User:NorwegianBlue/refdesk/biology

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[edit] Chromatophores

Hello Scientists. I know this isn't really the place, however, i wonder if i could get a few opinions on the chromatophore article? I requested a peer review, but have had no takers. I'm trying to get it to good or featured article status and would like the opinion of intelligent non-experts, on whether it is too technical, not technical enough or whether anything is missing. Would appreciate it any comments. Thanks. Rockpocket 06:25, 30 May 2006 (UTC)

Looks very nice! Don't see anything wrong with it. Good references, and external links. — The Mac Davis] ?? ญƛ. 07:51, 30 May 2006 (UTC)
I agree that the article is well-written, interesting, and visually pleasing. The article contains many technical terms, and it could be made more accessible to non-expert readers by making sure that the meanings of these are explained when introduced, or that there are links to pages defining them. An example: the sentence beginning with "Biochromes, such as pteridines and carotinoids...". In context, I suppose it is clear that biochromes are a subset of chromatophores, but I had to stop and think, and read the sentence a couple of times. There is a page (stub) called pteridines, a link would have been helpful if the link points to the correct molecule. Even a term such as de novo could have confused a non-expert reader, especially because the disambiguation page for de novo had a definition of de novo in the biological sense that was, in my opinion wrong ("newly synthesised", instead of "synthesised from simple building blocks". I have fixed that now.) But all in all, a very interesting and thorough article. --NorwegianBlue 19:04, 30 May 2006 (UTC)
Thanks for your comments. Actually, a 'biochrome' is defined as a 'pigment produced by an organism' (as opposed to a schemochrome, which is a colorless organic substance that reflects or refracts light). These aren't types of chromatophore per se, just different ways of generating colour. Its clear that that particular sentence can be improved, i'll work on that. Thanks again. Rockpocket 03:35, 31 May 2006 (UTC)
My pleasure. I have added some additional comments on the Chromatophore talk page. --NorwegianBlue 18:31, 31 May 2006 (UTC)

[edit] Gray Matter

Other than medicine, what kind of food can increase the amount of gray matter in our brains? Is there any exercise can increase the amount of gray matter?

Its matters more not how many neurons, it is how well they are connected. And how well you can use them. — The Mac Davis] ?? ญƛ. 00:11, 28 May 2006 (UTC)
Gray matter contains mostly cell bodies of neurons and glia; of the two, only glia are generally capable of dividing in the adult nervous system. I suspect you're more interested in the growth of new neurons, but currently this isn't possible outside of certain regions of the brain (ie, the olfactory bulb and hippocampus)—and even then the physiological significance of such growth is poorly understood. Why do you ask? --David Iberri (talk) 00:19, 28 May 2006 (UTC)

My question is what food, like lithium, can possibly increase the amount of grey matter, if we eat them? What I am looking for is somethings specific, like what kind of fruits or what king of exercises?

The brain is composed mostly of fat not gray matter. Let the first reply be your guide. Laziness can not be compensated for by increasing physical gray matter. What you need to do is to increase logical gray matter. There is a picture of a kid somewhere who got shot in the head and lost over half of his brain and yet he only had minor physical and mental incapability as a result! One of the TV documentary programs showed him being fitted with a prosthesis that was bigger than a grapefruit! The brain is like a wild cat - it may be born with physical agility and prowess and a bunch of other amazing attributes but if nothing is going on and it is sleeping all the time then those attributes only represent potential that is subject to atrophy. Put that same cat on a hot tin roof and all of those attributes will come alive and be put to good use dealing with a problem. Each time the cat is stimulated in such a way it is challenged to learn a better and faster way to deal better with a similar problem. What you need to do is look for challenges that will force upon you to the need to increase your logical gray matter. ...IMHO (Talk) 04:58, 28 May 2006 (UTC)
I very much doubt that any serious research has been done relating the amount of gray matter to nutrition. The only way to get this information would be do mri scans of a large number of people, calculate the gray matter volume (which I suspect in itself would be a difficult task), and do in-depth interviews of each person about their lifetime dietary habits. What has been done, however, is to study the correlation between nutrition and cognitive functions. There is no doubt that childhood malnutrition is related to lower scores in measurements of cognitive function. Omega-3 Essential fatty acids such as EPA and DHA are probably important. I suggest you follow this link to do a medline search. A search for "(epa or dha) and (cognitive functions)" gets some interesting hits. You might also want to try simply "nutrition and (cognitive functions)". As for exercising the gray matter, isn't that the same thing as using it? --NorwegianBlue 10:47, 28 May 2006 (UTC)

