Slashdot Mirror
Chimera Twins Story
skelley writes "Below is an audio link on this morning's story on NPR about Chimera twins, or people with two sets of DNA.
It turns out that every once in a while a set of fraternal twin eggs merge into one embryo. The resulting person has two sets of DNA.
The story says it is possible for a Chimera to have different sets of DNA in different body parts. This can cause complication for body identification, DNA typing for organ transplants, crime investigation, etc.
Researchers have no idea how common this is, but suppose that it is a reasonable percentage of all fraternal twin pregnancies, which would mean millions worldwide.
No text version. NPR often doesn't publish one.
"
Anther great aricle about Chimera twins that I was reading eariler is here. Man this stuff is really interesting. It actually says that about 8% of non-identical twins are chimera twins. That's actually pretty high.
Since NPR only provides an audio link, here are some text sites with info on chimeric twins (genetic mosaics).P ages/M/Mosaics.html
[Genetic Mosaics] http://users.rcn.com/jkimball.ma.ultranet/Biology
Google search for Genetic Mosaics
And for the non-biologists
Accentuate the positive, don't waste your mod points on the negative.
Why not try this, turn on the radio, the story should be on again shortly after 8:30 PDT.
No need to slashdot when the show is still available over the air.
My other sig is extremely clever...
Well, IANAMB (I am not a molecular biologist), but I would assume that if the blood or tissue types were incompatable, the embryo would very quickly become non-viable, and the body would take care of it in the normal way (remember - only about 15% of all inception results in birth; the rest are spontaneously aborted.)
I would imagine that the number of viable chimeric embryos is much lower than the total number of chimeric embryos; in fact, you could probably graph something like an inverse logistic curve of surviving chimeric embryos vs. days of pregnancy.
-Hentai [in vita non pacem est]
A good resource about Identical v. Fraternal twins states that fraternal twins develop when two separate eggs are fertilized and implant in the uterus. The genetic connection is no more or less the same as siblings born at separate times. They may look alike, or they may not.
Chimera twins would contain both complete sets of "sibling" DNA, which could theoretically be female and male DNA combined, let alone multiple blood types.
One might assume that multiple blood types would result in a (naturally) aborted fetus, and thus the person would not be born.
MORTAR COMBAT!
Read Stephen King's The Dark Half. :)
GET YOUR WEAPONS READY! --DR.LIGHT
they mention this in the audio. apparently chimerism can manifest itself in a hermaphroditic fashion - they mention a (mostly) male chimera who apparently had ovarian tissue.
Multiple blood types would in fact happin, but an effective blood type would exist which is simply the sum of given blood types. As all possible blood types actually occur already, no new blood types would appear in this manner.
The problem with mixing multiple blood types is that the immune system will target all foreign biological material, including unknown blood types. However, the method by which the immune system bootstraps itself involves sampling all material in the fetus, insuring that anything already present will not touch off the immune system.
In conclusion, mixed DNA does not have blood type problems.
it'll be cell-by-cell initially, although as the fetus grows the initial cells will divide and give patchyness
RE the Ford assembly issues discussed above, no problem at all unless the genomes conflict (i.e. one is saying "we're male" and the other says "we're female") - we're talking adaptive self-assembly here, the parts remake themselves as necessary. If you have a functioning immune system you're partly chimeric anyway, as (the short version) your immune cells mutate themselves randomly to be able to evolve new antibodies to new threats real time.
this concept is used all the time in making knockout mice (start with a normal mouse embryo, add some KO ES cells to get a chimera) they use cells from mice with different coat colours so when it works the mouse can be identified as it looks a bit like a Jersey cow in terms of coat patterns (but note as the next generation offspring come from single gametes they are one or the other, and so you can pick the mouse that came from your ES cells)
Was it this one?
The main thrust of the segment is about a woman who was told that her children could not be hers according to a blood typing. What was eventually discovered was that she is a Chimera, with the blood and eggs stemming from two different genetic strains (they would have been fraternal twins).
Try this explanation.
-- "Quidquid latine dictum sit, altum sonatur."
- Whatever is said in Latin sounds profound.
Presumably if you're mixing siblings you won't get stripes...
Sometimes you do
This link has some good information on Chimeras. Including a discussion of the lady featured in NPR.
g yP ages/M/Mosaics.html
http://users.rcn.com/jkimball.ma.ultranet/Biolo
You can tell a great deal about the character of a man by observing those who hate him.
Normal male = Xy (any extra "X" are abnormal, but even a XXXXy is still male - all but one X gets deactivated - but usually has serious medical problems). Normal female = XX (extra "X"s do not create supermodels, it creates medical problems)
Take a look at any calico or tortoise shell cat. What you are seeing is the result of random deactivation of one of the X chromosome early in the development of the embryo, and the random appearance of the colors (black or orange) on that chromosome. Humans have few easily testable traits that are testible for chimerism: one blood group is all I can think of at the moment, that "lives" on the X chromosome.
