One of the great chin-scratchers of modern physical anthropology revolves around blood type, in particular why most indigenous populations of the New World have such incredibly high percentages of the gene for type O. Sometimes, especially as you move south of the modern US-Mexico border, the percentages almost reach 100%.
Since almost everyone agrees that human habitation of the New World began with migrations out of the Siberia, across the Bering Sea, and the population on the Russian Asiatic side shows no similar high percentage of type O; if anything the percentage frequency of the type O gene drops as we move further and further north and east. Several theories have been advanced to explain the apparent 'Bering Sea Bottleneck'.
The most often suggested is the genetic drift theory. The basic idea behind genetic drift is easy enough to understand. If you flip a coin two hundred times, there is a very good chance that your results will be somewhere close to 100 times coming up heads, and another 100 times coming up tails. Indeed, the more you flip a coin, the more likely (given that you have an honest coin) the results will be 50% head and 50% tails.
However, suppose that you instead only flipped the coin seven times; would it not be feasible on any given Sunday to flip five heads and two tails? Sure it is. That is how Las Vegas stays in business. Genetic drift is like that: A small population may have an uncharacteristic gene distribution simply because the genetic coin did not flip enough to have things even out.
So the Genetic Drift Theory of the 'Bering Bottleneck Type O Anomaly' posits that a small band of folks swam, walked or boated over the Bering Strait, and because their numbers were so small, the genes for A and B did not come along with the coin flip. This small number of colonist determined the future gene pool for the continent due to their exerting a 'founder effect'.
It's not a bad theory, except that in order to accomplish this, the numbers of Asian immigrants to the New World must be very small; along the order of a dozen or less, so that there is an even slight statistical chance that they could all be type O. However, even if the original colonizers of the New World numbered, say ten or eleven, the odds of those entire ten or eleven colonist being type O is about one in a thousand. Even if the number of colonists is dropped to five the odds only drop to one in thirty-two. (1) And that also assumes that there was one boatload or band of colonists, when common sense tells us that there must have been numerous attempts, though perhaps not all successful, to migrate to the New World.
The second theory is that of Natural Selection, which a lot of people equate with evolution, but it's not. Natural selection posits that perhaps a mixture of all blood types were part of the original migration, but for some reason, probably infectious disease, the type A and type B colonists died out. Of the two, Natural Selection is perhaps the stronger theory since there a definite likes between ABO type and susceptibility to small pox, syphilis, E. coli and tuberculosis, all of which probably killed lots of people back then.
However, as any honest exterminator will tell you, it's hard to kill them all.
A.E. Mourant addressed this issue in his book Blood Relations
"Like the absence of B in the Australian aborigines, the lack of B in the northern zone and of A and B in the southern zone raises a problem of world-wide importance. Was the B gene totally absent from the original populations from eastern Asia that ultimately reached Australia and America, or was the gene lost on the way? If so, was this due to genetic drift in relatively small isolated populations, or to natural selection? Early blood-group workers suggested that when man left Asia for Australia and America mutations for the A and B genes had not yet occurred. However, analogous if not identical genes occur in the higher apes at least, and so are several million years old. In the light of the discussion of O frequencies in Europe it is not difficult to see how, as a result of the elimination of A and B fetuses of O mothers, first the gene B (which is rarer than A) could have tended to disappear, and then A itself."(2)
Now, it has been know for a while (3) that human and primate ABO genes are somewhat analogous, let's just say that they are similar enough for our purposes, which is to say that the individual genes for A, B and (by default) O are 'old'. However, does it automatically lead us to assume that just because genes share a long history, does that mean we can assume that they will always exist in the percentage numbers? Of course not, we just say that with Genetic Drift: percentages change.
With apologies to Edward Tufte, let's take a look at the snazzy graphic I just did:
What you are looking at is the northeast corner of Asia and the northwest corner of North America at the Bering Strait, across and under-which one day your kids may be able to drive their cars. Not surprisingly, the colors of the map mean things: For example, the darker green the land is colored, the higher the frequency of the gene for type O; the lighter the color, the lower the percentage (less type O genes)
Now, first of all, note that these are indigenous populations, so the modern-day Alaskans and Siberians don't figure here that much here. What sticks out at you? Yup, there is lots of O gene the further east (the right side of ther map) you travel! But what else? Normally we might expect the trail of O genes to drift nicely along, but in our map the distribution is bi-modal: The incidence of O gene is higher at both ends of the map and lower in the middle. You can see that by looking at the bar graphs below, which not only looks at the relative 'percentage if each percentage' but also the percentage of land versus water: Each bar graph is actually a snapshot of one of the sixteen 'slices' of the map, the black lines.
