Final answer:
A mutation in the regulatory sequence of the lactase gene allows for continuous expression in adults, enabling them to digest lactose. This genetic change is distinct from the mutations that would affect the enzyme's structure or its mRNA splicing, such as those in the coding sequence, exons, or introns.
Step-by-step explanation:
A mutation in the regulatory sequence of the gene encoding the enzyme lactase enables expression of this gene in adults. This mutation allows adults to continue to produce lactase, the enzyme necessary for the digestion of lactose, the sugar found in milk. In mammals, lactase expression is generally turned off after weaning, but in some human populations, a mutation has occurred in the regulatory elements of the lactase gene, allowing for continued expression into adulthood, a condition known as lactase persistence.
The regulatory sequence includes areas such as promoters and operators that are critical for the regulation of gene expression. Mutations in the coding sequence, exons, or introns would affect the enzyme's structure or splicing but not necessarily its expression. Therefore, the specific mutation that enables adult lactase expression would be in the regulatory region of the gene.
The lac operon is an example of such regulation in bacteria, where lactose availability and other factors influence the expression of genes involved in lactose metabolism. A constitutive operon, on the other hand, is one that is continuously transcribed, such as an operon encoding housekeeping genes.