[edit] cancer treatment and research 1450-1750 (Medicine)

I am looking for information related to the prevalence, attitude, understanding, and treatment of cancer during the time period 1450-1750. I think this is known as the early modern period. Any assistance would be appreciated.

Is this homework? I sense a lack of focus. You are asking several quite disparate questions about a broad range of diseases. An in-depth treatment sounds like a major piece of work. If that is what you are doing, and the problem is that you lack experience in browsing the internet, I suggest you start right here.
Google is an excellent tool for finding starting information. Hint: use Cancer as a search word, together with a term that you would be likely to find in the article you are looking for, and unlikely to find elsewhere, such as "century". If you add "-20th -20st", you will narrow it further down. The minus signs excude articles that contain 20th and 21st. Adding "History of medicine" (including the quotes) will narrow it further down.
A quick search yielded the following:
I cannot vouch for the quality of the sources.
If this is homework, feel free to cut-and-paste, but remember:
  • your teacher may be better at googling than you are.
  • your teacher has the advantage of being able to search for uncommon words or phrases that you have used.
Cheers, --NorwegianBlue 23:37, 1 June 2006 (UTC)

Thanks for the tips on browsing the internet. You are very perceptive. This was/is part of a homework assignment. I was doing some leg work for my 14 year old son. As a nurse I found the topic of some interest also. The hospital librarian put me onto the first site you mention. Apparently thru the 1500's the most common theory as to the cause of cancer was an excess of black bile. ( the bile I have encountered is yellow ) With the discovery of the lymphatic system this belief ended. Time, ability to do autopsis and the inquiring minds of many physicians " laid the foundation for scientific oncology during the 1700's. I have added this Reference desk to my favorites. I've found it fascinating reading, and the responses of the researches often amusing.Thanks again.

[edit] Amino acids

hello,

why are L-AMINO acids and not D-amino acids used in proteins?

I have searched through google and got only this fact but not the reason......can any one help me out...plz?

This is a fact of evolution. It is an interesting exercise to try and imagine alternative biologies, which with information-carrying molecules different from DNA and RNA, and structural molecules composed of something else than the amino acids we know. And even within the constraints of DNA and RNA, with 64 codons, there could have been 63 different structural building blocks (leaving one for a stop codon). Yet there are only 20.
Amino acids turned out to be good building blocks because of the peptide bond, which links the amino acids together in proteins. Polypeptide chains fold into complex structures. In parts of the chain typical folding patterns like alpha helices and beta-pleated sheets form stable substructures ("secondary structure") within the three dimensional structure of a protein. For such substructures to be stable, all the amino acids have to be of the same type, i.e. either L or D. Therfore, in the primordial soup, nature had to make a choice. And the choice fell upon L-amino acids, possibly for no particular reason. But it had to be one or the other. --NorwegianBlue 16:38, 31 May 2006 (UTC)
Are there cases where D-Amino acids are used rarely like reverse transcriptase in certian viruses? Or am I on the wrong track?
I'm not sure about reverse transcriptase, but a medline search for "(D-amino acid)" reveals that D-amino acids are indeed used to some extent for specialized purposes. "(D-amino acid) AND (reverse transcriptase)" gave no relevant hits as far as I could see. This article suggests that D-serine may be a regulator of glia cells, and thus indirectly control the exitability of neurons --NorwegianBlue 18:56, 31 May 2006 (UTC)
My read on this enzyme in Wiki suggests it is made from normal L-AMINO acids, but has a reverse function of making DNA out of RNA instead of the normal way of making RNA out of DNA. This is necessary for viruses to hijack the cells functionality.
Since viral proteins are made by the protein synthesising machinery of the host, they would be expected to consist entirely of L-amino acids, and your interpretation of the reverse part of the enzyme's name is correct. What I thought the questioner had in mind was whether there was something funny going on with this particular enzyme, such as a posttranslational modification, but as stated, no hits in medline. --NorwegianBlue 21:42, 31 May 2006 (UTC)
Chirality is often overlooked; see thalidomide. Isopropyl 20:32, 31 May 2006 (UTC)
The link should be Chirality (chemistry). --NorwegianBlue 21:48, 31 May 2006 (UTC)
I asked this on the Chirality page, but I am sure a lot more people read this one so I will ask it here. Is it possible to have an optical isomer using isotopes? Suppose the Alanine molecule has the Methyl group replaced with Deuterium instead of Hydrogen? Would that create an optical isomer?
Yup (also answered on the Talk:Chirality (chemistry) page wher eyou asked it). The carbon atom would be tetrahedral as usual and would have four different substituents on it, thus it is an asymmetric center. One might expect the optical rotation to be small--I can't find the exact number at the moment. DMacks 21:18, 1 June 2006 (UTC)
The optical rotation is expected to be small because the chemical difference between the two is small, right?
Yeah...from a non-scientific viewpoint, one might say "well, they're both hydrogen, so why is it considered a stereocenter at all (see the initial question), and even if it is in a technical or pedantic sense, would there be any rotation at all?" Until I found the actual ref (see my response below), I only remembered that indeed it was "a small number". DMacks 16:01, 2 June 2006 (UTC)
Tetrahedron 1959 6 338-344 reports measurements of optical rotation of several isotopically-chiral molecules such as RCHDOH, finding [a]D up to ~1°. DMacks 23:08, 1 June 2006 (UTC)