So if anything, the more constricted that land mass became, the less you find the type O gene.
Interestingly, look at the red numbers on the map. They are the percentages of type B gene. Notice as well that the Asian side of the Strait has some of the highest percentage of Type B gene on the planet. What about on the American side?
Virtually no type B gene.
Now, to me this implies that there may well have been two waves, a 'First Wave' that contained very high percentages of type O gene and which had a relatively easy time getting across the Bering Strait (which may well have still been a land bridge) and who created the 'Founder Effect' in America, and a 'Second Wave' somewhat higher in Type A and much higher in Type B which followed but got stymied by the ecological changes and the closing of the land bridge.
So what I think is that both the Genetic Drift and the Natural Selection theories are correct, but I'm more inclined to move both of their occurrences with regard to blood type further back in time and much further west. In that case, rather than having crossed before the advent of the genes for A and B, our first American colonists would have walked across before the rest of Asia had a chance to recover from the results of its own initial 'flip of the coin.'
By which time there was no more walking there.
1. L. Luca Cavalli-Sforza. Genes People and Languages. University of California Press, 2000
2. Mourant, AE. Blood Relations, Blood Groups and Anthropology. Oxford University Press, Oxford, UK 1983.
3. Saitou N, Yamamoto F. Evolution of primate ABO blood group genes and their homologous genes. Mol Biol Evol. 1997 Apr;14(4):399-411.
A long time ago I preceptored with a naturopath who was fond of having his handouts typeset by a local printer. He was an older style ‘nature-cure’ type healer, and his handouts contained some very far out stuff. When I asked him why he went to the great expense of having a printer typeset his advice, he replied that ‘when people see something in print, especially a format that they know is not homemade, they take it more seriously.’
Twenty years later we now would appear to know better. The easy availability of laser printers and desktop publishing software can make any would-be Hemingway look the part. Of course there is a price to pay for the ubiquity of it all. Nice-looking documents have become the very essence of banality and reader confidence further eroded by the inclusion of misspellings, bad punctuation and terrible font choices.
Many readers will remember that absolute reverence by which one beheld the evening news in our childhood. Walter Cronkite and The Huntley–Brinkley Report not only acted the part of impartial newscasters; they looked it as well.
In the arts we have recently seen the emergence of a new kind of artist. The conventional record labels, having seen their profits eroded by downloading and lack of consumer interest, can only play by the numbers and hope for another Britney Spears or similar mega-mediocrity. The industry crowns artless (but safe and cute) adolescents “American Idols” when in fact they have demonstrated no skills beyond what one would expect from a decent karaoke bar singer.
Composers and musicians who actually do have something to say have opted instead to release material direct to the public, often with a payment-optional policy. Although this would appear to be financial suicide, surprisingly, many of these ventures have been economically successful.
Have been re-reading Vivian Perlis' great book Charles Ives Remembered: An Oral history. I’ve drawn much comfort from Ives over the years; certainly through his music, but also with many of the corollaries between his life and my own. Our homes are within ten miles of each other, and we both shared the benefits (and challenges) of being the sons of men who were themselves way ahead of their time.
Ives was a musical genius, anticipating the serialism of Schoenberg and many other elements of modern music, such as microtones, by many decades. Unfortunately, this placed him squarely in the path of the conventional musical minds of his time. What frustration he must have felt reading reviews of his work, where instead of seeing the horizon line of a new art, the reviewer merely saw an amateur composer who just wrote down the wrong notes!
Ives had no patience for these people. On top of one review, he simply scribbled the phrase ‘rot and worse.’ To Ives, these were just mediocre minds, steeped in the traditions of the past. Problem was, they taught in the conservatories, wrote the reviews and set the standards.
"Stop being such a God-damned sissy! Why can't you stand up before fine strong music like this and use your ears like a man?"
