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Only women are capable of tetrachromacy because the gene that controls which wavelengths of light are detected by cones are on the X chromosome. Tetrachromat women have a mutation on one of their X chromosomes, and are heterozygous for "normal" cones (detect one wavelength) and "abnormal" ones (detect another wavelength). As it turns out, this means that some of their cones detect one wavelength of light, and others detect the other.

As men can not be heterozygous for traits on their X chromosomes, they can't express two cones simultaneously. They CAN have the mutation, but that means they see three colors that are somewhat different from the three colors that other men see, not that they see four colors.

All of the above as I understand it from a popular article on tetrachromacy. I am not a biologist.

The alleles being sex linked makes sense, although there are a couple of theoretical exceptions. One is the very rare male that has two X chromosomes because the SR-Y gene was transposed onto the X chromosome. The other would be having the reverse where the second gene is transposed onto the Y chromosome. The former are sterile while I would guess that the latter are not sterile, but unless they really do exist, are found and one that is found has the same combination as a tetrachromatic woman, there is no way to confirm that.

Genetics was one of my strong points when I was in school. I would have gone into bioinformatics if it were not for my poor performance in organic chemistry, which I felt crippled my ability to do the sort of work that I had wanted to do.

You don't need to be good at orgo to do informatics; I've worked with a number of very capable analysts who hadn't been anywhere near a lab since their undergrad days. (If you want to do lab work, things are different, but my experience is that it's kind of rare for informatic analysis and lab work to overlap.)

Another possibility for male tetrachromats would be people with the 47,XXY karyotype, who are phenotypically male but have the paired X chromosomes necessary to make tetrachromacy possible. Given the rarity of both variations, I can't imagine it being likely that any such person has been identified, but based on my (admittedly meager) knowledge of the subject, I don't see a theoretical barrier to such a person possibly existing.

While I call it bioinformatics because that is a handy umbrella term for biology and computers, what I really wanted to do was to figure out how to do in-Vivo gene splicing to correct genetic illnesses. Specifically, I wanted to build a compiler for generating gene sequences that could be synthesized and injected into bacteria to produce biological machines tailored to alter specific regions of a person's genome. You could call it a compiler for biological instances of sed commands on DNA. It would need to construct things that bypass with the immune system and two membranes with varying methods of entry and varying sized things permitted entry.

Others are making progress in that field, so I am hopeful that some day it will be a reality. The idea that molecular dynamics simulations are NP-complete is going to be a pain for anyone attempting to do that unless they are really clever in figuring out how to do construction like people were when they made the perfect ping. That should require Linus Pauling level or greater knowledge of chemistry, which I do not have. Building it would also produce an easy construction technique for a incredible number of biological weapons, so if it is never made, maybe it is for the best. Not to mention it would probably sow the seeds of the eugenics wars.

You are right about XXY. I had forgotten about it. I had no idea how to theoretically delete an entire chromosome without deleting all copies of it when I was studying with the goal of producing that tool, so it was easy for me to forget. Now that I think about it, it might be possible. Just append the entire chromosome in pieces to the end of another with the genes disabled and some trivial difference to enable final slicing into discrete chromsomes, delete all of the copies of the old trisomal/monosomal chromosomes and do final slicing and splicing to produce two new functioning chromosomes in what would hopefully be a large percentage of the cells while managing not to kill the organism in the process. It would need to be done when the organism is young to minimize negative impacts to its development and would be somewhat different when dealing with sex chromosomes. You would also need to be careful that genes that are needed for the procedure are not disabled by it (like not unlinking libc when trying to replace RHEL with Gentoo in in-place when using a script that calls commands that link to libc). If you manage to rewrite a sufficiently large enough percentage of the organism without killing it, that procedure should cure trisomy and monosomy conditions if done early enough in development. Doing it requires tools that we do not have though.

Anyway, that should make it clear why being bad at organic chemistry kept me from thinking that I could pursue this.

> the very rare male that has two X chromosomes because the SR-Y gene was transposed

Isn't Klinefelter's Syndrome (XXY) more common than that?

I had forgotten about klinefelter's syndrome. You are correct.

> Having three primary colors never made much sense to me either, but that could just be my own ignorance.

