allelic interaction which single gene deviated from mendel pattern

In incomplete dominance, the phenotype resembles a sort of blending of the two alleles. That is, if you have an allele for red petals and and allele for white petals, you end up getting pink petals with incomplete dominance.

Co-dominance is when both alleles are expressed separately. Human blood type is a good example of this. The A and B alleles are both expressed, so you get the AB blood type. In the petal color example above, you'd get a flower with white and red splotches.

I always found the best way to visualize this is to understand whats going on.

In most cases with true dominance, the dominant gene codes for a functional protein while the recessive allele does not. The recessive phenotype comes from the absence of the functional protein. However, even if one dominant allele is present, the protein is still being made so the dominant phenotype is expressed.

For incomplete dominance, the proteins being expressed work against each other and are usually dependent on expression levels. For example, if a flower had a genotype of RR then both alleles are expressing proteins that code for red. If the flower had the genotype WW then both alleles are coding for a white pigment. Yet, with the genotype RW, the proteins work against each other and the resulting phenotype is a pinking color. Hence "incomplete" since neither true phenotype of either the red or white allele is being expressed.

For co-dominance, the proteins expressed by the genes do not interfere with each other and as long as one allele is present the expression will result in the true phenotype. For example, with blood surface proteins, type A allele expresses the A antigen, type B expressed the type B antigen, and type AB expressed both antigens, with levels independent of each other.

Sure my explanation may be a little long and drawn out, but I think if you read it along with the other ones it might give you a better understanding.