- At a 1931 concert when a man booed during one his friend Carl Ruggles's works
Three decades ago Steward Brand said ‘information wants to be free.’ Brand’s WELL (Whole Earth 'Lectronic Link) was a precursor of the Internet, the greatest source of unfiltered information in human history.
When information is free, people get to choose what they want to hear and read about. When it is filtered, news organizations, corporations, professional societies and political parties choose it for them.
Years ago doctors would never think of explaining their premises and motives. To whom? The village blacksmith? What does he know of chemistry? Now consumers can harness the power of the Internet to research their health issues to any depth they desire. Yet most doctors still function in filter mode, thinking that the deck is still stacked in their favor.
Doctors have to learn about everything. A patient has to just learn about what is wrong with himself. You would be surprised by the speed in which a motivated patient can become a virtual expert in their condition.
In my vision of the future we will all become our own ‘aggregators,’ selecting information sources from an abundance of highly specific and single purpose ‘channels.’ Once aggregated into our lives, all these channels will fuse into a Multiverse of realities shared between like-minded individuals.
For example, you’re currently on the ‘Peter D’Adamo Channel.’
This will not stop filtering. Evidence suggests that we all filter out information that we disagree with. In True Enough: Learning to Live in a Post-Fact Society, Farhad Manjoo cites an experiment in which smokers and non-smokers could vary the amount of interference in static filled recordings of speeches. When smokers heard a speech about smoking and cancer risk, they did not try to improve the clarity of the recording. But they did push the button to get a clearer version of the recording when a speech was playing that said that there was no link between smoking and cancer. In non-smokers the exact opposite was true.
In Filters Against Folly Garrett Hardin writes about our so-called free enterprise system:
"What is the free enterprise system? Calling the system a 'profit system' is misleading, because it is truly a 'profit-and-loss system' as far as the competitors are concerned. The general public wins because competition ensures low prices. Unfortunately, the truth is not always so simple. A comprehensive history of great business fortunes would show a disconcertingly large number that were made in a quite different way: the enterpriser devised a silent way to commonize costs while continuing to privatize the profits -- but don't tell anyone. This has been a formula for success for centuries."
Truth be told, the last few years have been a painful, if eye-opening education in the reality of rent-seeking, the corruption (intellectual, spiritual and economic) that results when learning is wedded to bureaucratic authority and income. Competing with rent-seekers can be a wearying and scarifying experience and a note like Stephan's does a lot to reassure me, a least a wee bit, that I am not some type of evil lunatic.
'Many years have you have been snubbed and even mocked, your theories debased and reviled. People seem to offhandedly wave away the world of discovery you have achieved like an odd odor in the air. It would seem that tremendous psychological forces are interacting in peoples minds when it comes to change, specifically in terms of attaining concrete understanding of health. You scare people, they are not ready for the truth.
-Stephan (comment on one of my prior blogs)
Rent-seeking can take many forms. There was the time a major manufacturer of ephedra-driven diet pills, fronted by a now-deceased somnambulist reality TV star, advised me via FAX that they had been awarded the patent for developing supplements based on blood type and unless I 'played ball' with them, they would issue a cease and desist order. Investigating the patent quickly disclosed that the source material used in their application was in fact my first book. They were, in essence, using me again me. We rolled the patent back, but only at great expense. But what about people who can't afford to fight back against the well-heeled?
Maybe I’m just a libertarian (or just an aging hippy) but I would opt for choosing my own filters --versus having information filtered for me—- especially when the filtering is being done by individuals and organizations that I do not trust and for which I have no respect.
If you rob Peter to pay Paul, you've already got half the vote.'
Growing up in Brooklyn I remember many exciting and fun filled trips to Manhattan --or as anyone from Brooklyn calls it, “The City.” One of the features I always looked forward to seeing was a huge advertisement for a paint company that featured a can of paint pouring itself over a globe of the world, its byline proclaiming “We Cover the Earth with Our Paints.”
