Gene s controls the sharpness of spines in a type of cactus. cactuses with the dominant allele, s, have sharp spines, whereas homozygous recessive ss cactuses have dull spines. at the same time, a second gene, n, determines whether or not cactuses have spines. homozygous recessive nn cactuses have no spines at all. the relationship between genes s and n is an example . codominance.2. pleiotropy.3. complete dominance.4. epistasis.5. incomplete dominance.

4: epistasis

Explanation:

Epistasis is a form of gene interaction in which a gene on one locus modifies or suppresses the effects of another gene on a separate locus.

Epistatic gene interaction can be dominant, recessive, duplicate recessive, duplicate dominant, or polymeric gene interaction.

Recessive epistasis occur when a recessive allele at one locus suppresses the expression of alleles on separate locus/loci.Dominant epistasis occur when a dominant allele at one locus suppresses the expression of alleles on separate locus/loci.Duplicate recessive epistasis occur when recessive alleles at either of two loci suppress the expression of dominant alleles at the two loci.Duplicate dominant epistasis occur when dominant alleles t either of two loci suppress the expression of dominant alleles at the two loci.Polymeric gene interaction occur when two dominant alleles which have similar effect individually produce enhanced effect when they come together.

In this case, the expression of gene is controlled by the presence/absence of gene N on a separate locus. In homozygous recessive form, gene N suppresses the expression of gene S, a clear case of recessive epistasis.

The correct option is option 4.

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