The law of co-dominance: This means that when there are two alleles, which share a co-dominant relationship, the progeny will express both alleles. For example, if a black cat (CbCb) is crossed with a brown cat (CrCr), the kittens (CbCr) will be either brown with black spots or stripes, or black with brown spots or stripes, i.e., tabby cat. This means that both colors are co-dominant in this case, and both the alleles are completely expressed, and the kittens show both colors at the same time. In incomplete dominance: is the expression of alleles that are neither dominant or recessive. These alleles mix together and result in the expression of a physical trait, which is a mixture of both the alleles. For example, when you cross black mice (BB) with white mice (WW), their progeny is gray in color (BW). These colors blend together and express the phenotype of both alleles. This type of dominance is similar to mixing paints. When you mix one color with another color, the result is a completely different color. Similarly, when one allele is mixed with another allele, they blend together to give rise to a new phenotype. In short: In co-dominance, the off spring will show both alleles. In incomplete dominance: neither will be expressed but what will be seen is a mixture (as in mixing paint colors).