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Looking for Madam Tetrachromat Do mutant females walk among us? By Glenn Zorpette From the December 04, 2000 issue Preliminary evidence gathered at Cambridge University in 1993 suggests that this woman is a tetrachromat, perhaps the most remarkable human mutant ever identified. Most of us have color vision based on three channels; a tetrachromat has four. The theoretical possibility of this secret sorority -- genetics dictates that tetrachromats would all be female -- has intrigued scientists since it was broached in 1948. Now two scientists, working separately, plan to search systematically for tetrachromats to determine once and for all whether they exist and whether they see more colors than the rest of us do. The scientists are building on a raft of recent findings about the biology of color vision. The breakthroughs come just in time. "Computers, color monitors, and the World Wide Web have made having color blindness a much bigger deal than it ever was before," says Jay Neitz, a molecular biologist who studies color vision at the Medical College of Wisconsin in Milwaukee. Color-blind individuals, he explains, often lose their way while navigating the Web's thicket of color cues and codes. "Color-blind people complain miserably about the Web because they can't get the color code," Dr. Neitz says. (Just try surfing on a monochrome monitor.) Most people are trichromats, with retinas having three kinds of color sensors, called cone photopigments -- those for red, green, and blue. The 8 percent of men who are color-blind typically have the cone photopigment for blue but are either missing one of the other colors, or the men have them, in effect, for two very slightly different reds or greens. A tetrachromat would have a fourth cone photopigment, for a color between red and green. Besides the philosophical interest in learning something new about perception, the brain, and the evolution of our species, finding a tetrachromat would also offer a practical reward. It would prove that the human nervous system can adapt to new capabilities. Flexibility matters greatly in a number of scenarios envisaged for gene therapy. For example, if someone with four kinds of color photopigments cannot see more colors than others, it would imply that the human nervous system cannot easily take advantage of genetic interventions. For years now, scientists have known that some fraction of women have four different cone photopigments in their retinas. The question still remains, however, whether any of these females have the neural circuitry that enables them to enjoy a different -- surely richer -- visual experience than the common run of humanity sees. "If we could identify these tetrachromats, it would speak directly to the ability of the brain to organize itself to take advantage of novel stimuli," says Dr. Neitz. "It would make us a lot more optimistic about doing a gene therapy for color blindness." There have been very few attempts to find Madam Tetrachromat. The one that turned up Mrs. M in England, in 1993, was led by Gabriele Jordan, then at Cambridge University and now at the University of Newcastle. She tested the color perception of 14 women who each had at least one son with a specific type of color blindness. She looked at those women because genetics implies that the mothers of color-blind boys may have genetic peculiarities of their own. Among that somewhat peculiar group of women, one could expect to find the odd tetrachromat. It's almost as if the supersense these women enjoy comes at the expense of the men in their families. "I'm just sorry I've robbed my son of one of his color waves," Mrs. M says. Dr. Jordan reports that of the fourteen test subjects in her study, two showed "exactly" the behavior that would be expected of tetrachromats. "It was very strong evidence for tetrachromacy," she adds. The apparent tetrachromats were Mrs. M, who was identified in the study as cDA1, and another candidate, cDA7. Dr. Jordan set up an experiment in which subjects tried to determine whether a pair of colored lights matched. They used joysticks to blend two different wavelengths as they pleased. The resulting hues lay outside the spectrum of the blue photoreceptor, rendering it nearly useless, so that normal trichromats would have the use of only their red and green photoreceptors. Having hit upon a color, the subjects would then try to reproduce it by mixing two other wavelengths. Because the tetrachromats had the use of only two receptors, they found a whole slew of mixes that produced a matching color. However, any tetrachromat should have been able to use three receptors in this color space, and therefore make a single, precise match. In the experiment, cDA1 and cDA7 performed pretty much as a tetrachromat would be expected to. Nevertheless, Dr. Jordan declines to say that she has finally found a tetrachromat, partly because her testing is still a work in progress. The vast majority of us have no idea what tetrachromacy would be like. Anyone who had the supersense wouldn't know she did, let alone be able to describe it. After all, it is an exercise in futility for trichromats to try to explain their visual experience to color-blind people. Dr. Neitz and Dr. Jordan each plan a more definitive search for tetrachromats. Dr. Neitz plans to take advantage of the fuller understanding of the underlying genetics of color vision. His will be the first experiment that will use genetic techniques to identify women with four different color photopigments. What will he be looking for? Let's start with the basics. The genes for the red and green photopigments are adjacent to