What does a gene mutation primarily affect?

A gene mutation primarily affects a single gene because mutations involve changes in the nucleotide sequence of that gene. These changes can be caused by various factors such as environmental influences or errors during DNA replication. When a mutation occurs, it can alter the way the gene functions, which may lead to changes in the instructions for making a specific protein coded by that gene.

While the effects of a mutation can extend to influencing the organism's phenotype, the direct impact is on the specific gene itself and the protein it codes for. The consequences might be subtle or profound, depending on whether the mutation is beneficial, harmful, or neutral. Thus, understanding that gene mutations specifically target the nucleotide sequence of a single gene is critical in genetics and molecular biology.