Blood groups are inherited from our parents in the same way as other genetic traits (eg, eye colour). ABO and Rh are the most well-known among the blood group systems.
The ABO blood group system is determined by the ABO gene, which is found on chromosome 9. The four ABO blood groups, A, B, AB and O, arise from inheriting one or more of the alternative forms of this gene (or alleles) namely A, B or O.
Genetic Combinations of ABO Blood Groups
| Blood group | Possible genes |
|---|---|
| A | AA or AO |
| B | BB or BO |
| AB | AB |
| O | OO |
The A and B alleles are codominant so both A and B antigens will be expressed on the red cells whenever either allele is present. O alleles do not produce either A or B antigens, thus, are sometimes called ‘silent' alleles.
ABO Inheritance Patterns
| Parental blood groups | Child's blood group |
|---|---|
| O and O | O |
| O and A | O or A |
| O and B | O or B |
| O and AB | A or B |
| A and A | A or O |
| A and B | O or A or B or AB |
| A and AB | A or B or AB |
| B and B | O or B |
| B and AB | B or A or AB |
| AB and AB | A or B or AB |
Note: These are possible blood groups that children may inherit according to the combination of parental blood groups.
The Rh blood group system is attributable to two genes, RHD and RHCE, which are located on chromosome 1. The RHD gene is dominant, so the expression of the D antigen depends upon whether an RHD gene has been inherited from one or both parents.
Therefore, a person is considered to be D positive whenever the RHD gene is present, even though the gene may have only been inherited from one parent. Conversely, a person will be D negative if no RHD gene is inherited.
Slight differences, also known as polymorphisms, of the RHD gene exist. These polymorphisms result in D variant phenotypes, and somewhat complicate the inheritance pattern of the D antigen. Please refer to the reference list below for more information on RHD genotyping for D variants.
| Parental D phenotype | Child's D phenotype |
|---|---|
| Positive and Positive | Positive or Negative |
| Positive and Negative | Positive or Negative |
| Negative and Negative | Negative |
Note: These are the possible D phenotypes that children may express according to the combination of parental D phenotypes.
References
- Daniels G, 2013, Variants of RhD – current testing and clinical consequences. British Journal of Haematology, 161: 461-470.
- Reid ME, Lomas-Francis C, Olsson ML, 2012, The Blood Group Antigen Facts Book 3rd Edition.
- Sandler SG, Chen LN, Flegel WA, 2017, Serological weak D phenotypes: a review and guidance for interpreting the RhD blood type using the RHD genotype, British Journal of Haematology, 179:10-19.