Polymorphic Color Vision

A dichromacy-trichromacy hybrid that defines the color system of most new-world monkeys.

While most mammals have dichromatic color vision, diurnal primates have mostly evolved a form of trichromacy:

• Old world apes (e.g. humans) evolved a ‘true’ form of Trichromacy where all individuals have three separate genes for the three Photopsins.
• New world monkeys (e.g. squirrel monkeys) evolved Polymorphic Trichromacy, where there are only two genes (L- & S-Opsin), but the L-Opsin gene has multiple alleles with different Peak Wavelengths. Because this Opsin is on the X-Chromosome, males only have one allele of the L-opsin and are therefore always Dichromatic. However, females always have two alleles and therefore can be (functionally) trichromatic (if the alleles are of different varieties).

Male new world monkeys therefore have the potential for Trichromatic vision (the rest of the Phototransduction Pathway is present), only lacking the gene for the third Opsin (much like Dichromatic humans). This is why male squirrel monkeys were chosen for a now-famous study that used Gene Therapy to introduce functional Trichromacy into a Congenital Dichromat [Mancuso 2009].