Wikipedia:Reference desk/Archives/Science/2016 March 2

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March 2

Odor of Human Feces

My question concerns human feces which have been left standing until they are dry, and maybe have even crumbled. Do such dry human feces still have an odor, and if so how does the intensity of the odor compare with the intensity of the odor of fresh human feces, and what is the reason for such a change in the intensity of the odor. Thank you.Simonschaim (talk) 08:46, 2 March 2016 (UTC)

Did the references provided in response to your recent question on the subject help your research? AllBestFaith (talk) 12:12, 2 March 2016 (UTC)

Thank you.Yes it did help. What I am asking now is a different point and would be grateful if someone could please answer it. Simonschaim (talk) 13:55, 2 March 2016 (UTC)

Our articles Paleofeces and Coprolite do not address this question directly, but both contain links and references to external articles that might. {The poster formerly known as 87.81.230.195} 185.74.232.130 (talk) 14:43, 2 March 2016 (UTC)

Thank you. However all the references seem to deal with very ancient feces, and I am interested in feces about a week old. Simonschaim (talk) 12:38, 3 March 2016 (UTC)

You may be interested in the work of Henry Moule. Rapid desiccation due to his dry earth formulation stopped microbial action very quickly and the thus no unpleasant odor issued fourth.--Aspro (talk) 19:52, 5 March 2016 (UTC)

Simonschaim, someone once pooped on the floor in my building. It was near the roof so it decomposed I don't know how long until I saw it. It was absolutely desiccated (I think there was powder it was so crumbly dry) and full of disgustingly squirming maggots. It stunk horribly. Different temperatures and humidities and amounts of ultraviolet light etc. might cause different effects. Sagittarian Milky Way (talk) 01:40, 6 March 2016 (UTC)

Thank you. Simonschaim (talk) 05:27, 6 March 2016 (UTC)

Is cross-species surrogacy possible?

I presume that the genetic difference between the embryo and the surrogate mother, present even in the case of human surrogacy, isn't problematic. My question is, therefore: can we use non-human surrogates to give birth to our babies, thus avoiding the legal and ethical issues concerning "third parent's rights"? Or vice versa: can I have kittens after having feline embryos implanted into me? --217.140.96.140 (talk) 12:23, 2 March 2016 (UTC)

No and no. No non-human mammal has a likelihood of being able to carry a human fetus through a full term pregnancy of about 40 weeks through to a vaginal Childbirth. Even human mothers risk a 1 in 160 Stillbirth rate for reasons that are often unknown. See the article Mother about this irreplaceable biological role. AllBestFaith (talk) 12:59, 2 March 2016 (UTC)

AllBestFaith, I find your answer unreferenced, purely emotional, and entirely unhelpful:

• Plenty of mammals have the natural gestation period of 40 weeks or even more;
• It's not at all obvious whether stillbirth occurrence in non-human surrogates would be higher or lower than in human surrogates;
• The article Mother doesn't have even a single sentence about the supposed irreplaceability of humans for delivering human babies.

--217.140.96.140 (talk) 07:43, 3 March 2016 (UTC)

@IP user 217.140.96.140, you also find my answer is free of charge, you may have missed 3 of the 4 references that it contains, you cannot be interested in a remotely natural gestation with its attendant biological stresses for either an unpecified animal victim (with implanted human fetus) or human (with implanted cat fetuses), and stillbirth would be the blandest description of the predictable gruesome failure of your envisaged human experimentation for which the Helsinki Declaration is a normative reference on medical ethics. If there were any reliable source that confirms that a human mother has ever delivered kittens, piglets, lambs or other non-human infant, that information might be appropriately found at Mother#Surrogate mother. You have noticed that it is obviously not. AllBestFaith (talk) 13:38, 4 March 2016 (UTC)

"The Wikipedia article doesn't mention it, so it must never have happened" is certainly a paragon of a well-referenced answer.

