Which term refers to the influence of external factors on gene activity?

The term that refers to the influence of external factors on gene activity is epigenetics. Epigenetics involves the study of heritable changes in gene expression that do not involve alterations to the underlying DNA sequence. These changes can be caused by various external factors such as environmental influences, lifestyle choices, and even circumstances that affect gene expression.

Epigenetic modifications can include the addition or removal of chemical groups to the DNA or to the histone proteins around which DNA is wrapped, leading to changes in how tightly or loosely DNA is packaged. This, in turn, affects how genes are turned on or off, ultimately influencing an organism's phenotype without changing its genotype.

In contrast, phenotype refers to the observable traits of an organism that result from the interaction between its genotype and the environment, but it does not specifically refer to the mechanisms by which external factors influence gene activity. Genotype represents the genetic makeup of an organism, specifically the alleles inherited from its parents, and is not inherently affected by external factors. Mutation pertains to changes in the DNA sequence itself, which can lead to alterations in gene function or expression, but these are different from the influence of external factors on gene activity.