Ans. Co-dominance
Co-dominance is the situation where two different alleles of a gene pair express themselves completely, instead of showing a dominant-recessive relationship. As a result, the heterozygous organism shows a phenotype that is different from both homozygous parents
Example
An example of co-dominance is the expression of human blood group AB. The ABO 'blood group system is controlled by the gene l. This gene has three alleles i.e. IA, IB and i. The allele I produces antigen A in blood and the phenotype is blood group A. The allele I8 produces antigen B in blood and the phenotype is blood group B. The allele I does not produce any antigen and the phenotype is blood group O.The alleles IA and IB are dominant over i. When there is a heterozygous genotype of IA IB, each of the two alleles produces the respective antigen and neither of them dominates over the other Figure 15.9