In CODOMINANCE, both alleles express themselves independently in the phenotype. 

For example, in cattle, the allele for red hair is co-dominant with the allele for white hair. Cattle with both alleles are called "roan" which gives them a coat that is both red and white.

In some chickens, the allele for black feathers is co-dominant with the allele for white feathers. Heterozygous chickens then appear speckled with black and white feathers. 

Genes with more than two alleles are said to have MULTIPLE ALLELES. Blood type, for example, has three alleles: I^A, I^B, and i. I^A codes for blood type A. I^B codes for blood type B. And i codes for blood type O. 

Although there are three alleles, only two of them appear in a genotype, because chromosomes are in pairs, not triplets! 

In addition, the I^A, I^B, and i have different relations in terms of dominance with each other. I^A and I^B are co-dominant with each other, but i is recessive to both I^A and I^B. 

MULTIPLE ALLELES: Three or more alternative forms of a gene (alleles) that can occupy the same locus. However, only two of the alleles can be present in a single organism 

Blood Typing is an Example of Multiple Alleles AND Codominance! 

Blood types is  a very unique trait in humans! Not only is it an example of multiple alleles it is also an example of co-dominance! The I^A and I^B alleles are co-dominant and can both be expressed at the same time! That's why individuals with type AB blood have booth A-antigens and B-antigens on their red blood cells!