompatibility — Clinical Significance

When an Rh− mother carries an Rh+ foetus, foetal red blood cells can cross into the maternal circulation — most commonly during delivery, but also during miscarriage, amniocentesis, or trauma. The mother's immune system may produce anti-D IgG antibodies (alloimmunisation).

In subsequent Rh+ pregnancies, these anti-D antibodies cross the placenta and destroy foetal red blood cells, causing haemolytic disease of the foetus and newborn (HDFN). Severe HDFN causes anaemia, hyperbilirubinaemia, kernicterus, hydrops fetalis, and can be fatal without intervention.

Prevention: Anti-D immunoglobulin (RhoGAM) given to Rh− mothers at 28 weeks gestation and within 72 hours of delivery prevents alloimmunisation in the vast majority of cases. This is one of the most successful prophylactic interventions in obstetric medicine.

Worked Blood Type Inheritance Examples

Example 1 — A × B: All four blood types possible

When one parent is type A (I^Ai) and the other is type B (I^Bi), all four blood types are possible in offspring. This is the only parental combination that can produce A, B, AB, and O children.

Parent 1: A (I^Ai)

Parent 2: B (I^Bi)

25% A (I^Ai)

25% B (I^Bi)

25% AB (I^AI^B)

25% O (ii)

This cross illustrates both codominance (I^AI^B producing AB) and simple Mendelian recessivity (ii producing O). It is also why paternity testing by blood type alone has limited exclusionary power when one parent is type O.

Example 2 — O × O: Only type O offspring

Two type O parents are both genotype ii. They can only pass on the i allele. Every child will be ii — blood type O. No exceptions.

Parent 1: O (ii)

Parent 2: O (ii)

100% O (ii) — all offspring

Example 3 — AB × O: A and B only

An AB parent (I^AI^B) crossed with an O parent (ii) produces only A and B children — never AB or O. This is because the AB parent cannot pass on the i allele, and the O parent cannot pass on I^Aor I^B.

Parent 1: AB (I^AI^B)

Parent 2: O (ii)

50% A (I^Ai)

50% B (I^Bi)

Blood Type Transfusion Compatibility Chart

ABO and Rh compatibility is critical in blood transfusions. Receiving incompatible blood triggers an acute haemolytic transfusion reaction — the recipient's pre-formed antibodies attack the transfused red cells, causing rapid haemolysis, renal failure, and potentially death. The WHO Blood Transfusion Safety guidelines mandate ABO and Rh compatibility testing before every transfusion.

Blood Type	Can Donate Red Cells To	Can Receive Red Cells From
O−	All types (universal donor)	O− only
O+	O+, A+, B+, AB+	O+, O−
A−	A+, A−, AB+, AB−	A−, O−
A+	A+, AB+	A+, A−, O+, O−
B−	B+, B−, AB+, AB−	B−, O−
B+	B+, AB+	B+, B−, O+, O−
AB−	AB+, AB−	A−, B−, AB−, O−
AB+	AB+ only	All types (universal recipient)

These rules apply to red blood cell transfusions. Plasma compatibility is reversed (AB is universal plasma donor). Platelet compatibility depends on both ABO and HLA matching.

Frequently Asked Questions — Blood Type Genetics

How is ABO blood type inherited from parents?

ABO blood type is determined by the ABO gene on chromosome 9, which has three main alleles: I^A (produces A antigens), I^B (produces B antigens), and i (produces no ABO antigens). I^A and I^B are codominant — a person inheriting both gets blood type AB. The i allele is recessive — only genotype ii produces blood type O. Each parent passes one allele to the child.

Can two O-type parents have an A or B child?

No. Both O-type parents have genotype ii and can only pass on the i allele. All their children will inherit ii and have blood type O. This is one of the most reliable rules in ABO genetics — it is used in forensic parentage testing.

Can two AB parents have an O-type child?

No. AB parents have genotype I^A I^B. They can only pass on either I^A or I^B — never the i allele. Their children will be I^A I^A (type A), I^A I^B (type AB), or I^B I^B (type B). Type O (genotype ii) is impossible from two AB parents.

How is Rh factor inherited?

Rh factor is primarily determined by the RHD gene on chromosome 1. The Rh-positive allele (D) is dominant over the Rh-negative allele (d). A person is Rh+ if they carry at least one D allele (DD or Dd). Only dd genotype produces the Rh-negative phenotype. Two Rh-positive heterozygous parents (Dd × Dd) have a 25% chance of an Rh-negative child.

What is haemolytic disease of the newborn caused by Rh incompatibility?

Haemolytic disease of the foetus and newborn (HDFN) occurs when an Rh-negative mother carries an Rh-positive foetus. Foetal red blood cells can enter the maternal circulation during delivery. The mother's immune system recognises the Rh D antigen as foreign and produces anti-D antibodies (IgG). In subsequent Rh+ pregnancies, these antibodies cross the placenta and destroy foetal red blood cells, causing anaemia, jaundice, and in severe cases, hydrops fetalis. This is prevented by administering anti-D immunoglobulin (RhoGAM) to Rh-negative mothers after any sensitising event.

What does universal donor and universal recipient mean?

O-negative (O−) is the universal donor for red blood cells because type O red cells carry no ABO antigens, and the absence of the Rh D antigen means they won't trigger anti-D reactions. O− blood can be transfused to anyone in an emergency. AB-positive (AB+) is the universal recipient for red blood cells because AB+ individuals carry both A and B antigens (no anti-A or anti-B antibodies) and are Rh+. These terms apply to red cell transfusions only — plasma and platelet compatibility follow different rules.

Can blood type determine paternity?

Blood type can sometimes exclude paternity but cannot confirm it. For example, if a child has blood type O and the mother is type A, the biological father must carry at least one i allele (must be type A heterozygous, B heterozygous, or O). A type AB man cannot be the father of an O-type child. Blood type testing alone is insufficient to confirm paternity — DNA testing is required for definitive results.

What blood types can donate to each other?

For red cell transfusions: O− can donate to all types. O+ can donate to A+, B+, O+, AB+. A− can donate to A+, A−, AB+, AB−. A+ can donate to A+, AB+. B− can donate to B+, B−, AB+, AB−. B+ can donate to B+, AB+. AB− can donate to AB+, AB−. AB+ can only donate to AB+. For plasma, the rules are reversed — AB plasma is the universal plasma donor.