Co-dominance is the phenomenon in which two alleles express themselves independently when present together in an organism.

In other words, it is the phenomenon in which offspring shows resemblance to both the parents e.g. $ABO$ blood grouping in humans.

$ABO$ blood groups are controlled by the gene $I.$

The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.

The gene I has three alleles $I^{A}, I^{B}$ and $i .$

The alleles $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$ produce a slightly different form of a sugar while allele 'i' does not produce any sugar.

In humans, each person possesses any two of the three I gene alleles.

$\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$ are completely dominant over i.

When $I^{B}$ and i are present only $I^{B}$ express (because 'i' does not have any sugar) same is the case with $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{i}$.

But when $I^{A}$ and $I^{B}$ are present together they both express their own types of sugars, this is due to co-dominance.

Therefore, the red blood cells have both $\mathrm{A}$ and $B$ types of sugars.

$=$ Since, there are three different types of alleles there can be six different combinations.

Hence a total of six different genotypes of the human $ABO$ blood types are present as given below in the table.

Table : Table Showing the Genetic Basis of Blood Groups in Human Population

Allele from Parent $1$	Allele from Parent $2$	Genotype of offspring	Blood types of offspring
$\mathrm{I}^{\mathrm{A}}$	$\mathrm{I}^{\mathrm{A}}$	$\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{A}}$	$\mathrm{A}$
$\mathrm{I}^{\mathrm{A}}$	$\mathrm{I}^{\mathrm{B}}$	$\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$	$\mathrm{AB}$
$\mathrm{I}^{\mathrm{A}}$	$\mathrm{i}$	$\mathrm{I}^{\mathrm{A}} \mathrm{i}$	$\mathrm{A}$
$\mathrm{I}^{\mathrm{B}}$	$\mathrm{I}^{\mathrm{A}}$	$\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$	$\mathrm{AB}$
$\mathrm{I}^{\mathrm{B}}$	$\mathrm{I}^{\mathrm{B}}$	$\mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$	$\mathrm{B}$
$\mathrm{I}^{\mathrm{B}}$	$\mathrm{i}$	$\mathrm{I}^{\mathrm{B}} \mathrm{i}$	$\mathrm{B}$
$\mathrm{i}$	$\mathrm{i}$	$i i$	$\mathrm{O}$