Mono-zygotic, NOT Identical

Do these sweet little faces look identical to you?

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).