Excepting the obvious question as to why anyone would ever want to cover the world in it, paint is not a bad metaphor for how most scientists viewed inheritance before Mendel, it being a sort of “blended essence” --a mix of the features of both mom and dad, much like how we might combine white and black paints to make gray. In the late 1800s Charles Darwin proposed a mechanism of inheritance by means of gemmules, imaginary granules or atoms which are continually being thrown off from every cell or unit, and circulate freely throughout the system. Yet Mendel’s research showed that it was nothing of the sort; being in fact much more digital, like how a computer makes all sorts of interesting stuff out of what are essentially zeros and ones. Mendel’s theory nixed that notion completely, although after a while things started to be observed that appeared to indicate that genetics wasn’t all that black and white, on and off after all, but I’ll save that for a later story.
I’ve married a blue eyed woman, and have two daughters. The first daughter has brown eyes just like me. Simple enough: My brown-eyed alleles squash my wife's blue-eyed ones. However, my second daughter has greenish-hazel eyes, much lighter than mine or her sister, but certainly not bright blue like those of my wife, so it would seem like a little blending is going on over there after all. Eye color is not a simple dominant-recessive trait, although knuckle hair and tongue rolling are. The eye color trait is what geneticists call polygenic, which simply means that it is not decided by one single gene. In order to account for my younger child’s green-hazel eyes, we have to add other factors to the mix.
My wife is pure Irish on her mother’s side and a mix of Slovakian and Hungarian on her father’s. Hungarians have the highest percentage of green eyes of any population, close to 20%, so something in my wife’s blue-eyed world (the blue-eyed allele of her Hungarian father) produced a variant that refused to role over and die, but instead made alliances with other genes --including a recently discovered one that may go back to the Neanderthals--- that slips green eyes and red hair in between things, ultimately producing my younger daughter’s wonderful green eyes. Given that, you'd think I'd get the tongue rolling gene and she the knuckle hair, but alas, the results are quite opposite.
Many traits are polygenic, and when when added to the tremendously under-appreciated epigenetic effects on gene expression, explain why we have never found a single gene for diabetes, or cancer or Alzheimer’s disease. If it were that simple, we’d have had the answers to these questions already.
Another type of inheritance is very close to my heart. The allele (the set of alternate genes for any trait) for type O blood is recessive to the alleles for type B and type A. Again using my family as an example, biologically I am type A blood and my wife is type O. My daughters are both type A blood, so we know that they must have received a type O allele from mom and a type A allele from me. Their genotype for ABO blood type is A/o (recessive alleles are usually depicted in lower case, dominant in capitals, and genetic things are usually rendered in italics).
If I was instead type B blood and had provided a type B allele, the children would have type B, as type B is dominant to type O as well.
But here is where things get interesting. What happens if you were to receive one type A allele and one type B allele? Why, you would be blood type AB! The reason behind this is that although both B and A clobber O, they strike a tentative truce between themselves and split the kingdom and declare a dual monarchy. This is called co-dominance. There are not many instances of co-dominance in genetics, and ABO inheritance is almost always given as the example.
You may well ask why, if type O is recessive to types A and B, why hasn’t it disappeared, leaving only A and B to slug it out, and eventually producing a world of only type AB people? The reasons and proofs for this are mathematical, so I won’t bore you with them, but suffice it to say that if a population is large enough, and the individuals in that population tend to mate randomly, and there are no other major influences (such as one type being more resistant to an infectious disease), after one generation the gene pool will stabilize and reach a sort of equilibrium.
Since there is such a huge amount of o allele in the human population (so much so, in fact, that even though it is the recessive allele, individuals with type O blood constitute the majority of most populations around the world) it will keep propagating itself, whereas the type you’d have though would be replacing everyone else by now, AB, comprises at best about 2% of the population.
Most people probably have a negative concept of mutation, spawned by a slew of admittedly great science fiction. However, it might surprise you to learn that that vast majority of mutations, at least the ones that get incorporated into our genetic heritage, are not lethal and often don’t do very much at all. For example, let’s again turn to our trusty blood types. As we will explore in more later on in this book, genes are chunks of DNA that do things, like code for specific proteins. Although DNA is an incredibly long molecule (if all the DNA in all your cells was unwound and placed end to end it would produce a string capable of reaching to the sun and back several times) it is composed of a simple string of four repeating nucleotides abbreviated A,T,C and G. The sequence of these four repeating nucleotides is what contains the instructions for the protein.
