Which of the following represents a condition where one gene affects multiple traits?

The correct answer highlights the concept of pleiotropy, which is when a single gene influences multiple phenotypic traits. This phenomenon can often be observed in cases where a mutation in one gene leads to various physical or physiological changes in an organism. A classic example of pleiotropy is seen in the genetic basis for certain genetic disorders, such as Marfan syndrome, where a mutation in the FBN1 gene affects various traits, including skeletal structure, cardiovascular health, and eye lens.

In contrast, other options do not fit the definition of one gene affecting multiple traits. Polygenic inheritance involves multiple genes contributing to a single trait, while multiple alleles refer to the presence of more than two allele forms of a gene, such as in blood type variation. Epistasis is a genetic interaction where the expression of one gene is modified by one or several other genes, rather than one gene directly affecting multiple traits. Each of these concepts represents different genetic mechanisms and interactions, making pleiotropy unique in its direct influence of one gene on several traits.