Inheritance of blood groups
But which blood group do I belong to? This is determined by the presence or absence of the A and B antigens on the surface of your red blood cells (erythrocytes), which, in turn, is determined by the ABO genes you inherited from your parents. Each parent passes only one of their two copies of the ABO gene to a child. Thus, the child’s blood group can be the result of two copies of the same allele (form of the gene) or two different alleles (homozygous vs. heterozygous inheritance).
Mendel’s laws of inheritance
Everyone has a specific blood group profile, which is determined according to a set of rules known as Mendel’s laws of inheritance. These rules take their name from the natural scientist Gregor Johann von Mendel, who, in 1865, presented his findings from a systematic programme of pea and bean plant crossing experiments. The principles developed by Mendel would later be applied to all organisms that reproduce sexually. They were not recognised as generally valid laws of inheritance until 1900, though, when other researchers studying inheritance (C. E. Correns, E. Tschermak and H. de Vries), working independently of one another, rediscovered their significance.
Mendel’s laws describe precisely how human blood groups are inherited. Thus, the antigens on an individual’s red blood cells (erythrocytes) are determined by genes (pieces of the DNA). The ABO blood groups were the first erythrocyte blood groups to be described, thanks to a Viennese doctor named Karl Landsteiner.
Belonging to blood group A means the A antigen and anti-B antibodies (the antibodies that react against the surface of blood cells of the blood group B) are present in someone’s blood. People with the blood group B have the B antigen and anti-A antibodies. People with the blood group O have neither the A nor the B antigens. Those with the blood group AB have both the A and B antigens and neither the anti-A nor the anti-B antibodies.
Inheritance of the ABO blood Groups
Everybody has two copies of the ABO gene, and each parent passes one of these on to their child. Thus, an individual’s blood group is determined by the combination of alleles (the A, B and O variants of the ABO gene) they inherited from their mother and father. The A and B alleles are co-dominant relative to each other, and each is dominant relative to the O allele. These inheritance patterns result in the following blood group possibilities for children, depending on the ABO alleles they inherit.
| Mother’s blood group | Father’s blood group | Child’s blood group |
|---|---|---|
| 0 | 0 | 0 |
| 0 | A | 0 or A |
| 0 | B | 0 or B |
| 0 | AB | A or B |
| A | A | A or 0 |
| A | B | A or B or 0 or AB |
| B | B | B or 0 |
| B | AB | A or B or AB |
| AB | AB | A or B or AB |
Inheritance of the RhD blood group (also known as the Rhesus factor)
Inheritance of RhD status also functions according to Mendel’s laws. The RhD system classifies blood according to the presence (RhD positive) or absence (RhD negative) of the Rh D antigen. RhD positive means that someone has the D antigen on their red blood cells; if the D antigen is absent, the individual is RhD negative. When the RHD gene is missing, this can be indicated through the use of a lower case “d” (as in “the d allele”, though it can also indicate the presence of the gene in a form that does not result in the production of the D antigen).
Everyone has two copies of the part of DNA determining RhD status, one from each of their parents. Like the ABO blood group system, the RHD gene is associated with a specific inheritance pattern. The D allele is dominant relative to the (recessive) d allele, so D allele determines RhD status in a Dd individual. An RhD positive individual can have the gene combination Dd (heterozygous inheritance) or DD (homozygous inheritance), while RhD negative individuals always have the homozygous dd combination. Hence, any child of two RhD negative parents will also be RhD negative, as they can only inherit the d allele. Blood donor identification cards also contain a longer entry under Rhesus type, for instance, “CcD-ee”. This expanded notation indicates other markers in the Rhesus blood group system (the antigens C, c and E, e). However, many of the possible combinations are very rare.
Inheritance of the O marker
Unlike the alleles for the A and B antigens, the O allele is recessive. This means that when it is combined with either of the dominant A or B alleles, the dominant allele will determine the individual’s blood group. Thus, people with the blood group A have either two copies of the A allele (AA, and thus homozygous) or one A and one O allele (AO, heterozygous). The blood group B works the same way: people belonging to this blood group have the gene combination BB or BO. Individuals with the gene combination AB have both the A and the B antigens (blood group AB), because the A and B alleles are co-dominant.
Forensic medicine and proofs of paternity are two examples of areas in which the rules of blood group inheritance play an important role.