What is codominance? Explain it in detail with the example of human blood groups.

(N/A) Codominance is a phenomenon where both alleles of a gene pair are expressed equally in the heterozygote,meaning neither allele is dominant or recessive over the other.
In this case,the phenotype of the $F_1$ generation resembles both parents.
An excellent example of codominance in humans is the $ABO$ blood grouping system.
The $ABO$ blood groups are controlled by the gene $I$. The plasma membrane of red blood cells has sugar polymers that protrude from its surface,and the kind of sugar is controlled by the gene $I$.
The gene $I$ has three alleles: $I^A$,$I^B$,and $i$. The alleles $I^A$ and $I^B$ produce slightly different forms of the sugar,while the allele $i$ does not produce any sugar.
Since humans are diploid $(2n)$ organisms,each person possesses any two of the three alleles. The alleles $I^A$ and $I^B$ are completely dominant over $i$,so when $I^A$ and $i$ are present,only $I^A$ is expressed. Similarly,when $I^B$ and $i$ are present,only $I^B$ is expressed.
However,when $I^A$ and $I^B$ are present together,they both express their own types of sugars. This is codominance.
There are $6$ different genotypes possible for the $ABO$ blood group system.

Allele from Parent $1$	Allele from Parent $2$	Genotype of Offspring	Blood Type of Offspring
$I^A$	$I^A$	$I^A I^A$	$A$
$I^A$	$I^B$	$I^A I^B$	$AB$
$I^A$	$i$	$I^A i$	$A$
$I^B$	$I^A$	$I^A I^B$	$AB$
$I^B$	$I^B$	$I^B I^B$	$B$
$I^B$	$i$	$I^B i$	$B$
$i$	$i$	$i i$	$O$