each other on the X chromosome; strangely, blue is way off by itself on another chromosome. Women, of course, have two X chromosomes and therefore two sets of red and green photopigment genes. Men have only one X, so they have just one shot at getting the red and green photopigment genes right. Unfortunately for men, it turns out that those genes are prone to a kind of mutation that occurs when eggs are formed in a female embryo. When the eggs are created, the X chromosomes from the maternal grandmother and grandfather mix with each other in random places to make the egg's brand-new X chromosome. Because the genes for the red and green photopigments are right next to each other, those genes sometimes mix. That's perfectly normal. But every once in a while, the mixing occurs in a lopsided way, and the result, 30 years later, could very well be a man who has to check with his wife every time he dresses. A lopsided mix can have three outcomes: (1) the egg in the embryo has an X chromosome that's missing either a red or a green photopigment gene, (2) the X chromosome has two slightly different red photopigment genes, or (3) the X chromosome has two slightly different green photopigment genes. In any of these cases, if that egg gets fertilized and becomes a male, the man will get that X chromosome and be color-blind. Here it gets interesting. Suppose a woman inherits one X chromosome with two slightly different green photopigment genes. And let's say her other X chromosome has the normal complement of red and green photopigment genes. Because of a well-known biological phenomenon called X inactivation -- which causes some cells to rely on one X chromosome and others to rely on the other -- that woman's retinas would have four different types of photopigments: blue, red, green, and the slightly shifted green. (It would also be possible, through a different genetic sequence, to produce blue, green, red, and a shifted red.) X inactivation is only possible in women, so there has never been, and probably never will be, a male tetrachromat. True tetrachromacy would require a few other characteristics in addition to retinas with four different photopigment receptors. For instance, there would have to be four neural channels to convey to the brain the sensory inputs from the four receptors, and the brain's visual cortex would have to be able to handle this four-channel system. If a woman were born with four types of photopigments, would her brain wire itself to take advantage of them? No one knows for sure, but some experts strongly suspect it would. "Yes, definitely," says Jeremy Nathans, a pioneer in color-vision research at Johns Hopkins University School of Medicine. One reason to think so is the brain's great plasticity in other respects. People with special skills -- musicians, bilinguals, deaf people who learn sign language -- often show characteristic brain patterns. Dr. Nathans also believes, however, that for full-blown tetrachromacy, the fourth photopigment must not have a peak in sensitivity that is too close to the peaks of either the red or the green photopigments. That's the rub, as far as he's concerned -- he suspects that most female tetrachromats would have only mildly superior color vision, because the genetics indicates that the fourth photopigment would almost always be very close to either the red or the green. Every now and then, however, an oddball photopigment might appear, well separated from both red and green. "The genetics do not rule it out," Dr. Nathans explains. "It would be a rare event. But who's to say it hasn't happened? There are a lot of people out there." That idea finds support in the recent discoveries about the genetics of color vision, many made by Dr. Neitz's group. Those findings have shown that the genetics underlying color vision are surprisingly variable, even within the narrow range regarded as normal. "The variety in photopigment genes in people with normal color vision is enormous," Dr. Neitz reports. "It's enormous." Would there be any practical advantages to tetrachromacy? Dr. Jordan notes that a mother could more easily spot when her children were pale or flushed, and therefore ill. Mrs. M reports that she has always been able to match even subtle colors from memory -- buying a bag, for example, to match shoes she hasn't laid eyes on for months. And computers, color monitors, and the Internet raise a whole raft of possibilities. Just as someone with normal three-color vision surfs rings around a dichromat on the Internet, a tetrachromat, looking at a special computer screen based on four primary colors rather than the standard three, could theoretically dump data into her head faster than the rest of us. If Dr. Neitz or Dr. Jordan finally finds Madam Tetrachromat, the discovery will confirm that the human nervous system can handle four-channel color vision. And that confirmation would raise the possibility that, within a couple of decades, gene therapy will make tetrachromacy just another option that wealthy parents could check off on the list when they are designing their daughters. It won't be possible with male children -- not for quite some time, anyway. So as long as we're on this flight of fancy, let's take one more short hop: a few decades from now, men and women will still be seeing the world differently. But the expression might not be merely figurative any more. ADDITIONAL RESOURCES Q & A on the biology of human color vision. Web site for Jay Neitz's laboratory on color vision at the Medical College of Wisconsin. Includes images showing how the color blind would see certain scenes and objects. Explanation of the genetics behind color-vision deficiencies.
