It’s mentioned in this article:
http://www.newsoftheweird.com/archive/index.html
Assuming that the report was legit, of course.
Here’s the whole text, because as time goes on it’s going to get harder to find:
This seems really weird. How on earth could she be biologically unrelated to them? Wasn’t she biologically related to cells from her unborn twin sister? So wouldn’t that still make her biologically related? (Assuming of course, that one can believe the original premise in the first place.) Guess I’ll have to call up my sister and let her know that if she ever has any kids, I’ll have to deny being biologically related to them. roll eyes
Jejulife I think you’re being a little finicky about the term ‘biologically related’. The way you are using the term all humans are biologically related. In fact all organisms are biologically related. As you use it the term itself becomes oxymoronic because no one is NOT biologically related
Obviously the article is using the term in the more common context of bearing a biological relationship consistent with the social relationship. In other words she is the social mother of this child but biologically she does not appear to fill hat role. This is much as a surrogate or adopted mother would not be considered a biological mother despt being biologically related as a member of the human species.
Now that we have that sorted.
Basically Earl what happened was that this woman was conceived as one of a set of paternal (non-identical) twins. However before the embryos became very advanced they fused. The cells of both of the twins grew together to form one foetus, despite the fact that genetically the two were only as close as normal brother an sister. One of the embryos went on to produce the connective tissue cells that produced the blood systems and presumably the bones. The cells of the other embryo developed into the ovaries.
The foetus developed normally and the child grew into this woman. When she conceived only the cells form the ovaries played a part in producing the initial zygote (obviously). In the strictest sense the child is the child of only one of the twins that fused all those years ago. And yet the woman’s blood belongs exclusively to the other twin. As a result when a blood test was done the woman appeared o be no more closely related to the child than a sibling might be. That’s because her blood DNA is different to her ovarian DNA.
The process whereby to embryos fuse and go onto develop into a single organism is called chimaerism. Do a Google search on Chimaeras and chimaerism for more information.
Here is an excellent article on chimaerism, and a related phenomenon, mosaicism:
Human genetics: Dual identities
If the woman is a chimaera, then it’s highly misleading to say “she” is not related to her own children. (One could just as well say she’s not related to her own blood.) It’s simply that the cells in her body have two different genetic make-ups. All of them are part of a single woman. Note that one of the three children has the same genotype as the woman’s blood, which means that the ovaries themselves are chimaerical.
PS. I’m sure this is just a typo, but Blake means fraternal, not paternal, twins.
Colibri got it right (as usual).
It makes for good press to say the children are “not related to her”, but the “her” in question consists of different DNA in different parts of her body.
BTW, I couldn’t access the Nature article you linked to, but I’d really like to read it if you have another way of getting at it.
Not sure why you’re having trouble.
Try going to nature.com, go to “Today’s news,” and search on “chimaera.” It’s the fourth article.
Nature Science Update:
http://www.nature.com/nsu/ (Search on “chimaera.”)
Question is then: why is she related to even one them? Shouldn’t it be all or nothing?
Got it. Thanks.
That article got me thinking. I’ve always understood that conjoined (siamese) twins are always identical twins. This info would seem to imply that conjoined fraternal twins should be possible. And yet, every case of conjoined twins I’ve ever seen are clearly identical twins. Comments?
Colibri, thanks! That’s an extremely interesting article. I especially liked the speculation that this might be the cause of auto-immune diseases.
Would it be true that in adults with these conditions the two types of cells are fairly close? Otherwise, I’d think the immune system would reject the type of cell different from it. If the differences are great, mightn’t that even cause a miscarriage?
I was watching a show on twins a couple years ago (can’t remember the name, sorry) that said that depending on when during pregnancy an embryo divides into twins detemines whether you have identical twins, mirror-image twins, or conjoined twins. Fraternal twins, on the other hand, don’t come from a single embryo dividing but two embryos conceived from two eggs and two sperm.
Yes, I know that. But these chimeras are formed by the fusion of two fraternal twins. If they fuse at the level of just a few cells, it makes sense that they will form one individual. If they fuse later, they may be just like identical twins that didn’t split, eg conjoined twins.
Nope. The cells can be as unrelated as you like. The cells can even come from completely different species, as has been demonstrated by the creation o sheep/goat chimaeras.
The mammalian immune system isn’t created fully functioning. There is a prolonged period during development when the immune system is termed ‘naïve’. During this naïve period the immune system is still ‘learning’ what is self and what is non-self. Any material that the foetus is in prolonged contact with becomes recognised as self and the immune system will happily tolerate it for the rest of the organism’s life.
This naïve period, in addition to allowing for the creation of chimaeras, is very important in gene therapy. Defective cells form an organ can be removed, a functioning copy o the appropriate gene inserted and the cells re-implanted into a foetus without needing to worry that the functional gene will be creating what is technically foreign protein. The techniques for exploiting this vary. In some cases the foetus is simply exposed to the protein itself before the cells are implanted after birth.
No. With chimaerism any blend at all seems possible. It’s possible that one ovary comes from one twin and the other form another, or that part of one ovary is form a separate individual and so on. Exactly what parts of what organs come from what twin will determine the genetics of the offspring.
As I said, the use of the phrase “not related to” is highly misleading. She is equally related to both sets of children (or cells in her body are), it’s just that her blood genotype matches the genotype one child, but not that of the other two.
The cells in this woman’s body are genetically related to the same extent that siblings would be (both genotypes have the same parents, that is, her parents). Her children are related to her two genotypes exactly as they would be to an ordinary mother.
It might be pointed out that the children themselves are related to one another to the same extent as half-sibling cousins. That is, their maternal inheritance is the same as if they were offspring of two sisters, but their father is the same. Therefore they are somewhat less closely related than full siblings, but more closely related than first cousins with unrelated fathers.
This woman’s case is exactly the opposite of identical twins, in the sense that the twins are different indivduals with the same genotype, while she is a single individual with two genotypes.
Didn’t Steven King write something about this?
I read the linked Nature article. Let me get this straight: could a chimaera or a mosiac pass through a DNA screening? Do these conditions mean that two separate patches of tissue from the same person would test as being from two separate people? How about two separate blood samples from a single person who is a blood chimaera? If so, what are the implications for criminal or paternity cases that use DNA tests?
Could the answer to the conjoined twin question be that conjoined (late-splitting) identical twins are sometimes viable, while at least one of conjoined (absorbed) fraternal twins is never viable? The difference would be that one type of twin never separated, while another merged. Has there been no case of conjoined fraternal twins ever?
Yes, or at least they could.
Because the blood supply is constantly mixed the approximate genetic composition of the blood will remain constant over space and time.
In a word: tricky.
Depending on exactly what tests are done and how the two ‘twins’ were related some tests will identify the father/suspect at a lower probability level. Others will not.
For criminal tests this would be analogous to s sample contaminated with two separate person’s blood, and there are procedures for dealing with that. Provided such procedures were followed, and they wouldn’t be if the sample was not suspected of being contaminated, there shouldn’t be any problem although the results might be a surprise.
I don’t think it’s a case of viability. I think it’s more the case that fraternal twins can only become ‘entwined’ early in development before the two amniotic membranes have developed. Once the membranes are in place the two are physically separated and can’t merge. But at such an early stage there is no differentiation in the embryo. Even is a chimaera did develop into a conjoined each conjoined twin would have equal tissue from each fraternal twin. It’s not like each separate section of the twins would have unique tissues or appearance.