In co-dominance, both the genes of an allelomorphic pair express themselves equally in F1 hybrids. Such alleles which are able to express themselves independently even if present together in hybrids are called codominant alleles. (Thus, in co-dominance both express).
Classic example of co-dominance is coat coulur in cattle. There are two types, one with red coat (skin with red colour hair) and the other with white coat ( with white hair ). When red cattle (RR) are crossed with white cattle (WW), F1 hybrids (RW) have roan colour. Roans have the mixture of red and white colour hair. Thus both the traits are expressed equally. In F2 generation, (produced by interbreeding of roans) red (RR), roans (RW) and white (WW) are produced in the ratio 1:2:1. Thus in co-dominance also genotypic and phenotypic rations are identical.
In both, incomplete dominance and co-dominance genotypic and phenotypic ratios are identical but in case of incomplete dominance the phenotype of hybrids is intermediate between the phenotypes of parent while in co-dominance both the genes are expressed equally.