Question

Opsins are proteins that are found in the light-sensing cells in the human eye. Different opsin proteins are sensitive to different colors of light. The instructions for creating the type of opsin sensitive to red light are found on a gene known as OPN1LW, which is on the X chromosome.

A genetic mutation in the OPN1LW gene can lead to a form of color blindness that affects a person's ability to detect red light. However, women have two copies of the X chromosome, and they have normal color vision if only one copy contains the mutation.

Why do women who carry one copy of the mutated gene still have normal color vision?
A.
The non-mutated OPN1LW gene makes enough opsin to allow the cells to detect red light.
B.
The cell is able to use proteins other than opsin to detect red light when not enough opsin is produced.
C.
The body detects that a mutation has taken place, so it relies on the other copy of the chromosome for instructions.
D.
The mutated OPN1LW gene is repaired by the non-mutated gene on the other chromosome.

Answer 1

Answer: A.

Reason: OPN1LW is X-linked and a single functional copy produces enough red-sensitive opsin for normal color vision. Women with one mutated allele are typically heterozygous carriers and the remaining normal gene supplies sufficient opsin (X‑inactivation creates a mosaic but overall enough cones express the normal opsin). Options B–D are incorrect.