Question

Normally, purple people-eaters are purple. Researchers isolated a mutant that is pink and bred a large population of pink people-eaters for sale to the public as exotic (and dangerous) pets. Geneticists showed that pink is recessive. After many generations, a purple mutant appeared within the pure-breeding pink population. The enzyme that is supposed to convert pink pigment to purple was isolated from members of the original purple population (enzyme 1), from members of the pure-breeding pink population (enzyme 2), and from the new purple mutant (enzyme 3). The amino acid sequence of a portion of each enzyme is shown below:

Enzyme 1: ...Leu-Pro-Val-Ala-Pro...
Enzyme 2: ...Leu-Leu (truncated)
Enzyme 3: ...Leu-Leu-Leu-Ala-Pro

Which of the following mechanisms would best account for the production of the normal phenotype in the purple mutant that appeared among the pink population?


Reverse mutation to wild-type

Gene duplication

Intragenic suppression

Nonsense suppression

Intergenic suppression

Answer 1

The reappearance of the purple phenotype in the pure-breeding pink population of purple people-eaters is best explained by intragenic suppression, where an additional mutation partially compensates for the original mutation, restoring enough function to the enzyme to produce the purple pigment.

Step-by-step explanation:

The student asked why a purple mutant appeared within the pure-breeding pink population of purple people-eaters, where pink is recessive. The amino acid sequences provided for the enzymes from the original purple population (enzyme 1), the pure-breeding pink population (enzyme 2), and the new purple mutant (enzyme 3) suggest different mutations. Given that enzyme 1 has a sequence of ...Leu-Pro-Val-Ala-Pro..., enzyme 2 is truncated with ...Leu-Leu, and enzyme 3 has an extra Leu resulting in ...Leu-Leu-Leu-Ala-Pro..., the most likely mechanism for the reversion to purple is intragenic suppression. In this case, the third leucine might restore the enzyme's function enough to produce the purple pigment despite the original mutation causing the pink phenotype. The other options like reverse mutation, gene duplication, nonsense suppression, or intergenic suppression do not align well with the amino acid sequences provided.