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Biotech Science

Semi-Identical Twins Discovered 224

Posted by kdawson
from the my-brother's-a-keeper dept.
daftna writes in with a story from Nature about a pair of twins who are neither identical nor fraternal: they are semi-identical. Researchers discovered twins who share all of their mother's DNA but only half of their father's. Both children are chimeras — their cells are not genetically uniform, but include a mix of genes from two separate sperm cells that fertilized a single egg. This is, apparently, not as rare as one might think; but the resulting fetus is rarely viable. This report marks the first known incidence of two half-identical twins resulting from a double fertilization.
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Semi-Identical Twins Discovered

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  • by good soldier svejk (571730) on Tuesday March 27, 2007 @08:38PM (#18509953)
    Chimerism is also a source of the exceedingly rare brindle coat pattern in horses. [aol.com] In such cases the different color hairs will have different DNA. In one case this caused two consecutive DNA sample sent to a lab for pedigree verification to return negative parentage for both the sire and dam, [209.85.165.104] even though the owner had personally witnessed both the fertilization and the birth and hence knew for sure who the foal's parents were. DNA from the stallion's blood samples also showed no evidence of a Y chromosome.
  • Re:Serious question (Score:5, Informative)

    by reverseengineer (580922) on Tuesday March 27, 2007 @09:03PM (#18510185)
    Well, this case is primarily interesting in the sense of "this thing that very rarely happens, it just happened." The main scientific benefit is that further light is shed on the mechanisms of human reproduction. Obviously, the outlines of that process are well-known, but there's still a lot of uncharted territory when it comes to the non-normal functioning on this process. Reproductive biology is an area where animal models (even in other primates) tend to translate rather poorly to human beings, and is of course also an area with ethical limits on human experimentation. Conceivably, learning about cases like this can advance knowledge about things like infertility and birth defects.

    There's actually an interesting story, almost the flip side of this rare case in humans, running now in the New York Times about marmosets [nytimes.com], in which a form of chimerism is quite widespread:

    One of the most surprising results of the study is that over half of male marmosets have chimeric sperm. Dr. Ross and her colleagues discovered cases in which the DNA of male marmosets turned up in babies supposedly fathered by their fraternal twins. In other words, the sperm came from one male, but it had the DNA of the male's brother. A paternity test would show that the baby's genetic father was actually its uncle. The scientists were not able to isolate DNA from marmoset eggs, but they did find that 2 out of 21 marmoset ovaries were chimeric. It's possible that a female marmoset can give birth to nephews and nieces.
  • by wile_e_wonka (934864) on Tuesday March 27, 2007 @09:05PM (#18510207)
    I thought I would note why this doesn't happen all the time in humans (in some mammals it is common for an egg to be able to be penetrated by more than one sperm).

    According to my anatomy textbook, after the spem digests its way through the zona pellucida:

    The plasma membranes of the sperm and oocyte then fuse, and the sperm nucleus is engulfed by the oocyte's cytoplasm. This fusion induces the cortical reaction, wherin granules in the oocyte secrete enzymes into the extracellular space beneath the zona pellucida. These enzymes destroy the sperm receptors on the zona pellucida, preventing any other sperm from binding to and entering the egg.

  • by Grishnakh (216268) on Tuesday March 27, 2007 @09:09PM (#18510233)
    This is incorrect. From TFA:

    The 'semi-identical' twins are the result of two sperm cells fusing with a single egg -- a previously unreported way for twins to come about, say the team that made the finding. The twins are chimaeras, meaning that their cells are not genetically uniform. Each sperm has contributed genes to each child.


    According to this, in these chimaera-twins, each sperm contributes genes to each child. So if the sperm were from different fathers, the resulting twins would each really have two fathers, sharing traits from both (and the mother). It'd be interesting if some twins were created this way, with the three parents all of different races.
  • by brunes69 (86786) <slashdot&keirstead,org> on Tuesday March 27, 2007 @09:26PM (#18510343) Homepage
    I'll vouch for the parent, I saw the same show. But I think it was on TLC.
  • by DavidTC (10147) <[slas45dxsvadiv. ... ] [neverbox.com]> on Tuesday March 27, 2007 @10:14PM (#18510617) Homepage

    I don't even want to imagine the scenario that would be required to get two sperm from two different fathers to fertilize one egg at the same time.

    From what I remember from biology, almost all fertilization often happens when the egg drops into place, and the sperm is just hanging around already there. The 'sperm swims to the egg' is a fairly large over-simplification of the process.

    This is how the morning after pill works, it stops the release of the egg so it won't hit the sperm waiting for it, or at least screws the release up long enough that the sperm dies. (Sperm lasts like 24 hours inside a woman, IIRC.)

    So one egg being fertilized from two fathers is not that amazing, timewise. The egg (Or two eggs, or three, or whatever) shows up, and there are two men's sperm waiting for it. Considering the low amount of two sperm fertilizing one egg, the odds of them every happening with two fathers is improbable, but plenty of fraternal twins have different fathers. (Well, 'plenty' is probably overstating it, but it's happened enough that it's not even that neat a trick.)

    Incidentally, there's some study out that that demonstrated that men can sometimes produce sperm that is a good deal less viable, and the main purpose of it appears to be entangling other men's sperm, stopping it from reaching any eggs. And that men who are suspicious of affairs, or just know their lover has other lovers, are more likely to produce said combatant sperm.