[edit] No. of chromosomes in Neanderthals?

I know that it is not a well researched topic. But it'd be great if anyone could give me an idea about the number of chromosomes in the genome of Neanderthals? Thanks.nids(♂) 22:19, 28 January 2007 (UTC)

Neanderthals are now widely believed to have bred with homo sapiens, therefore they must have had a corresponding number of chromosomes, 46. Vespine 00:42, 29 January 2007 (UTC)
Widely believed? I thought this was merely a theory. Do you have some references? Clarityfiend 03:03, 29 January 2007 (UTC)
Isn't it truer to say that, if Neanderthals bred with homo sapiens then they must have had 46 chromosomes and therefore must have been human beings and therefore the distinction between Neanderthals and homo sapiens breaks down; or, if they didn't have 46 chromosomes then they were not humans and could never have bred with homo sapiens? JackofOz 03:09, 29 January 2007 (UTC)
Recent genetic work (e.g. the work of Krings [1], Bryan Sykes, and others) strongly argues that the humans and Neanderthals did not interbreed. Samples of mitochondrial DNA from well preserved Neanderthal fossils are too distinct from modern humans for the gene pools to have been mixing. Given that the two populations almost certainly had opportunities to interact, a lack of interbreeding would likely indicate that they were genetically incompatible, and one plausible (but by no means certain) explanation for that would be that Neanderthals had a different number of chromosomes from modern homo sapiens. Notably, homo sapiens have 46 chromosomes, while living great apes have 48, and that transition could have occurred after the Neanderthal population split from the pre-homo sapien line. Dragons flight 05:42, 29 January 2007 (UTC)
No it isn't, number of chromosomes isn't a unique species identifier, a domestic cat and a domestic pig has 38, doesn't mean they are the same species, nor that they can interbreed. But animals that DO interbreed do need to have the same number of chromosomes. Neanderthals had 46 chromosomes but that doesn't mean they were the same as humans, but they were probably close, it doesn't 'prove' that they bred. And I think recently it went beyond being just a theory, there was evidence found that Neanderthals could have actually just been bred into the population, not died out due to homo sapiens as previously thought. I'll try to find the sources. Vespine 05:15, 29 January 2007 (UTC)
We can have both the scenarios. i.e. Neanderthals had 48 chromosomes and they interbred with humans to produce Hybrids (they would be mostly sterile though). Also, Neanderthals may be having 46 chromosomes but still unable to interbreed with Humans. So, even if they interbred with humans, this does not prove that they had 46 chromosomes. But have you read somewhere that Neanderthals had 46 chromosomes. I am not asking for a source, just confirming if you had read that in a reliable source.nids(♂) 10:30, 29 January 2007 (UTC)
Skull suggests human-Neanderthal link. Anchoress 05:55, 29 January 2007 (UTC)
This skull, by itself, suggests nothing. For all we know, he could have been just a sterile hybrid.nids(♂) 10:11, 29 January 2007 (UTC)

