Environmental Effects on Phenotype Practice Test

Genetically identical yeast cells are grown in either glucose-rich medium or lactose-rich medium. In lactose medium, cells produce high levels of β-galactosidase enzyme; in glucose medium, β-galactosidase levels are low. Sequencing confirms the same DNA sequence in both groups. Which explanation best accounts for the enzyme differences between conditions?

A strain of bacteria with identical genomes was grown in either lactose-containing medium or glucose-containing medium. After 30 minutes, cells in lactose medium produced high levels of β-galactosidase, while cells in glucose medium produced very low levels. DNA sequencing of the β-galactosidase gene was identical in both cultures. When glucose-grown cells were transferred to lactose medium, β-galactosidase levels increased. Which explanation best accounts for the enzyme level differences between media?