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

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

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 a specific genetic interaction.
4. Recognize that 'epistasis' is the term used to describe this interaction where one gene masks or modifies the effect of another gene.
5. Differentiate epistasis from other options: 'pleiotropy' refers to one gene affecting multiple traits, 'linkage' refers to genes located close together on a chromosome, and 'codominance' refers to both alleles being expressed equally.