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You are here: Home / Uncategorized / Mono-zygotic, NOT Identical

Mono-zygotic, NOT Identical

August 6, 2010 by erinlittle

Do these sweet little faces look identical to you?

Finna.JPG
Sophie.JPG

Here are some interesting facts about “identical” twins.  They are not really identical.  That’s why the medical community, Multiple Births Canada, and other multiples groups prefer the more accurate term, mono-zygotic which means “from one zygote” with zygote being the cell produced by the union of two gametes (egg & sperm) before it undergoes cleavage.  Dictionary.com

Mono-zygotic twins (triplets or more) happen when the zygote splits during the first 12 days of pregnancy.  If the egg splits later conjoined twins may result. 

Here is some information I found at http://www.multipletreasures.com/facts-about-identical-twins.php

Differences in Identical Twins

There are two main factors that cause identical twins not to be truly identical:

  1. Identical
    twins share the same nuclear DNA – but do not have identical mitochondrial DNA. When the egg splits into two halves to form identical twins, the mitochondrial DNA may not divide
    equally between the two cells.  Mitochondrial DNA is in the Mitochondria which are outside the nucleus of a cell.

    Mitochondrial
    is DNA passed on to the child by the mother. A fertilized egg will have Mom’s
    mitochondrial DNA, half of Mom’s nuclear DNA, and half of Dad’s nuclear
    DNA. When the fertilized egg splits into two, the twins will each have
    identical nuclear DNA, but not EXACTLY identical mitochondrial DNA.
    Physical differences in identical twins are contributed, in part, to
    how much and how similar the mitochondrial DNA each twin inherited from
    Mom expresses itself.

  2. Environmental factors both prenatal (for
    instance, one twin having more room in the womb, viruses, genetic
    missteps, Twin-to-Twin Transfusion Syndrome, etc.) and after the babies
    are born (from something as minor as appetite to illness or accidents).
    Prenatal developmental differences can mean that one twin didn’t quite
    “finish” a developmental stage that his or her twin did.

One + two above = 

Gene expression.  Mono-zygotic twins have the same genotype but not the same                phenotype.  Phenotype = genotype + enviornment + random expression.  In twins, the phenotype for some traits, like fingerprints, are different expressions of the same genes.  The pre- and post-natal environment affects phenotype .  Even though the fetuses are in the same womb, the environment is not exactly the same and some genes get expressed differently. 

Fascinating stuff.  I’ve done some reading about the mitochondrial DNA and I definitely want to learn more.

If you look closely at my girls’ faces you can see some differences.  Sophie has a mole beauty mark, that developed at around four months.  Their noses are a little different.  Fiona’s lips are thinner.  Sophie’s face is fuller.  Sophie was one pound, five ounces heavier at birth (so it would seem she received more nutrients for some reason) and is now two pounds heavier and one inch taller. Their personalities were different from the moment they were born (probably before they were born but we hadn’t met them yet).   

 

 

Filed Under: Uncategorized Tagged With: DNA, identical twins, monozygotic twins

Comments

  1. buy acai says

    August 9, 2010 at 6:29 pm

    How are you, i I stumbled upon your website on yahoo and browse most of your other blog posts. I just added you to my Google News Reader. Keep up the fantastic job. Looking forward to reading more from you in the future. You know, It just so happens, I really enjoy this website and the insight from everyone who participates. I find it to be refreshing and very informative. I wish there were more blogs like it. Anyway, I felt it was about time I posted, I?ve spent most of my time here just lurking and reading, but today for some reason I just felt compelled to say this.

  2. Kath says

    August 7, 2010 at 9:46 pm

    They are amazing!

  3. Erin Little says

    August 7, 2010 at 8:08 am

    Thanks Christne.
    Just to clarify, my girls are what people commonly refer to as “identical”, they did come from the same egg & sperm.
    Mary Kate and Ashley are what are commonly called “fraternal” twins. The correct term for them is di-zygotic, meaning two zygotes. Genetically they are as similar as two siblings in any family, it just so happens they shared the womb.

  4. Christine says

    August 6, 2010 at 9:55 pm

    My most commonly used word when commenting on your posts?
    “fascinating”
    and once again – I find all the science behind twins/multiples – just fascinating.
    I always thought it was interesting the MaryKate and Ashley Olsen were not identical, yet they look SO much alike.

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