Co-dominance:  When two alleles neither show dominant recessive relationship nor show intermediate conditions but both of them express themselves simultaneously when present together, this condition is called co-dominance.

ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers on their surface and this gene controls the type of sugar. The gene (I) has three alleles IA, IB and i. The alleles IA and IB produce two different types of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. IA and IB dominate over i. when IA and IB are present together they both express their own types
of sugars: this is because of co-dominance. Since there are three different alleles, there are six different combinations of these three alleles that are possible and total six different genotypes of the human ABO blood types are possible.

Table showing the genetic basis of blood groups in human population:

Allele from Parent 1	Allele from Parent 2	Genotype of offspring	Blood groups of offspring
IA	IA	IAIB	A
IA	IB	IAIB	AB
IA	I	IAi	A
IB	IA	IAIB	AB
IB	IB	IBIB	B
IB	I	IAi	B
I	I	Ii	O

Single gene can also produce more than one effect. For example, starch synthesis in pea seeds is controlled by one gene. It has two alleles (B and b). Starch is synthesised effectively by BB homozygotes and starch grains produced here are large in size. In contrast, bb homozygotes are less efficient in starch synthesis and produce smaller starch grains. But, the starch grains produced are of intermediate size in Bb seeds. So if starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance A.