posted
Possible stupid question: Can specific maternal or paternal ancestry be ascertained from DNA generations down the road? If so, is there a point, a generation, at which it becomes too diluted, so to speak, to trace and the connection is lost?
Posts: 1810 | Registered: Jun 2002
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posted
If I remember rightly, mitochondiral DNA is passed down maternally, and can be used to trace back a lineage-- and it can be done *very* far back. I think they used that as the genetic proof for the "Out of Africa" theory of how humans spread, so as far as I know, dilution is not a factor.
Hope that helps...
[This message has been edited by tamalynn (edited November 06, 2002).]
posted
Mitochondrial DNA cannot be used to pin down a specific maternal ancestor because all human mitochondrial DNA is descended from a single ancestor. It can only be used to establish in approximate terms (millenia or greater) the period of time elapsed since a common maternal ancestor (I know that I've used "common maternal ancestor" as an event rather than person in this sentence, but that is how this technique is used).
DNA can establish parentage specifically, either paternity or maternity. It can also be used with a high degree of confidence descent from a grandparent or great-grandparent, if there are not a large number of siblings in the generation targeted (if your great-grandfather had more than about four brothers, it would be very difficult to establish by DNA testing that none of them could have been your actual great-grandfather).
Eventually, it becomes possible only to establish that you have some ancestral connection with the surviving family (or with the family of the target, if the a DNA sample can be obtained from the target). Depending on how many collateral lines of descent appear, you may be able to state with a high degree of confidence that you belong to a particular line of descent, but you cannot state with certainty that a particular person was your ancestor absent precise and reliable records of marriages, births, and other relevent information.
To summarize, yes a specific ancestor can be identified two or three generations "down the road." Yes, there is a point at which the genetic relationship becomes to tenuous to identify a particular ancestor, but that point is determined by how many collateral lines cannot be eliminated as possibilities (the more siblings or cousins of the target, the less certainty can be achieved). After more than four generations, even one same sexed sibling can make it impossible to identify the ancestor genetically.
posted
sort of off topic, but hasn't recent evidence been casting doubt on the theory of a single female ancestor? i haven't been paying very close attention to this area lately, but i'm curious.
posted
This is a little complicated and I may be headed in the wrong direction, so I hope I can explain it right:
A small number of clones are made from each of many people, the idea being that once in the population pool, the clones will be less likely to meet up with their physically similar donors. The resulting clones are mated with other clones to further obscure the DNA trail.
But would it? From what Survivor wrote, I don’t think clones from the same donor would qualify as true siblings or collateral lines.
posted
The single maternal ancestor hypothesis hasn't been seriously challenged, what has come into dispute is the dating implied by previous research. "Eve" may have lived millions of years further back than has been hypothesized (this has to do with the possibility that some lineages remained similar due to paternal mitochondrial "crossover" that may theoretically occur--a similar effect can happen with some types of retroviral DNA transfer).
Okay, clones are a different matter. A true clone should be genetically indistinguishable from the original organism or any other clone of the original. But if the originals were genetically diverse, then it should be possible to tell for several generations which original the parent clones came from, unless the group of original organisms includes some sets of siblings or other genetically similar individuals. But it would be impossible--no matter how few intervening generations "diluted" the genetic signature--to ever tell by genetic testing which clone was a parent, only the original organism could be identified.
I have no idea whether that helped at all. Here is a brief example:
Let's start with six originals [*] Adam [*] Bill [*] Charlie [*] Diane [*] Ellen [*] Francine For convenience we will refer to them by letter; A, B, C, D, E, F.
We then make three clones of each, thus A1, A2, A3, and likewise till F3.
Genetic testing cannot tell any F from any other F, but can easily distinguish any F from any E, and the same holds true for each progenitor and associated clones.