Symptoms and discomforts of pregnancy doesn't mention any stresses or discomforts in non-human pregnant animals; according to your own logic, it would mean there is none. --80.189.36.76 (talk) 20:42, 4 March 2016 (UTC)

Yes, Interspecific pregnancy is possible. Here [1] is an example of a house cat carrying a black-footed cat fetus. I'll leave the question of human interspecific pregnancy to others.--Wikimedes (talk) 13:54, 2 March 2016 (UTC)

Good stuff, thanks. The species problem will always be with us, but I wonder about Archaic_human_admixture_with_modern_humans. OP might consider some of these archaic humans to be non-human surrogates of human fetuses. SemanticMantis (talk) 15:14, 2 March 2016 (UTC)

I'd call archaic human admixture with modern humans an example of hybridization rather than surrogacy, as in those cases, the babies were genetically related to their birth-givers. --146.198.24.117 (talk) 16:30, 2 March 2016 (UTC)

Upcoming NASA mission to Mars

I keep reading about this would be mission. Today an astronaut who spent 340 days in a Russian capsule is coming home. What is the plan? One website said it would take 2 1/2 years for a round trip mission to Mars. So, it is a flyby. Is it a flyby? If so why do we need it? What would be an advantage versus a robot flying back and forth? The hardware has already been flying there for years. I cannot imagine they will land on Mars and spent some time there. How long? A year? What are their survival chances? NASA needs the funding keep coming in but has anybody evaluated realistic chances of success, dangers of radiation, the scientific benefits, etc. Why do we need Mars anyway? It is a dead planet. I don't think everybody understands what "dead" means. It is a fascinating subject, much more interesting than the odor of human feces somebody is inquiring about now. --AboutFace 22 (talk) 14:49, 2 March 2016 (UTC)

Last summer, this topic came up a few times. In August, I linked to Charles Bolden's talk at Keck on the roadmap for American space exploration. That video, and the NASA.gov website (which always contains a link to the present Mission Statement, the Strategic Plan, and so on) are great places to start. Nimur (talk) 15:25, 2 March 2016 (UTC)

I think the astronaut in question spend his time on the ISS, not "a Russian capsule"...but your question breaks down like this:

1. Is Mars 'dead'?
2. Is a manned mission justified?
3. Is a flyby mission needed?
4. Is it worth leaving a guy in the ISS for 340 days to collect information for that mission?

So...

• Is Mars 'dead'? -- We don't know. There could still be life there. But 'deadness' isn't the criteria for exploration. Is it of scientific interest (HOLY COW...YES!!!). It's the only planet within plausible travel time on which humans could possibly live for any reasonable amount of time - the only one where a self-sustaining colony might be possible. If we're ever going to expand our civilization beyond the rock we're sitting on - we have to go to Mars. Is there value to humanity to push ourselves off planet Earth and explore the universe? Could we eventually colonize Mars? These questions would apply just as well if Mars were "dead" or not.
• Is a manned mission justified? -- Can a man figure out stuff that a robot could not? (The common opinion is "Yes" - but it's definitely debatable) There are lots of things we need to know about this. For example, we know that very long periods in zero g is harmful to our bodies - but we have no idea whether a third of Earth-normal gravity is harmful, or 100% safe, or safe for longer periods than in zero g. Only a manned mission to Mars can get us that information.
• Is a flyby mission needed? -- Maybe. We need to test out a bunch of stuff before we commit to landing people there. I'm not so sure though. The cost of doing this is going to be pretty high - I'd vote for a bunch more robotic missions, then a manned landing.
• Is it worth leaving a guy in the ISS for 340 days to collect information for that mission? -- Yes. There have been astronauts left in orbit for this long before - but this time, it's better. The astronaut in question has a twin brother, who is also an astronaut. The twin who has been left on Earth is being used as a "control" in this experiment - and AFAIK, that's never been done before. It's a great opportunity to measure the effects of a year in zero g without other complicating factors.

