To solve this problem, we need to understand the concept of pleiotropism. Let's analyze the options:$\newline \bullet$Option 1: Presence of several alleles of a single gene controlling a single crossover.$\newline$ - This describes multiple alleles, not pleiotropism.$\newline \bullet$Option 2: Presence of two alleles, each of the two genes controlling a single trait.$\newline$ - This describes a dihybrid cross, not pleiotropism.$\newline \bullet$Option 3: A single gene affecting multiple phenotypic expression.$\newline$ - This is the correct definition of pleiotropism. A single gene can influence multiple traits.$\newline \bullet$Option 4: More than two genes affecting a single character.$\newline$ - This describes polygenic inheritance, not pleiotropism.$\newline$Therefore, the correct answer is Option 3: A single gene affecting multiple phenotypic expression.