Refer to the following information to answer the questions below.
Gene S controls the sharpness of spines in a type of cactus.Cacti with the dominant allele,S,have sharp spines,whereas homozygous recessive ss cacti have dull spines.At the same time,a second gene,N,determines whether or not cacti have spines.Homozygous recessive nn cacti have no spines at all.
-The relationship between genes S and N is an example of