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Ten-month-old identical twin boys (left to right) Callum and Marcus

chris bolin/ The Globe and Mail

My twin boys are almost one year old. Marcus has skin worthy of a soap commercial, chubby cheeks and a giant gummy smile he uses to woo anyone within beaming distance. Callum has spots of eczema, a narrow face and a shy grin that rewards only those he's comfortable with.

Callum is allergic to strawberries. Marcus is only allergic to waiting five minutes to eat. Their weights are distinct enough that Callum would have a hard time getting down to earth on a teeter-totter. Callum sat first. Marcus was the first crawler.

Our boys are different.

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And yet, even three months before they were born, the medical system told us they were likely identical – first neonatologists, then obstetricians, then a pathologist who sampled the placenta. ("Likely" is the key word, here, as only a DNA test is conclusive.) We accepted it, without question – when they were born, we were so worried they would be indistinguishable, we painted one's toe with nail polish.

But we never had a problem telling them apart. If they were made up of the exact same genetic building blocks, how could they be that different?

I figured the doctors had got it wrong – after all, ultrasounds are imperfect, and even the placental test misses some 10 per cent of the time. And in general we discovered a broad lack of knowledge in the medical community about identical twins – many doctors are simply more focused on health, than arcane genetic questions.

So I asked for a genetic sampling kit from the University of Alberta, swabbed my boys' cheeks, sent it off with a cheque for $180, and waited. Every year, 30 to 40 parents of multiples do the same – and that's not counting the Canadians who use private genetic services, of which there are many. There are medical reasons for such tests: knowing whether a twin pair is identical can be an important consideration when one is diagnosed with cancer, for example, and the other wants to know the chances of encountering the same.

Most parents, however, are driven purely by curiosity, and a lack of understanding about how truly unique even genetically paired siblings can be. After all, the term "identical" implies an overwhelming likeness, and the popular literature on identical twins focuses on their uncanny similarities, down to pairs raised apart who choose the same names for their own children. And yet, most parents of identical siblings can see little but differences.

"They look like completely different kids to me," says J.P. Jepp, the Calgary father of identical quadruplets. "It's bizarre when people go to me, 'How do you tell them apart?' Well, how can't you tell them apart?" J.P. and his wife Karen embrace the differences and actively avoided dwelling on the "identical" label. Their girls have freckles and birthmarks in different locations, not to mention dramatically different personalities.

"We have always tried to really see them as individuals – and we might have gone to the extreme," says Karen. "We never, ever dress them the same."

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For me, though, the questions nagged, and went beyond appearance: If Callum and Marcus were identical, why were they developing at different paces? Should we be concerned when one broke out in a rash, and not the other? Was there something wrong?

The answers, as far as science understands them, would shed light on some of the remarkable intricacies of the way we grow and develop.

I discovered that even those professionally obligated to find similarities between identical twins are confronted with how hard it can be. Martin Schoeller is a New York portrait photographer whose pictures illustrated a January National Geographic article on identical twins. He has photographed 120 such pairs, and several identical triplets, and intends to release his work in a book later this year – an assemblage of photos that will make clear the differences between such sets.

"You assume identical twins look exactly the same," he said. "But there's a lot of identical twins I photographed that look like two brothers or sisters that are not necessarily twins.

"Once you see them side by side in the same light and the same camera angle, you come to realize how different most twins look," he said.

The differences aren't just skin deep. The formation of life, from its very earliest hours, is fraught with all manner of avenues for things to slip up.

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Mostly, those slipups mean nothing. But between identical twins, they can manifest in more obvious ways. Pinched blood flow in the womb can leave one sibling permanently smaller and lighter. Female identical twins can have differences in which X chromosomes – one from each parent – are active. That's the reason, among the Dionne quintuplets, some were colour blind and others were not. Even the pattern of folds in the brain can be different. And in rare cases, identical twins have different hair and eye colour, a result of incongruities in how pigment-forming cells migrate through the body during growth.

Then there are mirror twins, which typically comprise one left-handed child and one right-handed. Geneticists believe that an egg, within days of fertilization, assigns sides. If the egg splits to form identical twins, those sides become independent, and mirror twins result. Among the Jepp girls, for instance, three are right-handed, the other is a leftie.

Of course, our own boys have their share of similarities. Their eyes are an identical bright blue. When they crawl, they use the same horribly inefficient commando dragging technique. They first rolled within four hours of each other.

So when I got the results of the DNA test, which compares 14 or 15 segments of the genome, I wasn't shocked. The test was unequivocal: "They were identical at each" of the parts they tested, said Fiona Bamforth, the twin expert who is chair of the University of Alberta department of laboratory medicine and pathology.

Which made it final. Marcus and Callum are identical.

In some ways, that makes their differences even more interesting. From the same genetic building blocks, they are each assembling something unique. If nothing else, it's an intriguing insight into the propensity life for diversity. No matter the genetic ties, the natural order favours a kind of irrepressible individuality.

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