Final answer:
Codominance is the relationship between genes S and N where both alleles are expressed equally in the phenotype of the heterozygote. An example of codominance is the MN blood groups of humans.
Step-by-step explanation:
The relationship between genes S and N is an example of codominance. In codominance, both alleles for the same characteristic are expressed equally in the phenotype of the heterozygote. An example of codominance is the MN blood groups of humans, where the M and N alleles are expressed as M or N antigens on the surface of red blood cells. Homozygotes express either the M or N allele, while heterozygotes express both alleles equally.