Codominant genes are two alleles of a gene which result in distinctly different phenotypes, but when they are both inherited together in an individual (one from the mother and one from the father in a condition called called heterozygosity), the individual has both of the phenotypes. For example, if one allele is for white scales and the other allele is for black scales, then the individual will have patches of black scales and patches of white scales. (This is in contrast to incomplete dominance, where the individual would inherit a blend of the two alleles and have gray scales).
This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/