When the interaction of genes alters expected Mendelian genetic ratios, this phenomenon is called:

• A. epistasis
• B. linkage
• C. codominance
• D. pleiotropy

Answer: (A)

Verified step by step guidance

1. Understand the question is asking about a genetic phenomenon where gene interactions change the expected Mendelian ratios.
2. Recall that Mendelian ratios are based on independent assortment and simple dominant-recessive relationships between alleles.
3. Identify that when one gene affects or masks the expression of another gene, altering these expected ratios, this is known as epistasis.
4. Differentiate epistasis from other options: linkage involves genes located close together on a chromosome, codominance involves both alleles being expressed equally, and pleiotropy refers to one gene affecting multiple traits.
5. Conclude that the correct term for gene interaction altering Mendelian ratios is epistasis.