For calico or tortie males (yes, they exist, and no they are not valuable) the division between the colors is a good indicator of how badly screwed up their sex chromosomes are. A male that is mostly orange with one small black patch probably acts like a tomcat and will show very few cells of the XXY pattern, and might even have that abnormality limited to that spot. One that is well-mottled with black and orange is probably not interested in breeding and will show mnay more abnormal XXY cells.
In order to test this for the possibility to screw up DNA identification, they could start by testing the known chimeras - cats.
Reading the post but refusing to listen to the audio clip got me interested in searching Google for all of this. I came across one link in particular, which is very interesting:
Genetic Mosaics
The writer discusesses a tetraparental mouse and a tetragametic human.
You're a dipshit spreading FUD.
From http://www.npr.org/about/place/corpsupport/financi als.html
There is no 'budget line' for NPR in the fedral budget.
No. Most hermaphrodites have DNA for one person - not two distinct cell populations from two separately fertilized eggs.
Usually cases of "ambiguous gender" are the result of "testicular feminization", ... genetically they are XY males, but because of an inherited trait on the X chromosome from their mom, they develop physically as female ... partially or to the extent that only their gynecologist could tell the difference.
The two I remember from doing lab tests in a fertility clinic were very "female" looking. And no, we didn't say "guess what, you are really a man" when the chromosome testing came back because they aren't. The default state for humans is female unless testesterone is produced by the fetus AND the fetus responds to it.
http://www.nature.com/nsu/020429/020429-13.html
Marques Johansson
Oh, that woman. She is a "blood chimera" ... the red blood cells in her body have a population from her and one from her twin because of merged blood vessels in the placentas. This can sometimes give some misleading tests when ordinary ABO/rH testing is used. She was not a "tissue chimera", which would be an individual with mixed populations in the other tissues, such as ovaries, skin, heart, etc.
Actually, as the chimeric embryo develops, the immune system would become tolerized to ALL self-antigens present. This is part of the normal development of a young immune system. The process is called "anergy", which makes immune cells unresponsive to self antigens. Since the two selfs are merged earlier than when the immune system develops, I doubt that autoimmunity is a problem.
The immune system will tolerate any antigens that it is exposed to at an early stage of fetal development. At a point that varies with each species, the immune system appears to take an inventory of "my cells and their surface antigens" and from then on it will attack anything not recognized as "self".
I believe the idea here is the vagueness of the Unborn Victims of Violence Act. It was introduced in the Congress in 1999, with the purpose of making it a crime if an individual causes death or injury to "a member of the species homo sapiens at all stages of development," including zygotes.
The confusion here is that tetragametic woman discussed on the NPR program is the product of two fused zygotes. This creates an interesting dilemma, since two eggs are fertilized but only one child is born. If the vagueness of the UVVA where to be upheld, it could be (naively) asserted that since the pregnancy began as two "unborn" members of the human species but ended in only one, that death or injury was caused to one of the zygotes.
While such arguments are poorly constructed, as is the religously-driven legislation, it can have legal ramifications for women who have had unviable fetuses naturally aborted. Such considerations belittle the complexity of human life in return for a simple minded religious explanation. Along with the quasi-legal arguments, much "deeper" philosophical issues relating to identity and self remain unsettling.
Nonetheless, it is an extremely interesting situation which only works to make life all the more interesting.
I _am_ a biologist, though this isn't my specialty. The way the body learns to distinguish self from non-self is by defining the set of things that the immune system sees pre- and neo-natally as "self", and all else as "non-self". This means that chimerism should, theoretically, not prompt an immune response.
-NotJeff
You wrote:
Every human has two copies of each kind of chomosome, making a total of 46, which is ONE complete set! For some traits, one of the chromosomes has a dominant gene that is expressed, in many cases. However, sometimes neither gene dominates and they are both partially expressed.Also, a man has one X chromosome and one Y chromosome, not two of each. Half of his sperm cells will have a copy of the X, and the other half a copy of the Y. And a female's DNA is finalized when the sperm meets the egg in the mother! Any egg cells produced contain merely half of the 46 chromosomes already present in the zygote at conception! Since the grandfather's Y chromosome could never be present in a mother's DNA, she will never pass a Y to her children, which is why we never see a YY-gendered person.
I design user interfaces for a free network management application,
You've got it backwards. It's exposure to testosterone which makes a (human) fetus male. However IIRC the sex organs form first. So you can have your XY female caused by not responding to testosterone; she can have apparently-normal external sexual characteristics (the testes will remain inside) but she's sterile (no ovaries). She also can't legally compete in the Olympics.
I suppose a mixed-sex-chromosome chimera could come out any way -- two testes, two ovaries, one of each, and with sexual characteristics of either or mixed gender.