We then marry up the clones, thusly: A1-D1, A2-E2, A3-F3 B1-D2, B2-E3, B3-F1 C1-D3, C2-E1, C3-F2
Now the clones start to have children. We can establish, without question, which couple produced any child in this first generation, since only the children of--for example--A1-D1 will produce children with a mix of A and D genes (this presupposes that we are certain that none of the A's are commiting adultery with any of the D's, since we cannot tell a child of, say, A2 and D3 from a child of A1-D1, except by the fact that it will be D3 rather than D1 giving birth, and there would be no way at all to tell a child of A2 and D1 from a child of A1-D1). Note that in order to tell which child is which, we are totally dependent on knowing who married who.
It gets a bit more complicated as you go along, and after a few more generations it becomes completely impossible (when exactly that occurs depends on how many original genotypes you decided to clone--in our example, after three generations everyone has the same set of great grandparents from a genetic standpoint) In any case, since the clones are genetically indistinguishable, you must rely on records of who each particular clone had children with to identify the parentage.
posted
Okay. That's why I framed my question the way I did -- my concern is the tracing of the originals. I figured that in some sense a clone, being genetically identical to its original, may as well not be in the chain at all since it would lead to the original anyway.
If I understand you correctly-- and not to belabor the point -- although you may not be able to trace parentage to a clone, you could do so with the clone's original, up to some generational point.
Also, even with original siblings, even though you may not be able to identify the exact original sibling, you could identify the original family group.
posted
Thanks, Survivor. (and others ) Besides helping with my original problem (no pun intended, believe it or not), you gave me some extra info I hadn't thought about.
From what I'm gathering from these and other readings, since the Y chromosome stays basically unchanged from father to son, and you need male descendants to trace the DNA Y chromosome, it sounds like a clone with only daughters would effectively stop the trace.
I'll have to think about this.
[This message has been edited by Kolona (edited November 08, 2002).]
posted
While I'm certainly getting out of my field of expertise, I think we've missed something. All of the following is based on the belief that from each parent, a child inherits an entire chromosome -- not parts of one chromosome and parts of the other. Is that right? If not, please ignore the following. :)
["The following" has been deleted, as my fallacy was pointed out for me. :)]
Luc
[This message has been edited by PaganQuaker (edited November 08, 2002).]
posted
okay, this may be a stupid question. i'm not very informed about genetics, but here i go....
why, Luc, would not receiving the X chromosome of your father's mother have anything to do with whether or not you received information from her other chromosomes?
posted
You do pass on whole chromosomes, but a person has 23 pairs (46 total), where only one pair contains the sex chromosomes (XX = female, XY = male). This is your standard, diploid cell.
In each pair, one chromosome came from the mother and one from the father. So they aren’t identical.
Depending how the diploid cell divides through meiosis to form the haploid egg and sperm cells with only 23 non paired chromosomes (one randomly from each pair of the diploid), you get a different mix each time. Probability favors that there is representation from both of the father’s or mother’s parents in the genetic material he or she passes on.
I’m thinking (Very vague memory from Bio here) too that chromosomes sometimes swap material across the pairs. If so that fuzzies up the picture as well.
[This message has been edited by GZ (edited November 08, 2002).]
posted
Chromosome pairs do swap genetic material at certain points when they cross over during meiosis. And I guess that the occational minor mutation would fuzz things up as well...
Posts: 35 | Registered: Dec 2001
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posted
Yes. A chromosome is a single large molecule, but the genes themselves are contained as alleles within the chromosome. Think of it like pages of a 23 page assigned essay and sentences on each page. Each page must deal with particular issues assigned in the essay, so you can swap the pages around with the corresponding pages of a similar essay and still get a complete essay. Further, you can recombine the sentances to get a new essay page.
Of course, while there are lot's of alleles, which actually code for something, there is also a lot of gibberish that doesn't code for anything, but serves as a handy way of identifying where some of the genetic material came from. Because they don't code for anything they are highly variable and can be quite unique. Some of these are introns, occuring within a single allele, and thus presumably providing identification of where a particular allele originated (although it has been theorized that introns actually serve some esoteric purpose of gene regulation and thus are not actually random).
