$(a)$ Co- dominance

Co-dominance is the phenomenon in which both the alleles of a contrasting character are expressed in heterozygous condition. Both the alleles of a gene are equally dominant. $ABO$ blood group in human beings is an example of codominance. The blood group character is controlled by three sets of alleles, namely, $I^{A}, I^{B},$ and $i .$ The alleles, $I^{A}$ and $I^{B},$ are equally dominant and are said to be co-dominant as they are expressed in $\mathrm{AB}$ blood group. Both these alleles do not interfere with the expression of each other and produce their respective antigens. Hence, $AB$ blood group is an example of co-dominance.

$(b)$ Incomplete dominance

Incomplete dominance is a phenomenon in which one allele shows incomplete dominance over the other member of the allelic pair for a character. For example, a monohybrid cross between the plants having red flowers and white flowers in Antirrhinum species will result in all pink flower plants in $\mathrm{F}_{1}$ generation. The progeny obtained in $\mathrm{F}_{1}$ generation does not resemble either of the parents and exhibits intermediate characteristics. This is because the dominant allele, $R$, is partially dominant over the other allele, $r$. Therefore, the recessive allele, $r,$ also gets expressed in the $\mathrm{F}_{1}$ generation resulting in the production of intermediate pink flowering progenies with $Rr$ genotype. (figure)