    The real amazing trick here is two chimera twins. Human chimeras are fairly unlikely to be healthy, as the two parts often react in bad ways. Although, now that I think about it, if one survived, that logically means the combination 'worked', and thus the other surviving is not that impressive.

    I'm confused as to why they're calling these 'semi-identical' twins, though. They'd be semi-identical if each of them contained a single genetic code that considered of the same part from the mother but with each having a different part from the father. However, they both contain the same genetic codes, so they are, indeed, identical twins. They just have 1.5 times the genetic code of anyone else, with cells in their body randomly containing one code or another.

    What they really are are two semi-chimeras. Parts of their body have half the genetic code differing, instead of all their genetic code differing like normal chimeras. Which is, I guess, why they managed to live. But I seem to recall that a lot of the surviving human chimeras are like this. So they're just unique in being a pair.

    I guess they're 'semi-identical' in the sense that different parts of their bodies might contain different genetic codes. One twin's heart might contain code A, and the other code B, whereas they both have code A eyes and code B hair color.

  • Already happened. (Score:5, Informative)

    by gcnaddict (841664) on Tuesday March 27, 2007 @10:23PM (#18510665)
    It just hasn't happened in humans yet.

    http://news.bbc.co.uk/2/hi/science/nature/6196225. stm [bbc.co.uk]
  • by NIckGorton (974753) on Tuesday March 27, 2007 @11:22PM (#18511041)
    Intersex is not the same thing as a hermaphrodite. All hermaphrodites are intersex, but the vast majority of intersex people are not hermaphrodites. To be a human hermaphrodite, you have to have ovarian and testicular tissue in the same person. Most people who are intersex have only ovarian OR testicular tissue. Instead of having both types of gonads there is some problem with sexual development in the womb that results in a person with physical characteristics somewhere on the spectrum between the poles of male and female bodies. (Hence the newer term for intersex conditions: DSD or disorder of sex development.)

    More importantly, if you call a person who is intersex a hermaphrodite many will likely be quite unhappy with you. Its akin to calling a Native American an 'Indian' - not only generally disliked by the people you are labeling but also factually incorrect because of a misunderstanding of what the term means (or on what continent you are located.)

    Nick
  • by kestasjk (933987) * on Tuesday March 27, 2007 @11:33PM (#18511127) Homepage
    Are you calling the virgin Mary a hermaphrodite?! Blasphemy!

    As we probably all know 'virgin' was indisputably a mistranslation; the Hebrew for 'young woman' (almah) was translated into the Greek for 'virgin' (parthenos). I wonder why we all still refer to her as the virgin Mary, now that we know she wasn't (necessarily, to be absolutely pedantic) a virgin.
  • by JeanPaulBob (585149) on Wednesday March 28, 2007 @01:20AM (#18511735)
    No, "virgin" was not "indisputably" a mistranslation. And it is most certain that your "young woman" is a poor translation--to my mind, worse than "virgin".

    The issue is not with the New Testament; there is no question that Mary is reported to have been a virgin in the New Testament. The issue originates with the Septuagint, the Greek translation of the Old Testament produced by Jews before the time of Christ, and it has to do with the translation of Isaiah 7:14, an ostensibly Messianic prophecy. (The Septuagint was the translation used by New Testament writers.) It translates the Hebrew "almah" with the Greek "parthenos". Parthenos almost always means "virgin", while almah has a slightly different but overlapping semantic range. It's closer to "maid, unmarried girl, young woman of marriageable age". (Your "young woman" leaves out the unmarried/of marriable age implications.) Culturally speaking, an almah most likely would be a virgin--that would be the strong expectation, and it's enough to make "virgin" a connotation of "almah". While parthenos is not a precise translation, it is not a mistranslation. At the very least, not indisputably so. My goodness, man, just read the Wikipedia entry on almah and follow the references! This is not obscure information.

    Sure, if you limit the meaning of "almah" to "young woman", it makes for a better game of "Hee hee, look at the silly Christians," but if you're interested in honest scholarship, you'll have to open your mind a bit.
  • by niktemadur (793971) on Wednesday March 28, 2007 @07:40AM (#18513421)
    Similar (but not, ahem, identical) cases have popped up before.

    I remember reading a long time ago, in one of the Wallace/Wallechinski Book Of Lists, an article about a case in the seventies in Germany, more mundane in that two eggs were fertilized, so that the kids were not chimeras, but extremely weird in outcome, as the woman gave birth to two boys, one fully african and the other fully caucasian!

    I believe it's the only recorded case of two simultaneous fertilizations from different ethnic gene pools, but that has more to do with the woman having sex with two men on a day she ovulated twice. So it's not as deeply unique as the case in TFA, but it must have been quite a spectacular sight.

    I would have loved to have seen the faces of the delivery room staff when the second kid was coming out. In fact, when mommy was presented with her newborns, she fainted in shock. I don't know if she was married, but if she was, her first thought when coming round must have been, however it's said in german, "I've got some explaining to do".

    Anyway, my point is that at least in Germany at least, there's a civil precedent for two simultaneous daddies.

IF I HAD A MINE SHAFT, I don't think I would just abandon it. There's got to be a better way. -- Jack Handley, The New Mexican, 1988.

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