SteveBaker (talk) 15:49, 2 March 2016 (UTC)

On the general 'purpose' issue, well, Mars is looking livelier than ever - there is recent evidence of actual, liquid water (in the form of brines), for example. Deep subsurface strata might well be warmer and wetter, though I don't think we have a way to see data on that. On Earth, there is life deep below the surface. Consider the commercial value of Taq polymerase, a single enzyme from an Earth organism living in a weird environment. If people find life on Mars, it may contain tens of thousands of such enzymes, that might be used in a wide variety of industrial processes. Discovering extraterrestrial life of any form also provides great philosophical insight on the universe.

But to uncover Mars life, well... there might be risks. It is easy to see advantages to leaving the bio lab on Mars and see if the astronauts start dying weird and terrible ways (or merely the plastic starts rotting or something less fatal but still economically devastating), rather than retrieving our best hope of biological samples back to this planet.

As the Sun warms, Mars warms. There is water there for some modest seas, and oxygen to be wrung from its minerals (e.g. chlorates). If it is uninhabited now, with minimal intervention, the planet might be set up to serve after the end of Earth as a place for Earth life to get some kind of a second chance to evolve something worth keeping. I'm not sure that's truly a deserved thing, yet its placement might be providential.

— Preceding unsigned comment added by Wnt (talk • contribs)

I don't think that any kind of life we might find on Mars would be likely to cause astronauts to "start dying in weird and terrible ways" - or to "start rotting plastic". Those are great science fiction tropes - but there are plenty of plastics on our many Mars rovers and landers - and there is no evidence of their plastic parts rotting in mysterious ways. Infectious diseases generally have to tailor themselves to the biochemistry and the vulnerabilities of their victims. The bugs here on Earth have co-evolved with us and our ancestor species, but Mars bugs that have never been within a million miles of a mammal would have a hard time reproducing inside our bodies. Diseases have a hard time jumping between species when our DNA is 100% compatible, and the chemical conditions inside our bodies are such a close match for theirs. To pick a simple example: How will lifeforms that evolved to live in a ph of 8.3 and zero free oxygen cope with a ph of 7.0 and a ton of free oxygen? I strongly suspect that we'd be a lot safer with hypothetical micro-organisms from Mars than the ones we have here on Earth. Put another way - how many human diseases are caused by extremophiles here on Earth? None that I'm aware of. Well, nothing is certain - but I don't think this is a big concern. SteveBaker (talk) 22:03, 2 March 2016 (UTC)

@SteveBaker: This is one of those situations where the odds are very small but the threat is very large. We don't know what kind of chemistry Mars life would have. To illustrate, consider CFCs. They haven't evolved self-replication, of course, but their impact in the atmosphere is catalytic. Free radicals can accomplish similar feats of catalytic degradation in lipid peroxidation, for example. What if there's some kind of chlorate-derived chemical assemblage on Mars that is somehow capable of using the energy of lipid oxidation to make more of itself out of oxygen and chloride and carbohydrate? Possibly its chemistry would permit it to treat carbon compounds more interchangeably than the life we know. I mean no, I would bet money against finding it, but that's only because I plan to welsh if I lose at billion-to-one odds. Wnt (talk) 11:57, 3 March 2016 (UTC)

The 'threat' is a handful of dead astronauts, the odds are quite small. This is definitely not a perfect situation, but the probability of dying in a Mars mission is already alarmingly high, there are MANY ways to die doing such dangerous things. The tiny probability of some bizarre chemistry doing this kind of thing only adds minutely to the overall risk of doing the mission. The odds of an astronaut contracting some horrible martian disease with an incubation time longer than the year it takes for the ride home, and thereby infecting all of humanity, is extremely close to zero. (That's why the people who went to the moon were only kept in isolation for a few days after they returned.) The loss of our brave Mars astronauts due to something this bizarre is a totally negligible additional risk compared to rocket explosions, equipment failure, etc, etc...and since we know that thousands of people will volunteer to go to Mars with a 100% certainty of dying up there - that seems to be an acceptable risk to those who volunteer to take the trip. SteveBaker (talk) 14:31, 3 March 2016 (UTC)