The difference between having the gene for type A blood or type B blood is a variation of a mere seven letters out of the total of 1,062 that make up the entire gene. We even know exactly where they differ: letters number 523, 700, 793 and 800. If you are type A blood, you have C,G,C,G in these locations, whereas if you are type B blood you have G,A,A,C there instead. Yet however slight this difference is, it is enough to cause a major problem if you were to receive the wrong blood in a transfusion. These are called point mutations because they are a simple one-letter misspelling in a gene, unless as in the case of blood type it is a consistent variation that is inheritable, in which case it is called a polymorphism.
The type O gene mutation is even more interesting. It derives from a frame shift mutation. If you are type O you may be surprised to discover that rather than having a difference of letters, like A and B, you're just missing one letter, number 258, entirely.
So hopefully by now you are comfortable with the notion that mutations are just part of life, unless of course you are unfortunate enough to have gotten a lethal one (and there are many) which probably would never have allowed you to get so far in life as to be able to read this blog. Many, if not most, of these mutations are spontaneously terminated while the sufferer is still an embryo in utero. Virtually all of the well-known genetic disorders are semi-lethal.
There are may causes of mutations, including viruses and radiation, but the most common cause is the simple fact that when our cells reproduce, they must make a complete copy of there DNA, and sometimes the copies don’t turn out so great. Think about the photocopy of that great joke that circulated around the office cubicle the other day. If it was barely legible, with bloated letters that ran one into the other, it was probably because someone made a photocopy of the original, which was quite likely a photocopy of the previous copy. Each time a copy was made of a copy, the writing was degraded a bit more.
Genes are like that. Often as we get older, we tend to get more and more of this “photocopy effect”. Perhaps what was once a word string of CAG became CAA. Even if it is copied correctly, it will be CAA from there on. Perhaps not unexpectedly these mutations are called “copying errors” and given the enormous amount of cell division that goes on over the course of a lifetime it is the real surprise is just how good of a job we do at it.
Fascinating presidential election; certainly a very unique and historic outcome. It will be interesting to see --given the perilous state of affairs we find ourselves in-- whether 2008 is also the first presidential election in which (come January) it is the winner rather than the loser who demands a recount.
Science is fact-based, but scientists can sometimes be charmingly naïve. One of the most common ways they display this naiveté is the coining of politically correct euphemisms. So, instead of the negatively charged term “race” you sometimes see the phrase “mutually inbred ancestral groups” which, at least to me, sounds even worse.
Despite the gloss, we at least now have a framework to allow us to collect and categorize those genes and polymorphisms that show different frequencies between races.
Called “Ancestry-Informative Markers” (AIM) this category of genes includes blood groups, markers of pigmentation and other SNPs that distinguish between races but don’t always result in some visually detectable difference. A collection of AIMs that distinguish African and European populations contains over 3000 highly differentiated SNPs. An example of an AIM gene is called “Duffy” and it codes for the Duffy blood group. A variant codes for a Duffy blood group type (Duffy Null allele) that is found 100% of Sub-Saharan Africans, but occurs very infrequently in other races. Interestingly, like some of the hemoglobins, this variant has been known to provide some resistance to malaria infection.
Looks like it’s time for another one of my semi-autobiographical digressions.
By the mid 1970’s I had completed the required college level classes to allow my application to a college of naturopathic medicine, since by then I had determined to follow in the footsteps of my father and enter this (at the time) obscure and curious profession. This was a time of great difficulties for this tiny healing art; more naturopaths were retiring and dying than entering the schools, and the future of the profession indeed looked rather bleak. There were tiny glimpses of hope however at least in the one remaining school, where the “Old Guard”--most often gentlemen who had learned their trade in the 1920’s and 30’s—- were giving way to “Young Turks”; aging hippies and other political rejects from the 1960’s. Unfortunately this was not at all harmonious, and at the time I was to apply we heard that the school was in uproar, as one faction or another had locked it opposite out, changed the locks, kidnapped the files –you name it.
