What does the expression of a gene refer to?

The expression of a gene specifically refers to the biological process through which the information encoded within a gene is utilized to generate functional products, primarily proteins, but also RNA molecules. This process involves two key steps: transcription, where the gene's DNA sequence is copied to produce messenger RNA (mRNA), and translation, where the mRNA serves as a template for synthesizing proteins.

Understanding gene expression is crucial because it directly affects how traits are manifested in an organism, contributing to its phenotype and allowing for the regulation of various biological functions. The other choices address related concepts, but they do not accurately define gene expression. For instance, the physical appearance of an organism relates more to phenotype, while discussing mutations pertains to genetic changes rather than expression. Additionally, inheritance of genetic traits involves the passing down of genes rather than their expression, highlighting the distinction between genetic encoding and functional output.