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    semi-identical twins?

    <DIV>Semi-identical twins discovered

    Twins normally share either half or all their genes
    Scientists have revealed details of the world's only known case of "semi-identical" twins.
    The journal Nature says the twins are identical on their mother's side, but share only half their genes on their father's side. </DIV>
    <DIV></DIV>
    <DIV>They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos - and each sperm contributed genes to each child. </DIV>
    <DIV></DIV>
    <DIV>Each stage is unlikely, and scientists believe the twins are probably unique. </DIV>
    <DIV></DIV>
    <DIV> Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say </DIV>
    <DIV></DIV>
    <DIV>David Bonthron, University of Leeds </DIV>
    <DIV></DIV>
    <DIV>These twins were born in the US, but neither their identity or their exact location is being revealed. </DIV>
    <DIV></DIV>
    <DIV>Their case is also reported in the journal Human Genetics. </DIV>
    <DIV></DIV>
    <DIV>Normally, twins either develop from the same egg which later splits to form identical twins - who share all their genetic material, or from two separate eggs which are fertilised by two separate sperm. </DIV>
    <DIV></DIV>
    <DIV>This creates non-identical (fraternal) twins - who share on average 50% of their genetic material. </DIV>
    <DIV></DIV>
    <DIV>Sometimes, two sperm can fertilise a single egg, but this is only thought to happen in about 1% of human conceptions. </DIV>
    <DIV></DIV>
    <DIV>Most embryos created this way do not survive. </DIV>
    <DIV></DIV>
    <DIV>Hermaphrodite </DIV>
    <DIV></DIV>
    <DIV>These twins, who were conceived normally, only came to the attention of scientists because one was born with sexually ambiguous genitalia. </DIV>
    <DIV></DIV>
    <DIV>The child was discovered to be a hermaphrodite, and has both ovarian and testicular tissue, while the other child is anatomically male. </DIV>
    <DIV></DIV>
    <DIV>But genetic tests show both are "chimeras", and have some male cells - which have an X and Y chromosome, and female cells - which have two X chromosomes. </DIV>
    <DIV></DIV>
    <DIV>The most likely explanation for how they were formed is that two sperm cells - one with an X chromosome and one with a Y chromosome - fused with a single egg. </DIV>
    <DIV></DIV>
    <DIV>The twins are now toddlers, and doctors say they are progressing well. </DIV>
    <DIV></DIV>
    <DIV>Another case 'unlikely' </DIV>
    <DIV></DIV>
    <DIV>Vivienne Souter, a geneticist at the Banner Good Samaritan Medical Center in Phoenix, Arizona who investigated the case, said: "Their similarity is somewhere between identical and fraternal twins. </DIV>
    <DIV></DIV>
    <DIV>"It makes me wonder whether the current classification of twins is an oversimplification." </DIV>
    <DIV></DIV>
    <DIV>Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina, said: "There's value in understanding that this can happen, but it's extremely unlikely that we'll ever see another case." </DIV>
    <DIV></DIV>
    <DIV>And David Bonthron, a geneticist at the University of Leeds, said: "The number of these cases is very small, but before they were reported, most people would have said this could never happen." </DIV>
    <DIV></DIV>
    <DIV>He added: "Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say. </DIV>
    <DIV></DIV>
    <DIV>"A lot of what we know about fertilisation is deductive, because we can't observe these events in humans."

    </DIV>
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    #2


    This was in the documentary about multiple births recently. Due to added interest in this house at the mo' this has been noted and logged.
    \"What is wrong with a drug that makes teen-age boys drive slowly?\"
    P. J. O\'Rourke

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      #3
      Interesting article Ruck. Done a Wikipedia search on this, frightening stuff.
      "Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too ?" - Douglas Adams

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