So instead, we looked across the Atlantic, to The British College of Naturopathy and Osteopathy, and upon acceptance, I duly relocated to the “Post-Swinging London” of the late 1970’s, which as it turned out was in a rather downtrodden phase, with escalating energy prices, joblessness and at times civil unrest. This was the era of the “Urban Punk” and “Anarchy in the UK”. One only had to look around to see heart-wrenching tableaus of its more hypocritical aspects: Homeless folks sleeping against under banners proclaiming the Queen’s Silver Jubilee.
Jobs were scarce, and as a foreign student, it would have been virtually impossible to get the few that were available. I had a small stipend, and made a “few quid” doing some odd jobs. Nonetheless the dire economic circumstances forced a series of relocations, each typically one level further down the social level than the one prior. Yet these were happy times, with great friendships and new experiences, more so when I landed at the charming London neighborhood of East Finchley, a quiet suburban backwater about five miles from London City Center.
Again, as long before, a pleasant and affluent suburb, East Finchley in 1977 was the infrastructure and architectural equivalent of a visit to an eccentric, wealthy, emphysemic great-aunt. While sipping tea and hearing of the “old days” you might gaze upon the fine wood details of the hand made furniture or the anonymous faces in the dulled and dusty photographs on the wall, often in the poses of stern solidity or in an exuberant moment of victory. It would seem that only the passage of time could dull the greatness of all that past glory.
If Great Britain was at its mercantile and military zenith by the beginning of the 20th century, even more so was its pre-eminence in the rapidly growing fields of genetics, statistics and evolutionary biology. In 1890, at the pinnacle of the gilded greatness that was Victorian England, doughty old East Finchley witnessed the birth of one the greatest of her sons, a man who in the words of a one historian was a “genius who almost single-handedly created the foundations for modern statistical science.” His name was Ronald Aylmer Fisher.
The son of a successful businessman, Fisher was had a precocious intellect, and because of his poor eyesight learned mathematics without the use of paper and pen; leading to a marvelous ability to visualize problems in geometrical terms, and to forever frustrating both teachers and students by being able to produce mathematical results without setting down the intermediate steps.
Fisher published an important paper in 1918 in which he used powerful statistical tools to reconcile what had been apparent inconsistencies between Charles Darwin's ideas of natural selection and the recently rediscovered experiments of the Gregor Mendel. Among many and varied later accomplishments, it was this singular achievement that gave birth to modern evolutionary science. This was completed with the publication of The Genetical Theory of Natural Selection in 1930.
In 1943 Fisher accepted the Chair of Genetics at Cambridge University. Photographs invariably show a bearded, white haired, bespectacled man, with very thick glasses owing to his extreme myopia. More often that not, a billowing pipe accompanies the picture. He was addicted to the crossword puzzles of the London Times, which in characteristic fashion he filled in only those letters where the words crossed each other. His eccentricities, termed by his student “Fisherania,” though sometimes embarrassing, where more often the source of great entertainment to his friends.
Johann Wolfgang von Goethe wrote that “Certain flaws are necessary for the whole. It would seem strange if old friends lacked certain quirks.” Certainly Fisher had his flaws. He was an early and enthusiastic proponent of Eugenics, a social theory advocating the improvement of human hereditary traits through various forms of intervention, including sterilization, prenatal testing and screening, genetic counseling, birth control. Fisher was also opposed to the developing argument that smoking caused lung cancer, partially due to his dislike and mistrust of Puritanism and perhaps also due to the solace he had always found in his pipe.
Although he would rapidly wash his hands of the more dunderheaded students, Fisher was a inspirational mentor to his acolytes, many of who would go on to stellar careers of their own in the field of genetics, statistics and anthropology. These ranks included the previously mentioned A.E Mourant, who did work with Fisher on the epidemiology of Rh blood group genetics; Robert Race and Ruth Sanger (who my friend Gerhard Uhlenbruck once described as 'Being married-- but to other people.') themselves later on co-authored an acclaimed textbook on blood groups; A.W.F. Edwards and Luca Cavalli-Sforza, who studied the “relatedness” among various population groups.
You are a collection of cells, literally trillions of them, each with a specific design and function. With a few exceptions, cells have a basic architectural design, most of the time being depicted as looking like a fried egg cooked sunny side up. However, in reality they are three dimensional beings, so it might be better to think of the average cell as a golf ball that you’ve cut across its midline. The “white” of our cell model is the body of the cell, and here are found many specialized areas called organelles that do particular jobs, much like our own internal organs have specific jobs as well. The “yolk” of our cell model is called the nucleus, and in this compartment there lies the object of our affections, the chromosomes.