@SteveBaker: We miscommunicated somehow on this one. I was suggesting the threat of contagion as a reason why it might pay off to send guinea pigs (real or metaphorical) to Mars, as opposed to what is otherwise the saner notion of creating a fully automated facility on Mars that returns samples to Earth for analysis. Wnt (talk) 18:14, 3 March 2016 (UTC)

The idea of a robotic sample-return mission to Mars is still on the table. We've done that with samples from comets before (See: Stardust (spacecraft)), and of course we've found at Martian meteorites (pieces of Mars that got thrown into space in some collision) that end up here in considerable numbers. We've found and identified around 120 of those (including the famous Allan Hills 84001 which caused President Clinton to announce the discovery of life on Mars!). That means that there must be thousands - possibly tens of thousands of Mars rocks out there that we haven't yet found - literally tons of the stuff. The Earth isn't isolated from Martian chemistry - if there were risks from bringing back a few hundred pounds of samples (as we did from the moon), it's a bit late too worry about it! SteveBaker (talk) 19:57, 3 March 2016 (UTC)

Here is NASA's main website for the Journey to Mars, and here is their October 2015 press release outlining next steps. Nimur (talk) 16:14, 2 March 2016 (UTC)

Thank you for the links. I glanced through most of them. The impression is not encouraging. The texts are a bunch of generalities packed with long words. Human mission to Mars is impossible unless it is a flyby. --AboutFace 22 (talk) 17:57, 2 March 2016 (UTC)

Of course NASA are not the only ones with plans to go to Mars, but the others don't plan for a return trip. Richerman (talk) 18:34, 2 March 2016 (UTC)

I can't vouch for its accuracy, but this article (from Wired) seems pretty good. It summarizes some major problems with getting to Mars (and staying there), with a lot of links. -- BenRG (talk) 18:42, 2 March 2016 (UTC)

Fundamentally, people want to go to Mars because they want to go to Mars. To some people, it's a goal in itself and an inspiration. -- BenRG (talk) 18:51, 2 March 2016 (UTC)

Yep, this @Wired paper is very impressive. I feel it instantly widened my horizons. Thank you, BenRG. This is what I would like to see eventually: an inventory of things the would be martians will need on the first day, the first week, the first month and year and when such an inventory is done it will become clear that the mission to Mars is both impossible and does not make any sense. --AboutFace 22 (talk) 20:49, 2 March 2016 (UTC)

It's not impossible - that's a ridiculous claim. It's expensive, it's difficult - but it's far, far from impossible. SteveBaker (talk) 21:51, 2 March 2016 (UTC)

It would be among the most epic achievments and adventures humans have ever mastered. We (all male atleast) want to line up, stand tall, sing glory, glory, hallelujah! and feel epic. Its the only reasonable explanation. Humans are so fragile and dependent that its basically nuts to even send one into orbit instead of a robot. --Kharon (talk) 22:02, 2 March 2016 (UTC)

Nobody said it would be easy. But consider this - it's only a matter of time until some giant metor/comet splats into Earth and wipes us out as efficiently as it did the Dinosaurs. The one sure defense against that is to have a 'backup' civilization out on some other planet or moon. Mars is the natural candidate for that. If humanity plans to be around for a few hundred million years - we need to get a decent colony going somewhere other than this particular rock. That may not happen for another 50 generations - but we have to start somewhere. SteveBaker (talk) 22:11, 2 March 2016 (UTC)

This blog post (and comments) goes into the various reasons why this argument is weak. At the least, I think that no one currently advocating for a Mars colony is really motivated by this argument; if they were, they would also advocate for cheaper and easier alternatives such as self-sufficient colonies in abandoned mines or in the middle of the Rub' al Khali. Really, they want to go to Mars because it's there. -- BenRG (talk) 00:57, 3 March 2016 (UTC)