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Those who are cocky about their great understanding of the universe (from atoms to ballots to galaxies) should find this article very humbling: most of us can't even see colors as well as some mutants - how much more there is that we just don't comprehend.
Wow, you're good. I was just about to post this.
Now, what were those creationists saying about complex mutations being so unlikely as to be unable to drive evolution?
"'Computers, color monitors, and the World Wide Web have made having color blindness a much bigger deal than it ever was before,' says Jay Neitz...."
Tell that to Vinny Testaverde.
Regards. S&W R.I.P.
Rattlesnakes use infrared. The Duckbill Platypus has some sort of electro-magnetic or ionic sense. My dog hears and smells a much wider range of events than I do, though I probably see more.
So what else is new?
The phrsse "secret sorority" suggests perparations for a whole new sub-sub-department of color coordination in the fashion industry.
"Be sure your colors really match—consult our tetrachromat."
Regards. S&W R.I.P.
I don't see how such an extremely rare and subtle genetic distinction could affect natural selection, which according to Darwin is the very engine of evolution.
I don't see how such an extremely rare and subtle genetic distinction could affect natural selection, which according to Darwin is the very engine of evolution.
Improved color vision can help you to survive. In this day and age, it may not be enough of an advantage to be selected for, but this is an unusual time. (This particular mutation may not have been carried even in prehistoric times, considering its deleterious effect on males, but that's beside the general point.)
Consider the difference between color blindness and color vision. Without color vision in the wild, it's tough to pick motionless animals out from the background, even at close range. With color vision, you can spot them at long range.
Add another color, and you add another entire dimension of variegation to the world. It would be almost as profound as the leap from color blindness to color vision. We trichromats can't even imagine what it might be like. Don't tell me you can't see how it would be a survival advantage in the wild!
I went to the automat
for lunch with Madam Tetrachromat.
She stared at each item
but never did bite them
unless each hue from the red to the blue
met with her fourfold approval.
"This one's medicinal!",
she exclaimed about the salad,
"And this one will kill you, lad,
in considerably less than a minute!
This one's got iodine in it,
That one's got arsenic—See
this little shade of grink—
that means it's all right to drink!".
Regards. S&W R.I.P.
Splendid! Did you write that?
"Did you write that?"
Gotta 'fess up—yep.
Thanks and best regards. S&W R.I.P.
those creationists saying about complex mutations being so unlikely as to be unable to drive evolution?
I don't understand ... unless you mean that men will forsake the needles in the haystack that are tetrachromates in case the sons they have won't be able to surf the web as well. =)
Can you explain further for me?
=)
Can you explain further for me?
Sure. I wasn't commenting on selection, but on the spontaneous appearance of a complex new ability.
A frequently heard creationist canard goes like this: "OK, you say that evolution is the change in allele frequencies over time. We all accept that, just look at the change in the prevalence of dark moths over light moths as tree trunks became sootier in England. The question, rather, is where do these alleles come from in the first place? Mutations do occur, but these are overwhelmingly simple replication errors that do nothing but debilitate the organism. The odds against a complex mutation giving rise to a new capability are so astronomical that this cannot be considered a realistic source of new alleles."
In this case, I agree, the survival advantages of tetrachromaticity are probably insufficient to offset the penalty paid by heterozygous males. But it still possibly may have supplanted trichromaticity in the wild, if it had happened millions of years ago!
Here's how. Consider that there is an analogous set of color sensor genes on the y-chromosome. If the tetrachromat mutation can happen once on the x-chromosome (and the article seems to suggest that it has happened more than once), it can happen on the y-chromosome. In fact, as the x-chromosome tetrachromaticity gene spreads through the population (which it may, as long as the color blindness problem isn't bad enough to eliminate it), such a mutation becomes even more likely, as there will be a greater number of chances for the gene to jump from one chromosome to another, as sometimes happens.
Once the tetrachromaticity gene is established on both chromosomes and becomes common in the gene pool, the chromosomes that carry trichromaticity become the de facto cause of color blindness: people homozygous for tetrachromaticity have 4-color vision, while heterozygous people have the problem.
That helps some. (I'll admit I usually need to read your posts a couple times. =)
A couple questions right off the bat:
And ... somewhat off the beaten path ... do you think it's really fair to attribute to evolution or natural selection the increasingly calculated moves that are aborting and/or purchasing children to meet a specific array of intelligence, colors, sex and such?
I mean, I believe in evolution (if not Darwinism) and I'm cool with natural selection as a means -- even among humans -- to choose mates based on aspects signifying strength and fertility and such. But given the increasingly arbitrary, atomized and FASHIONABLE (as opposed to standard and universal) criteria that are this or that hetero OR homosexual couple's, woman, man's or group's particular set of criteria for purchasing children, can we really continue to call it evolution?
