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The enzyme lactase breaks down the lactose sugar found in milk. Lactase is produced at birth in mammals, which makes sense because infant mammals drink their mother’s milk!

In most adult mammals, including most adult humans, the lactase gene is no longer expressed after the age of 6 years, resulting in the phenotype of adult lactose intolerance (inability to digest the sugar lactose due to the absence of the lactase enzyme).

However, some human adults, particularly those of European descent, are not lactose intolerant due to the presence of a genetic mutation that affects the expression of the lactase enzyme, resulting in prolonged expression beyond infancy and persisting into adulthood.

Based on this information, which of the following is a likely location of the lactose-tolerance mutation in the human genome?

A

In the coding sequence of the lactase gene because the adults who can digest lactose produce a different enzyme than everybody else.

B

In the coding sequence of the lactase gene because the adults who can digest lactose acquire a mutation in the sequence after infancy.

C

In the regulatory sequence of the lactase gene because the adults who can digest lactose have an altered pattern of lactase gene expression regulation that causes expression into adulthood.

D

In the regulatory sequence of the lactase gene because the adults who can digest lactose produce a different enzyme than everybody else.

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