What is 0 genotype

Inheritance of blood groups: AB0 system

The subscription system

The most well-known blood group system is probably the ABO system with the four blood groups A, B, AB and 0. Only one gene is responsible for the blood groups, which is not present in two but in three alleles.


The two genotypes AA and Ao can be responsible for phenotype A, and the two genotypes BB and Bo for phenotype B. So if a man has blood type A, he doesn't know whether he is homozygous AA or heterozygous Ao, both are possible. Unless he has a child with blood type B. Then he must have the genotype Ao, while the mother of the child must have the genotype BB or Bo. Should the couple have a second child, who then has blood group 0, the mother's genotype is also determined, namely Bo.

Only the children provide information about the genotype of the parents

The blood group antigens

Who or what actually causes the various blood groups in humans?

Let's look at the erythrocytes, the red blood cells. Erythrocytes have a very simple structure. Basically, they are "bags" with some cell plasma and a lot of hemoglobin (red blood pigment) that do not even contain a nucleus.

The membrane of the erythrocytes contains glycoproteins and glycolipids, i.e. proteins and lipids that are linked to sugar molecules. Two of the glycolipids are responsible for blood groups A and B.

Interestingly, these two glycolipids are closely related, they have a common ancestor. Infants only have what is known as the H antigen on their red blood cells. This is a lipid, or protein, that is linked to five sugar molecules:

The structure of the H antigen

The "correct" blood group antigens can then develop from this "baby antigen".

Formation of blood group antigens

  • People with the Blood group A produce an enzyme that attaches an N-acetylglucosamine component to the galactose unit of the H antigen. The antigen of blood group A is then produced.
  • People with the Blood group B produce an alternative form of this enzyme that attaches another galactose building block to the galactose unit of the H antigen. In this way, the antigen of blood group B is created.
  • People of Blood group 0 produce neither the A-enzyme nor the B-enzyme, so no further building blocks are attached to the H-antigen.
  • In people of the Blood group AB Both enzymes are formed and both antigens, both the A antigen and the B antigen, are incorporated into the cell membrane of the red blood cells.
For experts

The illustration above is not entirely correct, infants already have active glycosyltransferases (in the corresponding subtype of their AB0 blood group), which couple the necessary sugar residues to the H antigen. However, infants do not develop the corresponding antibodies against AB traits until they are one year old.

The blood group antibodies

So why are the blood groups so interesting at all? It is well known that blood groups are important for blood donations. A person with blood group A must not receive blood from blood group B because the blood then clumps, which is usually fatal. The antibodies that everyone carries in their blood plasma are responsible for the clumping.

A person with blood group A naturally has no antibodies against his A antigens, but antibodies against the B antigens. If he had antibodies against his own antigens, his blood would start to clump immediately.

Landsteiner's experiments (1900) and the explanation for the results

The figure above shows when a blood transfusion can clump. The figure below provides the explanation for this behavior.

A person with blood group A is seriously injured and now receives blood group 0. The plasma of the 0 donor blood contains antibodies against A antigens and also antibodies against B antigens. The next picture shows what happens during a blood transfer.

Blood group A is mixed with blood group 0 plasma

The A antibodies in the 0 plasma attach themselves to the A antigens on the erythrocytes of the A blood and thus lead to a clumping of the same. Even a person with blood group B should not be happy about a 0 blood donation, because the B antibodies present in the 0 plasma would lead to a clumping of the B erythrocytes.

A typical inheritance

As an example, we want to look at a typical inheritance.

Inheritance of the ABO system using the example of another A-father and a B-mother

Some of my students then always think that the parents must have exactly four children, one with blood group AB, one with blood group A, one with blood group B and one with blood group 0. That is of course complete nonsense, because the hereditary scheme shows only a) how these parents come to children with blood group AB, A, B or 0, and b) that each of these four blood groups is equally likely in the children (25%).

So if the two parents have three children, the probability that all three children have blood group 0 = 1/4 * 1/4 * 1/4 = 1/64. But that's not because the 0 allele recessive is. The probability of having three children with blood group AB is also 1/64, although both alleles are dominant over 0.


In the maternity ward of the Lübbeck hospital, four children were born in a turbulent night and accidentally left unmarked. However, their blood groups could be determined as A, AB, B and 0.

The corresponding examination of the four pairs of parents showed:

Couple 1: man 0, woman 0
Couple 2: man AB, woman 0
Couple 3: man A, woman B.
Couple 4: man B, woman B.

Play the assessor and save the family happiness of the four affected families by assigning the children to the parents.

First you assign the child with blood group 0 to pair 1, because these two parents cannot have any of the other children under any circumstances.

Couple 2 could have children with blood group A or blood group B, but no children with blood group AB or blood group 0. Whether child A or child B belongs to couple 2 cannot yet be decided.

The couple 3 can have children of the blood group AB, but also children of the blood group A (if the genotype of the woman is B0) or children of the blood group B (if the genotype of the man is A0).

Couple 4 can have children of blood group B or 0, but both parents would have to have genotype B0. Since the child with blood group 0 is already assigned to pair 1, only the child with blood group B remains for pair 4.

Since the child with blood group B is now assigned to pair 4, pair 2 can only have the child with blood group A. For couple 3, the child with blood group AB remains.

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01/27/2013: Page created
05/14/2015: page revised; Corrected graphics (many thanks to PD Dr. Jürgen Stolz), added links to cytology pages.
March 16, 2018: Expert part added after a suggestion by Prof. Dr. Tilmann Volk, University of Erlangen-Nuremberg. Thank you for this correction!