Just another note. It is not required that species that breed have the same number of chromosomes (see Donkey, Horse, Mule). If the homo sapien population exploded and interacted with Neanderthals, cross breeding (and the subsequent sterile offpsring) would have wiped Neanderthals off the planet quickly and with no genetic trace. Imagine 10,000 horses and 100 donkeys. Females are almost constantly pregnant and the large majority of female donkeys would be carrying sterile offspring. Homo Sapien/Neanderthal offspring wouldn't be sterile for the same reason as mules but they may be for other reasons. (This is my own pet theory, no pun intended :) ). --Tbeatty 06:25, 29 January 2007 (UTC)

The Cell paper from Svante Pääbo's group that was linked to, studied mitochondrial DNA. Humans are but a medium in which mitochondria propagate themselves. The fact that Neanderthal mtDNA is not found in contemporary populations, only shows that there is no unbroken maternal line between contemporary humans and neanderthals. If the carriers of particular mitochondrion variants had even the slightest disadvantage compared to others, they would be less likely to have children, and those mitochondria would die out. This does not mean that the people that carried these mitochondria have no ancestors today, only that there is no unbroken chain of maternal ancestors. Consider also, that the fitness of mitochondria reasonably might depend on climate, since they are the cells' "power plants". And the neanderthals lived in a climate that was very different from today's. Therefore, to me it proves absolutely nothing that neanderthal mitochondria are extinct. I also disagree with the statement made in the Cell paper, that the observation that the time of divergence between neanderthal mitochondria and modern human mitochondria, being much longer ago than than the estimated divergence time of modern human mitochondria, argues against interbreeding. The fact that all contemporary human mitochondria stem from the mitochondria that lived in a woman some 200,000 years ago only proves that mitochondria are subjected to evolutionary pressure. Studies of the Major histocompatibility complex of primates, show that humans, chimps, orangutans and gorillas share many polymorphisms, particularly in the class II region. This argues strongly against very narrow bottlenecks in the size of the human (and ape) populations, and also shows that speciation is not an event that happens in a single individual, but in large groups which, while diverging, gradually lose the ability to interbreed. Jan Klein wrote a very interesting review on this way back in 1987 (Origin of major histocompatibility complex polymorphism: the trans-species hypothesis. Hum Immunol. 1987 Jul;19(3):155-62.). Note, incidentally, that this also shows that a difference in the number of chromosomes cannot be an absolute obstacle to interbreeding. --NorwegianBlue talk 20:22, 29 January 2007 (UTC)

[edit] Magnetic Resonance Imaging

In trying to understand how MRI's work, I discovered that the wikipedia article on the subject is a bit dense. I would like to try and rewrite the physics aspects of the article so that it might be a bit more accessible to non-physicists. But, I still don't understand the physics myself. I posted the following message to the MRI talk page, but it doesn't seem to get much traffic. The following is a disection of Wikipedia's mri article section called 'Principal'. Any and all feedback greatly appreciated:

Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water and lipids.

Does the MRI work only with hydrogen atoms? Does it work only with hydrogen atoms in water and lipids? Are there any hydrogen atoms in a human (or other things commonly scanned) which are not contained in water or lipids? (i.e., could this be changed to state simply it relies on relaxation properties of hydrogen nuclei (or perhaps any nuclei with net non-zero spin)?

When the object to be imaged is placed in a powerful, uniform magnetic field, the spins of atomic nuclei with a resulting non-zero spin have to arrange in a particular manner with the applied magnetic field according to quantum mechanics. Hydrogen atoms (= protons) have a simple spin 1/2 and therefore align either parallel or antiparallel to the magnetic field.