Anyways...I guess that information isn't really helpful
posted
Hypothetical question: if three sets of non-paired chromosomes were to come into contact with eachother at the same time, with two being from the same species and the third being from a non-compatible species, what would the results be?
posted
A triploid set will usually lead to Down's syndrome or some such developmental abnormality if the extra chromosome comes from one of the parents. In a case where foreign genetic material were involved, I'd expect something more serious. Probably miscarriage early in pregnancey, or various severe congenital defects, abnormal organ development, some non-functional or possibly necrotic tissue, etc.
Or, just possibly, it would only result in a mild case of Down's Syndrome. It's hard to tell without knowing more specifics. If the foreign DNA were alien enough, it may not be activated within the cell at all, thus having no effect on development (i.e. if the chromosome came from an actual "alien" with basically different cellular mechanisms, the extra chromosome might simply be ignored).
posted
Actually, it would be easy, and we could do it today (well, it would actually take a few weeks).
The question in this case is why you would do that. If you code the chromosome so that it has no "tags" (sequences that act as hooks or keyholes or whatever metaphor you prefer), then it will never be cleaved by the cellular machinery and should never do anything (it might get in the way during meiosis, but this wouldn't harm the original organism, only progeny would be affected). Hmmmm, that's a suggestive possible use, but I'm not sure it would work. At least some of the time, the thing wouldn't get in the way, and if it did, the result should be failed meiosis, not mutant genes for the progeny. It would be a clumsey mechanism, in any case.
What exactly do you want this extra chromosome to do?
posted
Eventually, it would join with another of the same species that would be introduced into the body via a highly modified virus. This results in two separate sets of DNA within the same body. The virus would also carry the ability to encode the cells containing these dual DNA strands with the information necessary to shift from one to the other set on command, and to work with the other cells of the body to effectively metamorphose the body from one species to the other.
Hence you have a true, scientifically explainable shapeshifter. How the creature would get the massive amounts of energy needed to make this shift in a short time is another story entirely.
posted
Hold on here...I was just talking about a single extra chromosome. You're talking about an entire alternate set of genes. And switching the genetic codes in the nucleus...this would result in widespread tissue necrosis, not any change in the phenotype. Phenotype development can be directed by genes, but once your cells are where they are, doing what they do, the genes are out of the decision loop.
If you want a shapeshifter, then you need to go with an enhanced slime-mold or something of that nature, that already has the ability to physically transform from one shape (and form) to another. You would need to engineer the genetics to provide it with high strength connective and structural proteins that could be easily recycled within the organism, proteins analogous to those used in sensory organs, and some level of individual cellular sensitivity to a nervous system (which would be easier to provide seperately...actually, the whole thing would be easier to build from scratch, without using biological material, but getting the nervous system would be darn near impossible using genetic engineering).
If what you are looking for is the ability to genetically test as belonging to two or more different species, then the thing just can't be based on genetic material at all, and that's all there is to it.
I'm trying to explain, as realistically as possible, how werewolves could be created and how they would function, as a self-sustaining "race".
Their progenitors would have fantastically advanced genetic technology.
A werewolf would contain, within themself, both a wolf and a human set of DNA, between which they could transform when triggered by certain conditions (like, for example, a sudden adrenaline flow). The body would have extremely heightened capabilities for cannibalism and reformation of tissues as well as a highly efficient digestive system, and even then I imagine that the energy cost of such a transformation would be astronomical.
The idea I've been working with for how werewolves propagate and come into their own is this: a werewolf mates with a human, providing three sets of chromosomes to create a fetus. Only the two matching ones are used-- something about the werewolf's enhanced genetics must allow the body to ignore the third set but replicate it into every cell. The third set of chromosomes is ignored entirely until another, matching set is introduced from an external source at puberty. The fourth set is delivered, as I said above, by a highly modified virus, which also gives the body the information and ability to handle both DNA sets (without self destructing, I hope!) and to change between the two.
