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 various 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.

Rh positivity or Rh negativity is distinguished by testing for the RhD antigen, the expression of which depends upon whether an RHD gene has been inherited from one or both parents.

The RHD gene is dominant so a person is considered to be RhD positive whenever this gene is present, even though the gene may have only been inherited from one parent. Conversely, a person will be RhD negative if no RHD gene is inherited.

Parental Rh type	Child's Rh type
Positive and Positive	Positive or Negative
Positive and Negative	Positive or Negative
Negative and Negative	Negative

Note: These are the various possible Rh types that children may express according to the combination of parental Rh phenotypes.