What is the difference between co-dominance and incomplete dominance?

Alleles are alternative forms of the same gene. A dominant allele is a form of a gene that covers up the alternative recessive allele. Recessive alleles typically involve the absence of a specific phenotype.

In snapdragons or example, a white flower develops when two receive alleles are present. The recessive allele does not generate any pigment in the flower's pedals. There is also an alternative allele that produces red pigment. Two copies of the alternative allele will produce a deep red phenotype, but if only one copy is present the allele will not be fully expressed. Instead a pink phenotype will occur this is what is known as incomplete dominance.

During co-dominance two equally dominant alleles will both be expressed. A good example of this is roan cattle. Roan cattle have both red and white fur. The red and white alleles involved in this cross are both equally dominant, and both red and white phenotypes are visible. Co-dominance produces a mix of equally dominant phenotypes. Incomplete dominance, on the other hand, produces a phenotype that looks like a blend between the dominant phenotype and the recessive phenotype.