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:
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.
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.
- 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).
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Erin Little says
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.
My most commonly used word when commenting on your posts?
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.