Lecture 18: Genetics of Color Vision

(version 4 September 2006)

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The nature of color

Typical and atypical color vision

trichromatic

One in 5 million persons has only monochromatic vision seeing only shades of gray.

8% of males and 0.5% of females have dichromatic vision

Normally these individuals are red/green color blind.

Structure of the Eye

Rods

Contain the pigment Rhodopsin

Rhodopsin bleaches from pink to white when exposed to light

This bleaching converts the light image into shades of gray

Rods responsible for night vision as are very sensitive to low levels of light.

Cones

Roughly seven million cones/retina

Typical human eye has three types of cones,

Each type contains a single opsin pigment for either Red, Green, Blue.

Each opsin absorbs light at slightly different wavelengths

Genetics of Opsin pigments

Gene for Blue opsin is autosomal

Genes for Red and Green opsins are tightly linked on the X chromosome

Variation in Red/Green Opsin gene number

There is a single copy of the red-sensitive opsin gene and one or more copies of the green-sensitive opsin

The Red and Green genes are very similar in DNA sequence (96% identical), allowing for recombination between them. This can create individuals missing the green opsin

Crossingover can also create hybrid green/red genes that can be either red or green deficient

Evolution of color vision

Red-Green opsins 96% similar, but both only 43% similar to Blue opsins

Blue and Red/Green opsins diverged about 500 millions years ago

Separate Red and Green opsin genes in human those arose less than 40 million years ago, via gene duplication.

Old-world monkeys are trichromatic but New-world monkeys (South America) are dichromatic (have only blue and one Red/Green opsin). Old-world/new-world monkeys had last common ancestor at approximately 40 MYA.

Trichromatic female squirrel monkeys

But most females are trichromatic. Why?

The single Red/Green gene in squirrel monkeys has multiple alleles, some of which are red-sensitive, others which are green-sensitive

Seeing beyond the (human) visible spectrum

Some organisms have opsin genes that can see:

In the far-red (infrared), so can see heat (e.g., rattlesnakes)

In the far-blue (ultraviolet).