Mad Prime: Tag genetics

Color Blindness

Madeleine Ball — Mon, 04 Aug 2008 01:16:00 +0100

For some time now, I've been wanting to write about red-green color blindness, a dramatic perceptual difference with an interesting genetic and evolutionary story. This first post will mostly be an introduction to the topic. If you are color blind: I always feel guilty when I speak of this as a deficiency, or when I emphasize how profound the differences seem to the rest of us. I hope it doesn't bother you. I always wish I could pee standing up so... there.

Daylight vision in humans is mediated by the opsin proteins, which transmit signals that activate nerves when they are hit with light. Humans have three different opsins with different sensitivities to the colors of the spectrum -- it is the different color sensitivities that allow us to see color. You can call these the "blue", "green" and "red" opsins.

A normalized diagram of the sensitivities of opsins to different wavelengths of light.
"S" = "short wavelength", is the "blue" opsin.
"M" = "medium wavelength", is the "green" opsin.
"L" = "long wavelength", is the "red" opsin.

In its severe form, red-green color blindness occurs when a man is missing the "green" or "red" opsin - these conditions are respectively known as deuteranopia (1% of all males) and protanopia (another 1% of males). They are fairly similar in effect: a total loss of ability to distinguish hues in the green to red range. There are many less severe forms of color blindness -- 6% of males -- but that's a later post.

I say "males" because color blindness is almost always seen in men. This is because the "red" and "green" opsin genes are located on the X chromosome, which men have only one copy of. Women have two X chromosomes; even if one has inherited a deletion mutation, the other can serve as a back-up. For a woman to be color blind, both X's would have to carry the same mutation, which is much less likely to occur. (e.g. 1% * 1% = 0.01%)

I'll end this post by showing you what color blindness looks like. Vischeck is a service available online that simulates how images look to a color blind person. To a color blind individual the simulation and original images should look identical (or nearly so - computer monitors vary, so this cannot be perfect). If you're curious about the algorithm, the program is based on this paper.

Deuteranopia	Original	Protanopia

All colors in the red to green range -- green, yellow, orange, red -- are simulated here as yellow. As you can see, deuteranopia and protanopia are almost identical - the main difference is that red looks darker to the protanope (look closely at the picture of cars). Also interesting to note: the butterfly picture demonstrates how purple looks like blue to the color blind individual.

Credits: Opsin sensitivity diagram adapted from Wikipedia diagram, credit goes to User:Vanessaezekowitz and from the screenshot for Wavelength 1.3. Photos taken from flickr users Marshall Flickman, Teo, and Oneras under CC and CC-by-SA licenses.

Genetics

Madeleine Ball — Fri, 14 Mar 2008 02:49:00 +0000

Looks like my Genetics article was overdue for a "good article" rating. I think I'll work towards getting a featured article rating, the GA reviewer encouraged me to do this...

The double-blossom article was in the Did you know section of the front page for seven hours yesterday morning. It got the top spot, with the pretty double impatiens photo.

Did you know...

Madeleine Ball — Sat, 08 Mar 2008 21:51:00 +0000

... that double-flowered mutants (pictured) were first documented over two thousand years ago by Theophrastus and are found in many popular flower varieties — including carnations, camellias, and most roses?

I made this new wikipedia article in the last couple days and have submitted it to the Did you know project for display on the main page.

Past social hour posters

Madeleine Ball — Sun, 09 Dec 2007 08:23:00 +0000

Here are some previous posters I made for genetics department social hour, in reverse chronological order.

Social hour poster

Madeleine Ball — Thu, 29 Nov 2007 00:13:00 +0000

So I've been doing social hour posters for a while for our lab. Maybe I'm getting lazy. This round's poster comes in two varieties: raw and interpreted. (You can click to get larger images.)

The scientific kitty

Madeleine Ball — Sun, 22 Apr 2007 18:46:00 +0100

As Schrödinger so famously demonstrated, whenever one is illustrating fundamental scientific principles, the optimum choice for such an illustration is a cat. In that spirit, I'd like to present one of my favorite examples of how fundamental biology phenomena are visible in our everyday life.

Some background: Gene copy number is important. Variations in gene copy number are, perhaps, a subtle sort of problem -- having 50% more or less copies of a gene available for expression is conceivably a minor thing in the biochemical world, where feedback loops regulate gene expression to increase or decrease as necessary. (That's why so many disorders are recessive; as long as one functional copy of a gene exists, things seem to work fine.) Nevertheless, the duplication or deletion of entire chromosomes has a severe effect. Within the autosomes (non-sex chromosomes), no cases of chromosome loss are viable. The only extra chromosome that is mild enough to be viable in humans is trisomy 21, which causes Down syndrome. Chromosome 21 is the smallest autosome.

Because of this, when it comes to the sex chromosomes, mammals are faced with a copy number problem. Males (XY) have only one copy of the X chromosome, while females (XX) have two. The ways biology addresses this issue is called "dosage compensation". In mammals, dosage compensation is achieved by randomly inactivating all but one X chromosome in all cells. Thus, regardless of an animal being male or female, only one X chromosome is active in any given cell.

This X-inactivation occurs early in embryonic development. Once a cell has decided to inactivate a given X chromosome, that decision is inherited by all its daughter cells. As a result, female mammals exist as a "mosaic" of X-inactivations -- in their bodies, whole patches of tissue have one or the other X inactivated.

An interesting consequence of X-inactivation is that, unlike genes on other chromosomes, only one allele of a gene on the X chromosome is expressed in any given cell. This phenomenon is easily visible in tortoiseshell cats -- tortoiseshell coloration arises from X-inactivation, so these cats are almost always female. The coat color gene, which has alleles for orange or black coats, exists on the X chromosome. Because of X-inactivation, only one of the two genes is active in various patches of skin, giving rise to a pattern of orange and black patches. Since the process of X-inactivation is random, this pattern of patches is random.

I love this example of X-inactivation so much, I added a picture of a kitty to the wikipedia page on X-inactivation. It's always cool to have a everyday visualization of what would otherwise be an abstract genetic and developmental phenomenon.