Which of the following is an example of epistasis?

• A. A gene with codominant alleles results in both traits being expressed equally.
• B. A single gene with incomplete dominance produces an intermediate phenotype.
• C. Two different genes, when both have at least one dominant allele, produce the same phenotype.
• D. A homozygous recessive allele at one gene masks the expression of alleles at a second gene, resulting in a white flower regardless of the second gene's genotype.

Answer: (D)

Verified step by step guidance

1. Understand the definition of epistasis: it occurs when the expression of one gene is affected or masked by one or more other genes, rather than by alleles of the same gene.
2. Review each option to identify if it involves interaction between different genes rather than alleles of the same gene.
3. Recognize that codominance and incomplete dominance describe relationships between alleles at the same gene locus, so they are not examples of epistasis.
4. Focus on the option describing two different genes where one gene's homozygous recessive genotype masks the expression of the other gene, which fits the definition of epistasis.
5. Conclude that the example where a homozygous recessive allele at one gene masks the expression of alleles at a second gene, resulting in a specific phenotype regardless of the second gene's genotype, is a classic case of epistasis.