In horses, a single gene with two alleles determines skin color. The horse can be gray, white or appaloosa. An appaloosa horse is white with gray patches. Which of the following is true about alleles for color in horses?
The gray and white alleles are codominant.
The appaloosa and gray alleles are codominant.
The allele for white color is dominant over the gray color allele.
The allele for gray color is dominant over the white color allele.

We know that each feature of the horse's body is conditioned by two alleles, one from the father and the other from the mother. What is usually expected is that there is a relationship of dominance between them, that is, there are dominant and recessive alleles. Sometimes, however, this does not happen, and the two alleles end up contributing to the formation of the phenotype. This is called codominance.

In codominance, there are heterozygous alleles that are active and independent of each other. In this case a phenotype is formed that presents characteristics found in the other two homozygous phenotypes. An example of this can be seen in the question above, where the alleles that give the horse a white and gray color can be co-dominant and result in the appaloosa horse, which has both phenotypes being white with gray spots.