Why Are Identical Twins *Different*?

Twins Now_June 2014

I was one of the last to know that my twins are identical.

Via ultrasound, we knew they shared nothing but the womb–not a sac, not a placenta–making us believe they would be fraternal/sororal. When they came home from the NICU after an arduous 78 days of intensive care and observation, one had red hair and the other brown. Or so it seemed. Over time, their hair met somewhere in the middle, becoming a dark red, an unusual hue for chance to choose twice in children of mixed South Asian heritage.

Still, their father and I focused on their differences: their size differential, their unique freckles, and their individual personalities.

Their grandfather, my dad, was the first one in the family to tell us they were identical. After careful study, he said, “They have the same ears.” I’d never considered their ears before, but I couldn’t deny that our girls shared many strikingly similar features.

Ultimately, their father and I sprang for a DNA test to tell us once-and-for-all what should’ve been obvious: whether our little girls had come from a single zygote or two.

The test confirmed they are monozygotic, or in other words, identical twins.

Except they’re not really identical: One is left-handed, the other is the opposite; one wears glasses, the other doesn’t; one is silly, the other is serious; one loves math, the other prefers social studies; one enjoys Baby-sitters Club books more than Number of the Stars while the other’s reading preferences are the reverse; one adores salad, the other junk food; one likes peanuts while the other insists she’s allergic (she’s not); and they still look nothing alike (at least to their parents).

What makes them so different?

the geneThe answer lies in their environment. As author and physician Siddhartha Mukherjee has said, “[F]ate impinges differently on their bodies.” In his May 2nd article in New Yorker, drawn from his newest book (released today), The Gene: An Intimate History, Mukherjee attempts to explain to a lay audience how environmental factors change gene expression. Apparently, he did so with only questionable success. His New Yorker article has been heavily criticized as “misleading” by researchers in the field because it “ignored key aspects of gene regulation.”

The criticisms of Mukherjee’s New Yorker article make me wonder about the accuracy of his book, but at least one of the critics has said that an excerpt of the book “was more accurate and thorough than The New Yorker article.” Despite the book’s potential flaws, I’m looking forward to reading it in the hopes that it will shed light on the mysteries of genes and identical twins.

Twins are fascinating. They are also fun (as you can see from this short video of M & S when they were 12 months old!):


It’s been a treat to raise these two over the last eight and a half years.


*Dr. Mukherjee is also the author of the Pulitzer Prize-winning The Emperor of All Maladies: A Biography of Cancer, which I wrote about on this blog in Censoring Speech to the Detriment of Women’s Health.

**A few years ago, I asked my daughters about what made them different. Here’s what they had to say (in No Two Alike: Encouraging Individuality in “Identical” Twins).

***The picture is from the summer of 2014 when they lost their two front teeth at the same time. The video is from December 2008, when they received an animatronic dog for their 1st birthday. And here is how you’ll often see them now:



  1. Your girls are so cute and it is amazing they have such differences, especially one being left handed and the other not. They look so identical and I had no idea they could have not shared a sac or placenta. This must be quite unusual for identical twins.

    1. Hi Donna! Thank you! Only about 20-25% of identical twins are di-di (diamniotic and dichorionic, which means they share nothing in the womb). That’s why our doctors kept telling us our twins were fraternal.
      Thanks for stopping by! I hope you’ve been doing well.

  2. My best friend’s twins are identical and one is right- and the other left-handed. Apparently these are called mirror twins and are a subset of identicals. I remember the weirdness when they got identical but opposite first back teeth when they were teething!

  3. They’re so lovely! Alike or different, they’re both gorgeous and smart. And that video – too much cuteness! My heart just exploded.

    1. Thanks, Jaclyn! I love that video. My girls saw it for the first time (that they remember) when they were previewing the post. I let them to read anything I write about them now–they’re old enough to have a say in it.

  4. Your girls are beautiful. It must be a really wonderful experience raising identical twins. All those twins studies and you have your own to watch and love. I have heard stories about identical twins having such a special bond, do you notice anything like that with your girls?

  5. The font on one twin’s ereader is really large. Sure she doesn’t need glasses? 😉 They sure do have gorgeous hair. What a beautiful color! Doesn’t your third child also have hair this color?

    1. Ha! We’ve had her eyes checked. That was just her preference, for some reason (and it isn’t always that big). My youngest also has a lot of red in her hair, but it has more brown in it too. I joke that she’s my most South Asian-looking child. I look very South Asian–I’m half Sri Lankan–and I assumed my children would have almost black hair just like mine. Recessive genes are fascinating.

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