Final answer:
Protanopes can see yellow because the M and S cones can still be stimulated by yellow light, enabling perception of this color even without functional L cones, whereas green may not be perceived distinctly due to the lack of L cone input for contrast.
Step-by-step explanation:
The question why protanopes (individuals who have a type of red-green color blindness) can perceive yellow but not green, despite the fact that the M cone (or medium-wavelength cone) may have the same relative absorbance for these colors, is related to the way the brain interprets signals from the cones in the eye. Protanopes lack functioning L cones (long-wavelength cones), which are primarily responsible for detecting red light. However, they still have functioning M and S cones (short-wavelength cones).The perception of yellow occurs when both red and green cones are stimulated and send signals to the brain indicating that both long and medium wavelengths are being detected. Protanopes have a non-functioning L cone, but the M cone can still respond to the long wavelengths that yellow light emits.
Therefore, the M cone's response, along with input from the functioning S cones for certain shades of yellow, allows protanopes to perceive yellow.Green, on the other hand, is primarily detected by the M cone, and without the contrast of the L cone's input, the brain may not differentiate it as it would normally. Protanopes may perceive green differently or not at all, depending on how their M cones and brain interpret the wavelength of light.Color vision and the perception of colors like yellow and green are complex processes that involve not only the absorption characteristics of the cones but also how the brain processes and interprets the signals it receives from these cells.