Sunday, October 02, 2011

Lefthandedness and the vanished Monozygotic twin

 So there is a book about left-handedness that links this to the loss of a "mirror" twin before birth.

The puzzle of lefthandedness
Rik Smits presents an interesting theory of his own: Left-handedness is a side effect of identical twinning. He explains his ideas this way: When the embryos split at an early stage in the pregnancy -- around the first week -- this division would result in identical twins. And twinning may give rise to minor mirror-imaging effects, including left-handedness. But Smits suggests that most embryo splits don't always result in two viable fetuses, and the process often goes wrong. He proposes that perhaps a left-handed fetus survives and is born while the "clandestine" twin, the rightie, is lost early in the pregnancy, before would-be parents know of its existence.
Not sure about this - it would seem there is no particular advantage to being  the left or the right handed twin.   Many MZ womb twin survivors are right handed, but they may be mirror twins also. 

We can never know how many right handed  people are MZ mirror wombtwin survivors - unless of course we ask them how they feel about being a twin - they can usually tell if their twin was MZ or not, its amazing!   They may be the ones who have a preoccupation with mirrors, and there are plenty of those!




5 comments:

  1. Interesting all the same, I know I lost a twin in early pregnancy and the survivor, my son, is the only left handed person in our family, even extended family, no history of left handedness at all!!

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  2. I'm also the only left handed person in my extended family. Don't know about my twin for sure, though my mum has them in the family, and had signs of losing mine. I was also envious of twins growing up- but didn't know why, until now.

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  3. Just because you are the only left hander in the extended family doesn't mean you are the only womb twin survivor in your family. Left handedness is specific to MZ mirror twinning where the left-handed twin is the one to survive. That is only a few cases. There may be other right-handed MZ "mirror" womb twin survivors in your family - even your close family, and of course DZ womb twin survivors as well.

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  4. Althea- I agree that other members in my family may have also lost a twin. Im not suggesting that I definately lost one, or that they definately didnt based purely on handedness. It's just that from my viewpoint, I find this theory quite interesting.
    Also, the rate of LH is the same in DZ and MZ, so it's not quite clear cut to say it only applies to MZ pairs. Of course there may be different mechanisms at work in the two types of twins? Althea- I agree that other members in my family may have also lost a twin. Im not suggesting that I definately lost one, or that they definately didnt based purely on handedness. It's just that from my viewpoint, I find this theory quite interesting.
    Also, the rate of LH is the same in DZ and MZ, so it's not quite clear cut to say it only applies to MZ pairs. Of course there may be different mechanisms at work in the two types of twins?

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  5. I would need to have a citation for that statistic that left-handedness is the same for DZ and MZ live-born twin pairs, if we are to discuss this further. If indeed that is a true finding, then it probably means what I have always thought: that DZ liveborn twin pairs are often a reduced triplet set including a pair of mirror twins where one dies in the womb, with a 50:50 chance that the left-handed one will survive.

    The trouble with counting twins for statistics, is that twins are not always what they seem!

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