I will bet $1,000 payable in 10 years that there will be no meteorite threatening life on earth, however, I would say there is a much greater chance that once your mythical colony is established on Mars one day you will look at it through a telescope and see a nice, round crater in its place. I think NASA should add meteorite protection to the already overwhelming list of other problems to take care of. --AboutFace 22 (talk) 23:12, 2 March 2016 (UTC)

I'd make that bet too. But over 10,000 years, 10,000,000 years? Eh...not so much so. Indeed, the probability of our Mars colony being kersplatted would be pretty similar to our homeworld suffering that fate - but 2/N² is a lot smaller than 1/N when N is large - so the odds of both places being wiped out before we have a chance to re-establish our population is vastly smaller than the odds if we have just one location. Meteorite protection is actually even harder than getting men onto Mars...and even if we have it, it's not a 100% guarantee of safety. Two recently notable meteor impacts (the famous one seen and photographed across Russia and the smaller one that splashed down virtually unnoticed in the south Atlantic a couple of weeks back) should both have been spotted by existing technology - but they weren't because they came from the direction of the sun and were impossible to detect until they impacted.

It's always difficult to get people to understand that they shouldn't be too concerned about events with long odds and small consequences (I don't carry a spare tyre in my car because doing that improves fuel consumption - but most people do, even though they'll only suffer a flat while far from home maybe once in their lives, and they have a cellphone). But when the odds are high and the consequences low - or when the odds are long and the consequences are off-the-charts dangerous (such as the end of all human civilization), we should be taking action. Attempting to deflect giant rocks is one approach and Earth-2.0 is the second. Because the consequences of failure are so enormous, and neither approach is technologically easy, we should be working in both directions. SteveBaker (talk) 14:20, 3 March 2016 (UTC)

The "wired" piece exaggerates several of the difficulties. Of course it can't be one rocket taking off from Florida to Mars. It's an in-orbit assembly job, like ISS only smaller. Yes, storing hydrogen and oxygen in space for years is a dumb idea. Even for short storage, the Space Shuttle Orbital Maneuvering System and, in the more distant past, RM-81 Agena used storable propellants as do modern Orbital maneuvers. Yes, Mars dust is toxic but that's when it's dry. Recycle your shower water. Yes, the whole thing is expensive but Earth people are richer than we were half a century ago. Yes, unknown dangers are surely lurking. That identifies the problem; the world more fears the unknown than we did during the Cold War. One might say, we were crazy back then, and have become more sensible in the past half century. Or overly nervous. Jim.henderson (talk) 20:14, 3 March 2016 (UTC)

In which does the cardiac cycle begin?

Does the cardiac cycle begin in the 4th phase or in the 0 phase? It's not clear to me, because the 4th phase represent the phase before the constriction of the cells (before p wave in ECG), and the 4th stage is considered -if I'm not wrong- as the 1st part of the ECG (before p wave, and it's called isoelectric line). 93.126.95.68 (talk) 18:05, 2 March 2016 (UTC)

When any cycle begins is always a matter of opinion. Ruslik_Zero 18:08, 2 March 2016 (UTC)

Color of human feces (Melena)

Our article on melena (black tarry feces caused by gastrointestinal bleeding) could use some improvement. This is one of the places where a valuable symptom is available for anyone to self-diagnose, yet a person would not readily go to a doctor over a question of color and consistency of feces, so if we can make Wikipedia a better resource we have a genuine chance of saving a life somewhere down the line. There are some questions I really want answered by our article:

1) What exactly counts as black feces and tarry/sticky appearance? We need actual pictures, the more the better, or at least links to medically certified photos. At the very least some kind of color reference. I found a bunch of crap on the web, like YouTube videos, that unfortunately come with zero provenance and zero reliability. In general, can merely "dark brown" or "very dark brown" feces be melena, or would it be limited to those which are pitch black? When they're called "tarry" or "sticky", does that mean any feces that adheres to the toilet and won't readily flush, or something different?

