Color vision and X-inactivation.

[Lionel Mandrake]

12. A couple of comments about those curves.

1. The curves for birds and humans show different types of data. The curves for birds show the spectral absorptions of different types of cones, whereas the curves for mammals are the effective spectral absorptions taking into account absorption by other structures in the eye. The most important other structure is the lens, which effectively cuts off ultraviolet light before it can reach the retina. That's why the curves for humans go to zero at very short wavelengths.

2. The peaks of the curves for the L and M cones in humans vary quite a bit among individuals, because of the many polymorphisms in the genes that control the pigments in these cones. Evolution has moved these two peaks as far apart as possible in the most fit individuals. Mildly deleterious alleles have the effect of moving the L and M peaks closer together. If L peak is normal and the M peak is abnormal, the individual has deuteranomaly. If M peak is normal and the L peak is abnormal, the individual has protanomaly. (There is no name for the type of deficit in which both peaks are abnormal.)