A phenotype is controlled by gene A and gene B. Gene B controls the expression of gene A. This is an example of _____.

• A. Incomplete Dominance.
• B. Epistasis.
• C. Codominance.
• D. Multifactorial characters.
• E. Alleles.

Answer: (B)

Verified step by step guidance

1. Understand the concept of epistasis: Epistasis occurs when the expression of one gene is affected by the presence of one or more 'modifier genes'. In this case, gene B controls the expression of gene A, which is a classic example of epistasis.
2. Differentiate between the given options: Incomplete dominance, codominance, multifactorial characters, and alleles are different genetic concepts. Incomplete dominance involves a blending of traits, codominance involves both traits being expressed equally, multifactorial characters involve multiple genes and environmental factors, and alleles are different forms of a gene.
3. Identify the relationship between gene A and gene B: Since gene B controls the expression of gene A, this indicates a hierarchical relationship where one gene affects the expression of another, which is characteristic of epistasis.
4. Eliminate incorrect options: Incomplete dominance and codominance do not involve one gene controlling another. Multifactorial characters involve multiple genes and environmental factors, not a direct control of one gene over another. Alleles refer to different versions of a gene, not gene interaction.
5. Conclude that the correct answer is epistasis, as it best describes the scenario where gene B controls the expression of gene A.