The virus itsself actually carries two sets of chromosomes, one for wolf and one for human, and being a "smart virus", it injects one and the other disintigrates along with the virus package once it has served its purpose. The newly complete werewolf then begins to manufacture the same sort of virus, composed of mixtures of its own chromosomes (much like sperm, but with that extra info), in its salivary glands. Thus other potential werewolves-- people with that third set of chromosomes waiting to be activated-- must be bitten (or even possibly just kissed), in order to become full-blown werewolves themselves.
Please, do poke holes. But also, please PLEASE offer other possible solutions. This does, after all, have to work.
posted
Pardon me for being a plebian here, but were I reading a book about werewolves, I wouldn't need every DNA "i" dotted to enjoy the story. I know we've been through this on another thread, but I can't help wondering how many good stories never make it to paper because of constricting research findings. To me, Shasta, your idea already works. Just dot the "i's" in fiction and go with it.
Posts: 1810 | Registered: Jun 2002
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posted
Okay, well, first off, drop the entire hybrid genetic angle. Wolves and humans are not genetically dissimilar enough for the type of gene expression shift you're talking about to cause anything as dramatic as a physical transformation (if you suddenly transposed all your current genes with wolf genes, it would take a couple of days, maybe weeks, before you noticed anything, and what you would notice would probably be something along the lines of flu-like symptoms followed by a progressive autoimmune dysfunction as wolf proteins began to accumulate, probably resulting in eventual death).
Since you are going with the idea that a viral component is involved anyway, what I would suggest is that the human is genetically normal, but is host to a symbiotic organism (like mitochondria, but different ) that is capable of reshaping the body to a certain degree under certain conditions (adrenaline would be an ideal trigger). It would only have to do three things. 1st, provide cellular energy, possibly by increasing the intracellular ATP levels and extracellular oxygen and glucose pathways. 2nd, it would have to act to heal damaged tissue, possibly by templating healthy cells and mass producing them while acting as or providing a synthetic connective tissue substrate. 3rd, it would have to be transmissable by saliva to other compatable human hosts (I won't bother to detail that one, most microbal organisms that live in humans can pull this off).
The first and second features would increase strength and endurance, as well as resulting in rapid growth of hair, fingernails, skin etc. (just how rapid is a matter I'll leave to you, although you might want to play with the idea that it takes a couple of days of "werewolfiness" to get the full effect of long claws and shaggy hair and bestial skin--as well as the idea that returning to human involves razor, emory board, and exfoliants liberally applied ). The third feature would result in the transmission of the symbiont via the bite of the werewolf. Sorry, but I can't do fangs plausibly, unless you want them to be permanent.
There is no real need to explain such an organism as a genetically engineered creation either (though it could be). Many microbal organisms in nature have evolved symbiotic relationships with host organisms (current theory holds that this is where mitocondria--and indeed, all the other features of the eukaryotic cell--come from). Much of your digestion relies on the assistence of microbal communities, both in your saliva and in your gut, and termites would quickly starve without the microbes that allow them to digest cellulose. A symbiotic organism of the complexity described above might well be more selective in its requirements for a particular host (the genetic compatability thing) than is usually the case, but since some specific microbes are limited to a single species or sub species, this would not be unrealistic.
But I'm not sure I understand why it has to "work" if your doing a story about werewolves, since they are an accepted convention. Unless you are planning to have a scientist (possibly a werewolf?) investigating the biological mechanism underlying the transformation as part of your storyline.
posted
I forgot to mention, the first feature of accelerating the metabolic pathways would also cause extreme hunger and an increase in body temperature. Don't know if that helps.
Posts: 8322 | Registered: Aug 1999
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posted
i love stories that view myth with a scientific mind. i once considered doing a piece about vampirism caused by a rare bacterial infection. i was thinking of something that could increase platelet production (wounds heal quicker) and consume plasma (the need for blood) among other things. i put it aside because i really don't have the expertise (nor the time) to pull it off right now.
TTFN & lol
Cosmi
[This message has been edited by Cosmi (edited November 22, 2002).]