2) Can melena be chronic and consistent without noticeable anemia symptoms? I read that it takes 100-200 ml of blood to cause melena, which I don't think the body can consistently put out on a daily basis without ill effect, but I don't actually know that. With conditions like peptic ulcer or duodenal ulcer or (see below) colon cancer, is melena something continuous that gets gradually worse over time, or does it come in episodes?

3) Can melena originate in the ascending colon? Our article says yes, but according to the Merck Consumer Version "The black color of melena is caused by blood that has been exposed for several hours to stomach acid and enzymes and to bacteria that normally reside in the large intestine." [2] Yet their professional version [3] agrees with the article that right colon bleeding can also be the cause. Is there some distinction in appearance between melena that comes from the stomach vs. ascending colon? Or is the consumer version just wrong to mention stomach acid?

4) I see inconsistent indications online, so: how urgent is melena? In the absence of other symptoms, do people need to get immediate medical help, or can they see if it goes away or wait until a later checkup?

Wnt (talk) 18:29, 2 March 2016 (UTC)

Regarding "if we can make Wikipedia a better resource we have a genuine chance of saving a life somewhere down the line": That is expressly not a goal of Wikipedia, indeed it is the exact OPPOSITE of the goal of Wikipedia. See Wikipedia:Medical disclaimer. People are encouraged to AVOID using Wikipedia to self-diagnose their medical problems, and editors are encouraged to AVOID writing articles with a tone or style that would lead to such use. --Jayron32 18:59, 2 March 2016 (UTC)

Ok fine, then let's just ignore intent, and then Wnt's questions are valid and improvement of articles is fine use of the ref desks. Actually, upon further reflection, I don't think question 4) is something that our article or reference desk can handle in general, and of course anything specific to a certain case is also right out. By analogy, some cuts are very urgent, and some can wait indefinitely, and we can't say whether a cut is urgent. SemanticMantis (talk) 20:28, 2 March 2016 (UTC)

I thought using the ref desks as a place to request help for wikipedia article improvement was discouraged. I think there's other more appropriate places specifically for that, like the article's talk page. Vespine (talk) 22:23, 2 March 2016 (UTC)

Must...answer...question....' 02:40, 3 March 2016 (UTC)

... and anyway melena is a sign not a symptom. Richard Avery (talk) 07:48, 3 March 2016 (UTC)

True ... unless the patient flushes his toilet rather than summoning a doctor to his home. The policy discussion explains why comedians tend to bypass Wikipedia as a target ... how could their art improve it? Wnt (talk) 11:46, 3 March 2016 (UTC)

The policy is there for a good reason. If you want to find reliable online information about about a medical problem you should go to a website written by medical professionals - not an encyclopedia that can be edited by anyone. Richerman (talk) 23:57, 3 March 2016 (UTC)

It disturbs me that people have no problem with having a bad article, but object philosophically to the notion of making it better. Wnt (talk) 00:41, 4 March 2016 (UTC)

We should have a better article. Better does not mean "teaches people to diagnose and treat their own diseases instead of seeing a medical professional". --Jayron32 13:48, 4 March 2016 (UTC)

Yet people do. Have you ever gone to a doctor and had him or her ask the color of your feces? You might get a diagnostic test aimed at colon cancer or some other conditions that cause it, but the symptom itself has no use, unless individual people use it. We can't provide a true diagnostic how-to guide, but we could provide information useful for self-diagnosis. Wnt (talk) 10:54, 5 March 2016 (UTC)

Sex chromosomes are found in somatic cells as in the sex cells?