Tree primary colors thing seems to be mostly ignorance of the educators teaching that stuff to children. There are no absolute "primary colors" - basically, you can pick any colors at random and use them as a base for linear combinations; the set of colors you can express as such linear combinations form a color space defined by those base colors you picked. Red, green and yellow are a common historical picks, RGB is more common now because that's what is put on the computer screen, but CMY form a color space too.

C.f.: https://en.wikipedia.org/wiki/Primary_color#/media/File:CIE1... - the triangle inside is the color (sub)space of RGB overlayed on CIE 1931 color space (https://en.wikipedia.org/wiki/CIE_1931_color_space).

Source: I educated myself when I got asked by a local drunk to resolve a bet between him and (presumably) another local drunk while I was shopping in a local grocery store. Those two got into a bet over whether RGB or RGY are primary colors.

So who won, based on your exposition? Or was it a draw?

I left that for the two to settle between themselves. I just explained the thing to the guy that asked me first. He was on the RGB side, having shown me a picture similar to this one[0] on his mobile phone. My guess is that he ended up claiming victory, because when I tried to explain the details of the problem, he was going all "so this means RGB can be considered primary colors, right?". I never met the other guy.

[0] - https://en.wikipedia.org/wiki/RGB_color_model#/media/File:Ad...

RBY is what is commonly taught as primary bases in school, due to color compositions and the ease of expressing it through mixing pigments.

R + B = Purple R + Y = Orange B + Y = Green

Right, I probably meant RBY not RGY; I wrote from memory, and meant the one most commonly taught in primary schools in Europe.

I always understood it as RGB being additive -- each color directly stimulates a given color cone in the eye. Whereas process colors (CMYK) are subtractive, because they are a tint. White light hits the paper, but each color (Cyan, Magenta, or Yellow) takes out a certain RGB color leaving the remaining two colors to reflect off the paper. For example, Cyan is a filter for Red, leaving Green and Blue to reflect.

I'd say it's a property of the medium, not of the colors. Computer screens blend additively because you emit a mix of light waves; pigments blend substractively because they are filtering out wavelengths from incoming white light. Color spaces and "primary colors" are orthogonal to that.

http://news.nationalgeographic.com/news/2012/09/120907-men-w...

Men and women seeing slightly differently is in line with our biology. Just how it is, really. Men are more prone to color blindness too.

https://nei.nih.gov/health/color_blindness/facts_about

That explains the LED thing. Thanks.

>One involves "green" status LEDs on electronics, which seem more in common with yellow than green to me.

Almost all "green" LEDs emit at a wavelength of 565nm, which is almost equidistant [1] between green and yellow. Most "green" LEDs are thus green solely by convention, and seeing them as greenish yellow (or yellowish green) isn't unusual at all, because that's the color they actually emit. You can find green LEDs that actually are green, but they're relatively uncommon, and tend to be costly unless you order in bulk direct from a manufacturer.

[1] http://ledmuseum.candlepower.us/35/m-565re2.gif

Simplified version:

X chromosome can express 1 or 2 color receptors, Y only expresses 1.

Women (XX) can get up to 4, but the X occasionally only produces 1, since women have two X they mostly have 3, sometimes 4 and very very rarely only get 2 (both X only express only one)

Men (XY) can get 3 but when the X chromosome only gives 1 they only have 2 and are color blind. Depending on which one they didn't get, that determines their color blindness.

[1] http://www.radiolab.org/story/211193-perfect-yellow/

https://en.wikipedia.org/wiki/XY_sex-determination_system is a useful start for anyone not familiar with the very strong link between some genes and sex determination.

The next step from there is that in humans the X chromosome carries more genes than the Y chromosome. Since women have two X chromosomes, they can express multiple variations of genes located there. Men will only possess one variation.

This is the reason that color blindness is so much more common in men, some of the genes related to color vision are carried on the X chromosome so a single error in one of the pigment genes is sufficient. Women are only color blind if both copies are defective.

I see some things that people identify as orange as more yellow.

I am certain is has to do with all cones not being equal. You may have a cone that is more sensitive to a wider or narrow range; or one that has a peak sensitivity to a slightly different wavelength than the norm; or both.

it could be a recessive trait carried on the X chromosome, and women have two copies of X, whereas men have an X and a Y, so could never satisfy the two copies criterion for recessive traits.