My understanding of spin is a little rusty; "with resulting non-zero spin" - what does the word 'resulting' mean here? The spin is created as a result of applying the magentic field? Any given proton may obtain either spin 0, 1/2 or -1/2? Is the direction of alignment (parallel/antiparallel) arbitrary or a result of some other property of the proton which will precisely determine the direction (+ or - spin)?

Common magnetic field strengths range from 0.3 to 3 T, although field strengths as high as 9.4 T are used in research scanners [2] and research instruments for animals or only small test tubes range as high as 20 T. Commercial suppliers are investing in 7 T platforms. For comparison, the Earth's magnetic field averages around 50 μT, less than 1/100,000 times the field strength of a typical MRI.

Is this paragraph appropriate/needed in the principal section?

The spin polarization determines the basic MRI signal strength. For protons, it refers to the population difference of the two energy states that are associated with the parallel and antiparallel alignment of the proton spins in the magnetic field and governed Boltzmann's statistics.

Is this saying that spin polarization is the integral difference in protons in one state versus the other? So, if I have 500,001 protons that are parallel and 500,000 protons that are antiparallel then my spin polarization is 1? What is Boltzmann's statistics and what does that have to do with the calculation of spin polarization? What is the cause for the descrepancy; is it arbitrary or is it a result of some fundemental property of protons?

In a 1.5 T magnetic field (at room temperature) this difference refers to only about one in a million nuclei since the thermal energy far exceeds the energy difference between the parallel and antiparallel states. Yet the vast quantity of nuclei in a small volume sum to produce a detectable change in field.

The thermal energy causes protons to switch between parallel and antiparallel? (which seems to argue that the state is arbitrary) If the selection of parallel and antiparallel is arbitrary than each "small volume" would statistically cancel out each other "small volume" (since that would not be helpful; seems to argue that the selection isnt arbitrary and must favor either parallel or antiparallel statistically. If so, why?) '...detectable change in magnetic field...'?

Most basic explanations of MRI will say that the nuclei align parallel or anti-parallel with the static magnetic field though, because of quantum mechanical reasons, the individual nuclei are actually set off at an angle from the direction of the static magnetic field. The bulk collection of nuclei can be partitioned into a set whose sum spin are aligned parallel and a set whose sum spin are anti-parallel.

This is describing the heisenberg effect? Each proton could never be precisely in a specific alignment, but simply has a probablity cloud of alignments which is centered along the magnetic field? If thats the case then there is a certain probablity that a given proton is precisely aligned with the magnetic field and the probablity cloud is statistically parallel with the magnetic field? Can be partitioned theoretically or is partitioned in practice during the scan?

The magnetic dipole moment of the nuclei then precesses around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor frequency.

Ok, starting to get really lost... Is this saying that similar to -- electron orbitals around a nuclei--, the proton's axis has multiple quantized "orbitals" which effect the probablity cloud of its angle of incidence with the magnetic field? Ok, all my previous theories start to breakdown when adding that the proton precesses at a specific rate. And the angle of incidence with the magnetic field is a function of energy level. Perhaps the higher level orbitals are donuts centered around the magnetic field which allows for a specific frequency, but then it seems that the 0 level orbital should still have no frequency (and should still include the magnetic axis itself). ...the proportion is nearly equal... Proportion of 0 level to 1 level? Why would it be nearly equal? Why are the protons in the level 1 state at all? Aren't there states above level 1? Larmor frequency is specific to the level 1 state or all states precess at the same frequency?

The tissue is then briefly exposed to pulses of electromagnetic energy (RF pulses) in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state. Or in other words, the steady-state equilibrium established in the static magnetic field becomes perturbed and the population difference of the two energy levels is altered. The frequency of the pulses is governed by the Larmor equation to match the required energy difference between the two spin states.

Is the EM applied from a single direction or radially? The EM waves hit the protons and move them from level 0 to level 1? How are the states measured; photons created when the proton relaxes and released in a random direction as opposed to the original direction of the source beam? What does that information tell one about the material being scanned? What is really being measured; proportion of hyrdrogen atoms at every given location? Different regions are detected by measuring intensity of photons detected bouncing randomly? I have no idea how far off from reality I am at this point, but if that's the case how does measurement work in 3D?

