Why Don’t Identical Twins Always Look Alike?

My daughters, J and M, are identical twins. This means that they both grew from a single fertilized cell, splitting into two bodies what was once one. They have the same DNA.

MomandGirls1

They do not look the same. This weekend, we went to a knitting class for kids in Austin. The other mom there asked which of the girls was mine; she had assumed that they were friends whom I’d brought to class together.

People frequently ask me how my girls can possibly be identical if they don’t look the same. After all, don’t they belie the very definition of identical? The same question was asked by another mother of identical twins in a MoM Facebook group recently. She wondered how it was that only one of her twins had crossed eyes so severe that he needed surgery. Shouldn’t both have the crossed eyes if one did? As you may know, M has a condition called frontonasal dysplasia that impacts how she looks. J doesn’t have this condition. I felt compelled to answer the other mom’s question.

I thought my response to the other mom was worth sharing with you.

Why Don't Identical Twins Always Look Alike? from hdydi.com

The confusion comes from the usage of the word identical. Used colloquially, it means that two things are alike in all ways. Used scientifically to describe two organisms, it means only that they share their genetic makeup. My daughters’ DNA is identical (more or less; every time a cell divides in two, there’s the potential for something to get miscopied, resulting in a minute genetic change). That’s what makes them “identical” twins. Personally, I prefer the term monozygotic to identical. It makes clear that my daughters started as a single (mono) fertilized egg (zygote).

DNA doesn’t dictate everything. In the case of my girls, the sides of J’s face came together exactly on schedule, so she’s symmetrical . M’s facial formation was behind schedule for some reason we don’t know, so she has a cleft running down the center of her face and an adorably unusual nose. Even though they can look very similar, their noses, foreheads and chins are different. Plus, J has a dimple, an interruption in her cheek muscle, that M doesn’t.

Baby wearing an eye patch for amblyopia from hdydi.comMy M also had amblyopia. Essentially, her brain ignored the message from her left eye, rendering her functionally blind in that eye. We caught it early, and we were able to resolve it by having her wear a patch over her “good” eye several hours a day for a few months. Her twin J has always had perfect vision in both eyes.

The biology of differences between identicals can be pretty complex, but think of it this way. The DNA that our identical kids share is the blueprint from which they are built. However, the actual process of fetal development (just like house construction) introduces teeny tiny differences. When those differences comes early in development (think during the framing of a house) they can have a really major impact on the final product, whether human or house.

Especially when our kids have medical challenges, it’s really easy to blame ourselves and wonder if there’s something we as moms could have done differently. There usually isn’t.

And really, would I want my kids to be exactly alike? M’s sparkling wit is a foil for J’s earnestness. J’s emotional maturity balances M’s academic precocity. M’s musical and artistic ability are a match for J’s literary talents.

My unique girls are not identical people even though their matching DNA makes them identical twins.

Sadia (rhymes with Nadia) has been coordinating How Do You Do It? since late 2012. She is the divorced mother of 7-year-old monozygotic twins, M and J. She lives with them and their 3 cats in the Austin, TX suburbs and works full time as a business analyst. She retired her personal blog, Double the Fun, when the girls entered elementary school and also blogs at Adoption.com and Multicultural Mothering.

9 thoughts on “Why Don’t Identical Twins Always Look Alike?

  1. My boys had different head shapes in the beginning (and still different today – one of the easiest ways to tell them apart) and I remember the doctor explaining that identical twins start with the same DNA and then are shaped by other forces in the womb (such as placement during pregnancy, during labor/birth, even how much amniotic fluid they had affects head shape and other things.)

    On the other hand, medically my boys have been the exact same, even so much as getting their teeth on the exact same day, milestones within hours of each other, same exact illnesses, same concerns, etc…

    • When I was in college, I actually had the opportunity to work on maternal effect genes. These are genes expressed in the mother in the womb that impact fetal development; the offspring don’t have to inherit the genes themselves to be impacted. For example, differentiation between the head end and the feet end of an embryo is at least partly determined by a gradient of chemicals in the womb. If the chemical are evenly distributed, you can end up with two heads and, obviously, the embryo isn’t viable. Super cool, how our bodies work!

    • Sure thing! I added a little note in the text of this post about mutations that can occur after conception. People definitely tend to think that our DNA is written in stone, but if it were, there would be no cancer, right?

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  3. We had mild TTTS, so one of my twins was bigger at birth. That’s still the case now that they’re almost 8! She is 8 pounds more and two inches taller than her identical sister, which makes it easier for people to tell them apart when they’re together. They also have different hair–one has very curly hair, the other is straight. I think their different experiences in the womb is what has caused them to not be so identical.

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  5. Wow, what a well written article! I was looking for the answer to this question and you explained it so well. Thank you!

  6. My sister and I are identical twins, and get told we are not, how can it be? Etc, etc. because we look so different. But mum had the testing done when we were born. Was 99% or something along those lines accurate that we are in fact identical twins. We are 30 now. :)

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