Generally speaking

(i) most characteristics are determined by several genes,

(ii) most genes have two possible alleles - dominant, recessive or codominant with each other.

However, as in the case of blood groups, you sometimes get more than two alleles for a single gene.

In the case of human blood, two from three possible alleles control which blood group you inherit.

The three alleles controlling blood type are denoted by codominant A and B and recessive O.

Codominant means that when an individual has both dominant alleles, they have the blood type AB, NOT A or B, because one cannot dominate the other. Allele O is recessive.

Therefore you can potentially have one of four blood types: AB (from AB), A (from AA/AO), B (from BB/BO) and O from (double recessive OO). (allele pairings).

 

Possible genotypes and phenotypes of blood groups (simplified e.g. there are subgroups of A alleles)

Phenotype blood group	From possible genotypes
A	AA or AO
B	BB or BO
AB	AB
O	OO

In some genetic diagrams the genotypes are indicated with a different notation for dominant alleles

e.g.  I^A, I^B and recessive allele  I^O, but I've used the 'usual' simpler notation i.e.

AO  as I^AI^O,  BO as  I^BI^O,  AA as I^AI^A, BB as  I^BI^B, AB as  I^AI^B  and OO as  I^OI^O

Worked examples of the genetic inheritance of blood groups

Example 1: Parent of blood group AO crossed with a parent blood of group BO.

Genetic Punnett square table for crossing a parent of blood group A with a parent of blood group B
Parent genotypes cross: AO  x  BO
Gamete alleles: A, O , B and O
Genotypes of parents - alleles	A	O
B	AB	BO
O	AO	OO

Since O is a recessive allele and A and B are dominant alleles,

this parental cross will give the ratio of the phenotype blood groups as:

AB : A : A : O or 1 : 1 : 1 : 1, in other words a 1 in 4 chance (25%) of any of the offspring having ANY of the four possible blood groups. A matching diagram is shown below.