Anyway, I'm sure I'll have more questions and if this goes well I'd like to try and tackle the k-space section also. Aepryus 18:02, 25 January 2007 (UTC)

Hmm - that's a lot of questions. MRI is a pretty involved topic, and I don't think you'll get far without reading a lot of more basic material that makes up the technique. I'll try to answer some of the questions and reference you to pages that you could read to understand nuclear magnetic resonance better. First of all, as you can see from the MRI article, there are many different variations on MRI technology which are used to image different aspects of a sample, but I'll go over the basics. The first article you should read is quantum spin, which should clear up many of your question. You could also take a look at a somewhat poor article on spin 1/2, and at another poor article on the Bloch sphere, which is an essential theoretical tool for understanding NMR. Speaking of which, Nuclear Magnetic Resonance is the basic theory, and the article is decent - you should read the "Theory of nuclear magnetic resonance" section.
Ok. Here's the basic idea. Protons are spin 1/2 particles, meaning they are a two state quantum system. That is, you can find only two orthogonal eigenstates, and the proton will be in some superposition of the two eigenstates. For instance, / sqrt2( | z + > + | z − > ) is an equal superposition of the eigenstate in the positive z direction and the negative z direction in the basis of the z-direction eigenstates, and | x + > represents a proton only in the x+ direction eigenstate. The idea behind the Bloch sphere representation is that any superposition of eigenstates can be represented by a vector in space which represents the direction of the magnetic moment of the spin (each spin produces a magnetic field like a little bar magnet). In the absence of an external magnetic field, your choice of basis is arbitrary - there is no eigenstate with more energy than any other eigenstate. However, when you apply a magnetic field (say, in the z direction), the z+ eigenstate gains a higher energy than the z- eigenstate. It is the peculiar property of a 2-state quantum system that when it is put in a superposition of the energy eigenstates, it precesses. I am amazed that we don't have an article on spin precession - perhaps I'll write one. In the Bloch sphere geometrical representation, that means if you point the spin off axis, it will spin about the magnetic field axis at the larmor frequency. Now: the idea behind NMR. If you have a bunch of these proton spins in a magnetic field, then you zap them with a circularly polarized RF pulse at the larmor frequency, then an interesting thing happens. Imagine a small magnetic field component (the RF pulse) rotating with the precessing spins at the larmor frequency. The spin sees a small constant magnetic field perpendicular to it (the RF field in it's rotating frame), and precesses about it. The result is a spiral down the Bloch sphere which, depending on the duration of the RF pulse, can put the spin at any latitude on the sphere. Once the spin is precessing at a nonzero angle to the z-axis, it acts like a little antenna, and can be detected by placing an induction coil around the sample. Thus, you can zap the sample, then detect a response.
How do you use this for imaging? That is another very long story, and you'll have to do some reading before you'll understand the fourier space idea. -bmk

[edit] Do animals have nationality?

For example Knut (polar bear). Is it German? or Ah Meng, Sumatran Orangutan, Is it Indonesian?. Should we categorize them together with nationality? Frankedjsjs (talk) 18:30, 8 February 2008 (UTC)

Governments may opt to enforce a national ownership over animals, but the animals themselves rarely regard this enforced ownership as part of their identity. -- kainaw™ 18:34, 8 February 2008 (UTC)
Saying that Knut is German does not imply that he is a German nacional. So as you can say that Berlin is a German city, Knut is a German bear. Mr.K. (talk) 19:38, 8 February 2008 (UTC)
A pleasant instance of polysemy. German may mean, among other things, (1) "having the German nationality" and (2) "of, pertaining to, originating from Germany". These shades of meaning obviously differ from one another (or else they would not be shades). It is unlikely that a piece of Swiss cheese would be issued with a Swiss passport on application. Bessel Dekker (talk) 00:46, 9 February 2008 (UTC)
Our article states: "Nationality is a relationship between a person and their state of origin, culture, association, affiliation and/or loyalty. Nationality affords the state jurisdiction over the person, and affords the person the protection of the state. Traditionally under international law and conflict of laws principles, it is the right of each state to determine who its nationals are."
If we accept this, the question boils down to whether a non-human animal is a person or not. A closely related question has been tested in court in Austria, see here and here. The chimpanzee Hiasl was denied the right of having a legal guardian. I have not read the ruling documents, but one reason it was denied, was that acceptance of a chimp having a legal guardian could ‘create the public perception that humans with court-appointed legal guardians are at the same level as animals’. Wild animals cross borders without passports. How would you define the nationality of a migratory bird? So, IMO the answer is no, non-human animals do not have a nationality in any legal sense. --NorwegianBlue talk 12:51, 9 February 2008 (UTC)

