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. "

Comments

Another great article

[Blangopolis]

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 you can't RT{F}A

[zubernerd]

Since NPR only provides an audio link, here are some text sites with info on chimeric twins (genetic mosaics).
[Genetic Mosaics] http://users.rcn.com/jkimball.ma.ultranet/BiologyP ages/M/Mosaics.html
Google search for Genetic Mosaics
And for the non-biologists

Re:Odd

[Hentai]

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.

Re:Women already do this.

[wwest4]

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.

Re:It's not uncommon

[timbloid]

Was it this one?

Re:finally, a valid excuse

[socrates32]

Try this explanation.

And another genetic anomaly heard from

[Tsu Dho Nimh]

It doesn't take "chimerism" to get this effect. The well-known variations in the number of sex chromosomes can also do it, and they can happen in the cell division process.
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.

Re:Heehee

[killthiskid]

You're a dipshit spreading FUD.

From http://www.npr.org/about/place/corpsupport/financi als.html

NPR's annual revenues come primarily from member station dues and programming fees, contributions from private foundations, and corporate underwriting. A long-standing board policy prohibits NPR from soliciting listeners directly: on-air fund raising, direct mail, and telephone solicitations remain a prerogative of member stations.

The only direct government funding NPR receives is through competitive grants from government agencies for specific projects. Such grants are awarded by the Corporation for Public Broadcasting, the National Science Foundation, and the National Endowments for the Arts and the Humanities, and typically represent only 2% of total revenues.

There is no 'budget line' for NPR in the fedral budget.

Re:1 Car, 1 Part Source

[baz00f]

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.

Re:Hmm

[robolemon]

I think you're oversimplifying the issue and in the process coming to some false conclusions. Sure, each human has 46 chromosomes... 23 from each parent.

You wrote:

"Well every human actually has 2 complete sets of DNA. Even the Y chromosome. Only half are expressed at a time."

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.