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 -- called heterozygosity), the individual ends up with a blend of the two phenotypes. For example, if one allele is for red hair and the other allele is for blue hair, then the individual will have purple hair. (This is in contrast to codominance, where the individual would inherit patches of blue and patches of red hair).