[edit] What is the resistance of the human body?

Italic text —Preceding unsigned comment added by 59.93.198.63 (talk) 06:08, 3 February 2008 (UTC)

It's quite variable, depending on where you measure, skin condition, fat/lean content, the connections used, and other factors I can't think of offhand. This article should be illustrative. — Lomn 06:39, 3 February 2008 (UTC)
It also depends on what you're trying to resist.--Shantavira|feed me 09:43, 3 February 2008 (UTC)
Presumably electricity (electrical resistance). —Pengo 14:22, 3 February 2008 (UTC)
Maybe Shantavira meant what type of signal? I doubt skin is an ohmic conductor. Trimethylxanthine (talk) 04:28, 4 February 2008 (UTC)
1200 Ohm was published on some obscure website.--Stone (talk) 13:02, 4 February 2008 (UTC)
That is clearly wrong. Although we're not supposed to, here's some original research. I found my ohmmeter and a box of resistors. Measuring from arm to arm, after having licked my fingers to reduce resistance at the surface, gave a reading of slightly over 100 kOhm. To verify this, I checked against various resistors, the closest being 130 kOhm. With dry skin, the resistance is considerably higher, and I was unable to get a consistent reading, but it is certainly higher than 500 kOhm. --NorwegianBlue talk 14:03, 4 February 2008 (UTC)
I had the same experience when measuring with an ohmmeter, but I think that it's lower at higher voltages (if it weren't then it would be safe to touch household power supply - you need about 50 mA to die or so I learnt). BTW, I've heard rumors that someone killed himself with a 9 V battery by inserting the contacts into the veins in his right and left hand .... Icek (talk) 02:03, 6 February 2008 (UTC)
Wow! Do you have a source? Googled it with no luck. Should qualify for a Darwin Award! --NorwegianBlue talk 09:10, 6 February 2008 (UTC)
Unfortunately not - I read it somewhere on the internet a few years ago. What is the electrical conductivity of blood and lymph? With a salinity of 0.7% it's maybe about 1 S/m (seawater's is 5 S/m according to our article, but other ions like phosphate will probably make blood's conductivity larger). If the current is 50 mA, then the conductance should be 1/180 S. If the distance between the contacts is 1.5 m, and we assume equal thickness along the conductor, its cross section should measure 83 cm2, e. g. a cylinder about 5 cm in radius. At least it looks as if it could be true. Icek (talk) 14:41, 6 February 2008 (UTC)

[edit] The biggest family-tree you've ever seen...

I was kind-of a fan of the Fox series New Amsterdam (apparently I was the only one, it got canceled after 8 episodes), which was centered around a character named John Amsterdam. In 1642, this fella was a Dutch soldier who saved the life of a young native American girl (in a battle in the place that would become New York) who blessed/cursed him with immortality (he will become mortal again when he meets his true love). Unlike many other immortal people in fiction, he was able to father children. In one episode he states that he has fathered a total of 63 children. When he said that it struck me that he probably was the ancestor of a large chunk of the population in New York.