Chromosomes were first discovered at the end of the 19th century by a German biologist named Walther Flemming. Flemming was looking at cells under a microscope and got the idea to use colors to dye the cell to make it easier to see things. The idea must have worked better than anticipated since he at once began to see spaghetti looking things in the nucleus that dyed a very deep color. As is the fashion, he named these entities chromosomes which is Greek for “colored bodies”.
Chromosomes are one of the more dynamic faces of Nature; they have to be, since they are responsible for the passing on of the Baton of Life that we call reproduction. The number of chromosome in the cell nucleus differs somewhat from species to species. We human have 46 chromosomes; dogs have 78; alligators 32; cabbage plants 18.
Your chromosomes are both the governess and chauffeur of the most important molecules in your body; DNA. Like any blueprint, DNA needs to read in order for the work order to be constructed. Now, DNA is a long, long molecule. If it were completely unraveled it would be about six feet long, yet so thin that it would be invisible. If the entire DNA, in every cell of your body, was stretched out and laid end-to-end in a straight line, it would reach to the sun and back over one thousand times.
I think an effective way of describing the dynamic qualities of the chromosome is to use a few metaphors. My older daughter likes to knit, so we often visit the knitting supply shop in town for fresh yarn. Yarn usually comes wrapped in skeins, a length of yarn wound around a reel. Most yarn comes in lengths of 80-150 yards. One of the nice things about buying yarn this way, rather than just as one long unwound string, is that you can put it under your arm and walk to the car. This is certainly better than tying a knot to the rear bumper and pulled the unwound string all the way home. Thus, the first important lesion of chromosome dynamics; if you’re going to reproduce you’ve got to stuff that entire DNA into a very small, tight package. Chromosomes are just that: tight packages of DNA.
On the other hand, it is very difficult, if not downright impossible to knit anything if the skein of yarn still has the paper label wrapped around it. In order to use the yarn, you have to unwind it. That’s the formula: when the cell needs to use DNA to get information about how to make a protein, it has to unwind it. When it needs to reproduce, or turn off the DNA information flow, it needs to concentrate and condense it.
DNA is packaged and concentrated by special proteins termed histones. This concentrated DNA is called chromatin, which is the DNA plus the histones that package DNA within the cell nucleus. Chromatin structure is also relevant to DNA replication and DNA repair.
Histones are very cool bead-like proteins that spool the DNA in a way that makes it either tighter or looser, sort of like the cardboard around which our skein of yarn is wrapped. Histones respond to changes in their structure by tightening the DNA wrap or loosening it. Whenever a cell needs to access the genetic information encoded in its DNA, the histones on the section of the DNA that is needed undergo a chemical reaction called acetylation by which a molecule called an acetyl group is stuck on the histones, causing them to relax and unravel. When business is concluded for the day, special enzymes come along and chomp off the acetyl group cause the histones to become de-acetylated, which makes them tighten up again, sending the DNA in the region back to its resting state. Think of it like this; when your DNA needs to work its histones chow down on acetyl groups for breakfast and they do yoga; when it needs to reproduce or shut down, the histones lift weights --the strain of which causes the acetyl group to pop out of their mouths.
Only until recent times have we understood this mechanism, and of its supremely paramount importance: That it is used by the environment to influence gene function and that influence, for either good or bad, can be passed on as inheritance. Amazingly, we not only inherit the genes from our parents, but state of histone acetylation of the genes as well. Thus, the histone acetylation patterns of the genome are a prime mechanism of epigenetic inheritance, along with DNA methylation.
Scientists have given each human chromosome a number, according to its size; thus chromosome number 1 is the largest, then number 2, etc. Chromosomes come in pairs, one from each parent. So there are 23 pairs, for a total of 46 in us humans. Numbers 1-22 are non-sex chromosomes called autosomes, and pair 23 contains the X and Y sex chromosomes.
In the few minutes it has taken to read up to here, this, around 400 million of your red blood cells were depleted and replaced, consistent with the set of genetic instructions contained in your DNA. This is where the genetic code comes in.