Shouldn't it have more of an economic or "market force" moniker instead?
As for the moths ... why isn't that just a case of the dark alleles being left as all the white moths were picked off faster and dark moths were free to fool around at leisure as trees became sootier?
That's what evolution is, so yes.
Is it actually a case of spontaneous "appearance" of some anomaly (is that the right word?) or enhanced ability to ferret out the anomaly?
Can't get anything past you. I was wondering whether somebody would notice that possibility. Yes, it may well have been there all along. That's a testable hypothesis, however: stretches of DNA mutate at a predictable rate. If you gather all the...well..."tetrachromasomes" (to coin a gratuitous portmanteau), you should be able to identify how (or whether) they are all interrelated. It should be possible to establish how long it has been in the genome. This work has not yet been done.
do you think it's really fair to attribute to evolution or natural selection the increasingly calculated moves that are aborting and/or purchasing children to meet a specific array of intelligence, colors, sex and such?
You've hit upon an interesting philosophical question. When peacocks choose each other based upon color, shape and song, we call it natural selection. If dogs chose dogs based upon color and shape, we would also call it natural selection. When people choose dogs based upon color and shape, we call it artificial selection. But what is it when people choose people according to color, shape, intelligence, whatever? Is it different from what a peacock does, because we apply reason and not mere instinct to the selection? And if we acquire different, more perspicuous ways of seeing--ultrasound, amniocentesis, polymerase chain reaction--and apply them to selection, is it a qualitatively different process? Do all human choices ipso facto qualify as artificial, unnatural selections? I'm not telling you, I'm asking you.
I mean, I believe in evolution (if not Darwinism) and I'm cool with natural selection as a means -- even among humans -- to choose mates based on aspects signifying strength and fertility and such. But given the increasingly arbitrary, atomized and FASHIONABLE (as opposed to standard and universal) criteria that are this or that hetero OR homosexual couple's, woman, man's or group's particular set of criteria for purchasing children, can we really continue to call it evolution?
Evolution it unquestionably is; Darwinism it probably is. Darwin himself, of course, never specified anything about the mechanism by which new traits arise; his theory concentrated on how they fare after they arise. "Survival of the fittest" doesn't specify what the organism is fit for. "Fit for coping in the wild" is one answer; "fit for market success" is another (witness the Granny Smith apple).
There is one implication of this new universe of selection besides which everything pales to irrelevance in comparison: the potential, nay, the inevitability of the deliberate genetic enhancement of human intelligence. This will be a runaway process: some of this new breed of geniuses will devote themselves (to so much greater effect!) to the enhancement of their children's intelligence, and so on, and so on... Human intellect will achieve a Moore's Law of its own, reinforcing and reinforced by the computational Moore's Law that has been in operation these past 150 years. To facilitate this phase transition is, more than anything else I can think of, what I would like to do with my life. It is for this reason of the utmost importance to foster genetic technologies and protect them from government interference.
I want that!
To facilitate this phase transition is, more than anything else I can think of, what I would like to do with my life.
What? And make your kids obsolete?
Is Testaverde color blind? His name means "green head" in Italian.
Sho nuff on both counts, and a bit ironic, as you say: Testaverde:
Testaverde threw 35 interceptions in 1988, his second season, and revealed afterward that he was colorblind, prompting fans to turn him into a punch line. A radio station bought a billboard near the former Tampa Stadium, painted it green and wrote: ''Vinny thinks this is red.''
I haven't found a good account on the net yet, but I recall the whole brouhaha. I also remember later efforts made to use game uniforms that he could more easily distinguish from those of the opposing teams. What I don't recall is whether his interceptions went down markedly after they started doing that.
Most colleges, of course, used to wear visitor and home team jerseys, usually light or white with team color trim, and solid team color respectively. I don't remember whether the pros always did this, but I do recall several games where, except for color, often both light, the two pro teams on the field had almost indistinguishable uniforms.
At any rate, it's entirely possible the problem never occurred for him in college.
Must have been a headache for all concerned, and fit for a Malatesta too, eh?
Best regards. S&W R.I.P.
It gave the coach agita, I'll betcha.
LOL. Tossing and turning. And the fans had fits.
Best regards. S&W R.I.P.
I assume he throws fewer interceptions for the Jets. I wonder if their basic color being green makes a difference. The home jersey would probably look very dark to him.
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