His last kid (well, latest) is called Omar and was born in 1943. If we assume that he had kids with some regularity between 1642 and 1943, that would mean he fathered a kid every four years ((1943-1642)/63=4). My question is this: approximately how many living descendants would he have? Making some reasonable assumptions, we could say that everyone of his kids had two kids at the age of 25, who themselves had two kids, and so on. I tried calculating this, but I couldn't quite nail down the sum properly (haven't done real maths in a looooong time), could you guys help me? I realize this is a fairly trivial question, but I thought it would be fun to find out. I also realize that the answer will be highly approximate, but I'm really just looking for an order of magnitude here (1,000? 10,000? 100,000? 1,000,000?). Cheers! --Oskar 19:46, 15 May 2008 (UTC)

I get \sum_{k=1}^{63} 2^{\frac{61+4k}{25}-1} \approx 27950. So that's about 30000 or so, using your formula and ignoring possible intermarriages between his descendants. —Ilmari Karonen (talk) 20:03, 15 May 2008 (UTC)
Which is, of course, just the size of the last generation, and doesn't include surviving members of the previous generations. Assuming the three latest generations are still alive on average means multiplying the figure by 1 + 1/2 + 1/4 = 1.75, giving a total of about 48912. Four surviving generations would make that 52406. That rounds to about 50000 either way, given the crudeness of the approximation we're dealing with, and I wouldn't be too confident about that 5. Or about the number of zeros either. But for a quick back-of-the-envelope calculation, which is what you were asking for, it should be within a few orders of magnitude. —Ilmari Karonen (talk) 20:08, 15 May 2008 (UTC)
One of the biggest problems with this kind of calculation is that you're assuming his descendants all marry people that aren't descendants. If two descendants get married, their two kids end up being counted twice, and if a large chunk of the population are descended from him then the chances of that happening become quite large, so you get an overestimate. Although 2 children per family is probably an underestimate, since that would involve 0 population growth. Perhaps we're lucky and it cancels out! These problems become more apparent if you go back further - consider the people that claim to be descended from Jesus. If Jesus had two children and they each had two children when they're about 25, 2000 years later you end up with about 1024 descendants alive today - 15 orders of magnitude larger than the total world population! Over a 300 year period, the effects are significantly less, but it still shows how enormously wrong such a simple model can end up being. --Tango (talk) 20:59, 15 May 2008 (UTC)
As a computational problem: One child every 4 years from 1642 to 1943 and each descendent having exactly 2 children with an unrelated spouse at their 25th birthday, I get 336,261 descendents through 2008. All the above caveats of course apply. Dragons flight (talk) 04:51, 16 May 2008 (UTC)
On the other hand, even though Tango is right in that calculating the number of descendants that way is an overestimate, it is true that many people who lived ages ago have lots of descendants because everyone has two parents. You don't have to be immortal for this. I think, if you take a human from a few thousand years ago, then most probably he or she either has no descendants or a significant proportion of the contemporary population is his or her descendant. On the other hand, if you count only male-line descendants, this is far from true, because everyone has only about two male-line ancestor in any generation. – b_jonas 07:46, 16 May 2008 (UTC)
Surely everyone has precisely one male-line ancestor in each generation? --Tango (talk) 13:18, 16 May 2008 (UTC)
I believe b_jonas means something like 'people of whom you are a male-line descendant', in which case you get two per generation, barring incest. Algebraist 13:45, 16 May 2008 (UTC)
You'd only be a male-line descendant of two people per generation if you're male: otherwise zero (well, I suppose it depends on your definition). Anyhow, b_jonas is touching on the identical ancestors point, a point in history where every person then alive is either the ancestor of everyone now alive, or of noone now alive; our article says this likely happened 5000 to 15000 years ago. (Related are most recent common ancestor, mitochondrial Eve, and Y-chromosomal Adam.) Eric. 86.153.201.165 (talk) 00:10, 17 May 2008 (UTC)
The difference in mine and Dragons flight's results appears to stem from the original poster's assertion that (1943-1642)/63 = 4, which of course isn't true (it's actually 43/9 = 4.777…). Besides, the correct divisor isn't 63 (the number of children) but 62 (the number of gaps between children), giving an average interval of about 4.8548 years. Combining this figure with the other assumptions given, I get a total number of 202,689 descendants, of which the latest generation of course makes up exactly half, or 101,344. With three surviving generations, the number of surviving descendants would then be about 177,353. —Ilmari Karonen (talk) 01:11, 19 May 2008 (UTC)