Four, six.

The phenomenon of expression of both the alleles in heterozygote is called codominance. As the result the phenotype is different from both homozygous genotype.

Examples

$1.$ Blood group is the good example codominance

$2.$ $ABO$ blood groups are controlled by gene $I.$ The gene $(I)$ has three allele $I^{A}, I^{B}, i, I^{A}, I^{B}$ produce slilghtly different form of sugar while i does not produce any kind of sugar.

$3.$ $I^{A}, I^{B}$ are dominant alleles where as $i$ is recessive alleles

$4.$ Since, there are three different allele, there are six different combination of these three alleles are possible and four phenotypes $(A, B, AB$ and $O )$

Genetic Basis of Blood Groups in Human Population

Allele from Parent $1$	Allele from Parent $2$	Genotype of Offspring	Blood Types of Offspring
$I^{A}$	$I^{A}$	$I^{A}\,I^{A}$	$A$
$I^{A}$	$I^{B}$	$I^{A}\,I^{B}$	$AB$
$I^{A}$	$i$	$I^{A}\,i$	$A$
$I^{B}$	$I^{A}$	$I^{A}\,I^{B}$	$AB$
$I^{B}$	$I^{B}$	$I^{B}\,I^{B}$	$B$
$I^{B}$	$i$	$I^{A}\,i$	$B$
$i$	$i$	$ii$	$O$

When $I^{A}$ and $I^{B}$ are present together they both express their own types of sugars this is because of co-dominance. $ABO$ blood grouping also provides a good example of multiple alleles.

Here, you can see that there are more than two,$i.e,$ three alleles governing the same character. Since, in an individual only two alleles can be present multiple alleles can be found only when population studies care made. Dominance is not an autonomous features of a gene. It depends on much on the gene product