Mendelian inheritance does not apply to the inheritance of alleles that result in incomplete dominance and codominance.

Why? How are the alleles in these two situations different from the alleles that Mendel studied?

Phenotypes that Mendel studied was controlled by one gene that had two possible alleles, dominant and recessive allele. This made it possible to see just one or the other phenotype (total of 2 phenotypes).

Well now we know that a characteristic may be controlled by one gene with two alleles, but the two alleles may not have a traditional relationship of dominant and recessive. Which is why we see the patterns like codominance and incomplete dominance.

Codominance is when both alleles are expressed equally in the phenotype of the heterozygote. For example, you have a black flower and red flower, a codominant flower would have both black petals and red petals. Do you see how the traditional relationship of one being dominant and other being recessive no longer holds because you express both alleles.

Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the phenotype of both parents. SO a black and red flower would make an offspring with purple petals where neither of the alleles reigns supreme over other rather we get a mixture of both alleles.