Differences between identical twins

Factual Questions
21

I have (had, actually) a pair of uncles, my mother’s brothers who were identical twins.

I didn’t know them as children, obviously. But in my earliest memories of them, I never confused one for them other.

They’re a case study in nature vs. nurture. One is still living, in his eighties. Quite healthy, mentally just fine. The other died more than fifteen years ago.

The deceased uncle was a lifelong hard drinker and heavy smoker (the last smoker on the plant who still smoked unfiltered cigarettes). He died, ultimately, of cirrhosis. He basically drank himself to death. His brother had tried to help, had sent him to drying-out places a couple of times, to no avail.

He was what you’d call a high-functioning alcoholic, I guess. Never lost a job, and in fact did fairly well in the corporate world, being (after a career in federal law enforcement) VP of security for a multi-national corporation.

But he was an alcoholic. I never in my life saw him without a drink in his hand. Never.

The surviving brother had a very different personality. He wasn’t a teetotaler, he enjoyed a drink, but responsibly and appropriately. He quit smoking when he had kids (in the early sixties). He had to be careful about his weight, but otherwise had no serious health issues (as I said, he’s still around, still in fairly good shape).

I remember my mother (also deceased) telling me than when they were kids, her brothers were altar boys (of course – so was I. Big Irish-Catholic family) and were in very high demand for weddings in the parish, because the identical twin altar boys at the wedding Mass looked great in pictures. So at one point, before their lifestyles diverged so much, they must have looked very alike.

Also, I have a friend, a contemporary, who is gay. He has an identical twin brother who is straight. I have no idea what the significance of that is.

Anyway, I’ve rambled, but I think my point is that genes aren’t absolutely determinative. Not saying genetics don’t matter, but it’s not everything.

22

These are just an anecdotes; I don’t have any data to back it up, because it’s nothing we’ve rigorously collected.

People are occasionally mistaken about whether their twins are identical (MZ) or fraternal (DZ). This seems to come from a few different places. Sometimes it is parental desire. “Identical twins are the cool ones, so my twins are identical,” or “my twins are separate people, so they are fraternal.” In both cases, regardless of the twin pair’s actual zygosity.

Doctors will often tell the parents they have identical twins if there is a single placenta, and fraternal twins if there are two placentas. That is usually true, but there are many exceptions. Identical embryos may split early enough and implant far enough apart to get two placentas, and fraternal embryos may implant close enough together that the placentas fuse. So at birth the doctor says something, and that’s what is believed.

Now that it is easy enough to determine zygosity with a genetic test, we occasionally find twin pairs that classify themselves wrong.

Anyway, the point of all of that is that some of these identical twin pairs who are very different from each other, may be fraternal twins. Of course not always, or necessarily even often. Identical twins are separate people who just happen to share the same genetic makeup. That tends to make them more similar than two other siblings, but still not the same.

23

I know twins who are fraternal, but look really, really similar, and their parents thought they were identical when they were born (I’m guessing about 1963, because they were about 4 years older than I was). I’ve seen pictures of them as babies and toddlers, and I could see how they were really hard to tell apart. They were actually a little bit hard to tell apart even as adults-- when you ran into just one of them. If you saw them side by side, you could see they weren’t identical, but they were enough alike, that it took a minute to be sure of which one you’d just run into.

Mostly, I figured it out by how friendly each one was, because I saw one really frequently, and one rarely, so the one I saw more often (I’d been friends with his girlfriend in high school, and that’s how I knew them in the first place), was always much more familiar with me. If I said “Hi!” and he looked like he was having trouble remembering my name, Mike. If he came right over and hugged me, Steve.