In this thead, Yllaria talks about “mosaic people.”
I didn’t want to hijack, so I decided to start another thread. What the heck is this? Do “mosaic people” really exist, or am I being whooshed?
Yeah, they’re called chimeras. We did a few threads on this a several months ago, but I couldn’t find any doing the search. There’s also a good article in NATURE on this, but I can’t find it either.
Basically, it’s a reverse identical twin-- two fraternal twins fuse at an early stage of develpment and the person ends up being a mosaic of two different individuals. It can even happen with twins of different genders!
No it’s perfectly true.
I’ve never heard the term mosaic person before in this context though. Mosaic people are generally women and occasionally people with genetic abnormalities. They are considered mosaics because not all the cells in the female body have the same X chromosome active thus females are mosaic of genetically different cells.
The generally accepted term for what you are talking about is chimera. If you do a Google search on chimerism/chimaerism you will get heaps of websites. Or you could search old threads for the same subject. It’s been discussed several times in the past. Basically it’s the result of two twins fusing at a very early stage.
I’ve most often heard mosaicism used to refer to people who, for example, have some cells that have the typical 46 chromosomes and other cells that contain an abnormal number, either a monosomy(45 chromosomes) or a trisomy(47 chromosomes like in Down’s syndrome). Someone with Mosaic trisomy 21 will have many of the same features as someone with full Down’s syndrome, but the degree to which they are affected depends on how many cells have the trisomy.
Here’s a cool site on mosaicism.
http://www.medgen.ubc.ca/wrobinson/mosaic/mosaicism.htm
But I think you’re talking about chimeras.
Here’s a bbc news article about them including a really cool thing called microchimerism where a woman absorbs some of her baby’s DNA into her bloodstream during pregnancy and carries it around for years afterward. Wacky.
http://news.bbc.co.uk/2/hi/health/3264467.stm
-Lil
Kinda makes you wonder how many people are fused identical twins. 
Seriously, though, bona fide human chimeras are very rare, like a handful-per-large-country rare. There was a paper in NEJM a few years ago, I think, about a hermaphrodite child who was really a XX+XY chimera, the odd result of IVF gone a bit wrong. I think one of the poor kid’s “testicles” didn’t descend, and when they looked closer, they saw an ovary and part of a female urogenital tract instead. This led to the discovery of the chimerism. And, as percypercy notes, a fetus can swap cells with its mother, and she can carry some of those cells around for years, even decades. She might even pass some of those cells on to the next child! There’s speculation these fetal cells can even be mesenchymal stem cells that could hang out in the bone marrow or other places and have regenerative properties. Now that is some weird and wild stuff.
I’ve actually worked with chimeric animals, and sometimes the blended traits are distinct enough from one place to another that the animal really does appear to have “mosaic” patterning in its tissues (it can be a fascinating thing to see), but you sometimes have to look closely, down to the microscopic level, even; and other times it’s quite obvious. It can be necessary to make chimeras, because an animal with 100% of a certain trait isn’t viable; but as a heterozygote doesn’t have the desired phenotype, a chimera is a pretty good trade-off (though the approach is relatively crude compared to other methods in common use today). It’s also a powerful tool for studying embryogenesis and patterning.
Disturbingly, there’s talk of making human-animal chimeras, even some patents on the idea, I’ve heard. There’s an ethics can-o-worms I’m glad I have nothing to do with.
I’ve read somewhere (no cite, sorry) that in this case, if the fathers are different, the second child could have TWO biological fathers DNA-wise. :eek:
I’d love the SD on that.
[urk=“http://boards.straightdope.com/sdmb/showthread.php?t=236092”]Here is the earlier thread.
What’an urk 
FYI, there was a CSI episode last year that used this as the premise of a murder mystery. The murderer knew he was a chimera and that DNA taken the usual way wouldn’t match his blood (or maybe it wouldn’t match his sperm). The CSI team knew the guilty guy was a brother of one of the suspect, but kept running out of brothers. Grissom, of course, figured it out. He noticed the guy’s skin had a wavey color pattern under certain light conditions and remembered an article he read, similar to the Nature article, that had a picture of that exact skin condition.
There’s a good thread in ATMB at the moment about ‘in jokes’ and oft-repeated phrases here on the Boards that can baffle newbies. I know we don’t want too many of these, but I fondly hope that ‘Do you have an URK for that?’ and ‘Please provide the correct URK’ etc. will catch on. URK is just too good a ‘Boardism’ to let it fade into oblivion.