Are there 23 chromosomes in each somatic cell of the human body? (22 pairs of autosomes and 1 pair of allosome). If there are, that says that in each cell has a sex chromosome. and my question what is the purpose of being a sex chromosome for example in a myocyte (muscular cell) of the biceps or in epithelial cell of the lung which are never take place in human sex action (or: Sexual intercourse) 93.126.95.68 (talk) 23:09, 2 March 2016 (UTC)

There are 23 *pairs* of chromosomes in the somatic cells of the human body, or 46 chromosomes per cell. (There are exceptions, but this is the general case). Thus everyone, male or female, will have at least one X chromosome in each cell. The X chromosome contains about 2000 genes, and most of these have nothing to do with sex determination. To address your broader question, genes don't occur only in the cells they directly affect, they appear in each cell. - Nunh-huh 23:20, 2 March 2016 (UTC)

Thank you. You wrote: "genes don't occur only in the cells they directly affect, they appear in each cell." Is there any reasonable reason for this fact? 93.126.95.68 (talk) 00:09, 3 March 2016 (UTC)

It seems eminently reasonable to me without further explanation. Genes in general have rather broad effects not limited to one specific cell or even kind of cell, and nature makes no fetish of tidiness. We still carry a non-functional gene that would, if functional, allow us to produce Vitamin C rather than rely on getting it in our diets. Duplicated genes and those coding for non-functional proteins may provide the substrate for useful mutations. If we don't remove this junk DNA, why would you think we would remove genetic information at the gene level on a cell-by-cell basis? - Nunh-huh 00:39, 3 March 2016 (UTC)

Actually my question is bigger than you got me. I'm asking about two entire chromosomes (pair) that found in places they should not be apparently, their place is gonad (testis or ovary). it's not a single gene or something like that, it's a complete set of sex. I begun to think that it's just a historic result of the zygote which all the cells are made of it. What is your opinion about this idea? 93.126.95.68 (talk) 00:47, 3 March 2016 (UTC)

Your basic premise—that sex chromosomes contain genetic information that only effect gonads—is simply wrong. You are letting the way they are named affect how you are thinking about them, instead of acknowledging the fact that they contain genes that affect both somatic and germ cells. - Nunh-huh 01:05, 3 March 2016 (UTC)

To further elaborate, in therians (which includes humans), the presence or absence of the Y chromosome is what drives sex determination. The SRY gene located on the Y chromosome triggers development of a fetus into a male when expressed. Female is the "default" phenotype for therians. The X chromosome doesn't "do" anything to produce the female phenotype. To produce male phenotype, SRY "switches" the fetus to developing into a male. The X chromosome contains many genes that have nothing specifically to do with reproduction, and that are essential for the functioning of most cells. We're pretty sure the X and Y chromosomes are descended from a pair of autosomes that were just like any other chromosome pair. Somehow the predecessor of the SRY gene developed on one of the pair, which became the Y chromosome. Over time, genes not essential to maleness migrated to the X chromosome. See XY sex-determination system for some more details. --71.119.131.184 (talk) 01:39, 3 March 2016 (UTC)

The classic example of non-sexual genetics linked to the sex chromosomes is color blindness; one of the most common genes to cause color blindness is a recessive gene on the X-chromosome. In women, with two X-chromosomes, only one of the dominant genes (which allows proper color vision) on either of the chromosomes is necessary. In men, with only one X-chromosome, doesn't have the opportunity (if they have the recessive gene on that chromosome) to have a dominant gene suppress their color blindness. That's why color blindness (at least some forms) predominantly effects men: about 8% of men have some form of color blindness, for women it's only about 0.5 %. --Jayron32 11:38, 3 March 2016 (UTC)

And you can demonstrate this result very easily with some simple arithmetic: If 8% of women have a color vision gene defect on the X chromosome they got from their mothers (same probability as with men) and 8% of those have colorblind fathers and therefore got the same defect on their other X chromosome - then the number of women who got two bad genes is 8% of 8% - which is (0.08x0.08)x100% = 0.64%, which is very close to the 0.5% commonly quoted. SteveBaker (talk) 13:58, 3 March 2016 (UTC)