This is the second in a multi-part series. If this is way too long and detailed for you, possibly all you really need is part 1: how to tell identical twins apart from fraternal twins.
How to actually understand the difference between identical twins and fraternal twins
Fraternal twins are easy. Everyone understands fraternal twins: two babies, siblings, are born at (mostly) the same time. Each one, most likely, started out exactly the same way that every other baby starts out. There’s something to do with an egg meeting a sperm, and then once the egg is fertilized it divides and divides until eventually you end up with enough cells that they start to get organized, and at the end of about nine months, if everything goes well, you’ve got a healthy newborn baby.
So, if there happen to be two eggs that get fertilized at the same time, you can end up with two healthy newborn babies. (Often you end up with two slightly premature babies, actually.) They can be a boy and a girl or two boys or two girls. They are just as likely to look alike as any two siblings. So, they might look a lot alike or they might look very different. They are two babies with the same birthday, unless it’s a really long delivery or it happens over midnight.
The vast majority of twins are fraternal. They are unusual, but not that unusual. Having fraternal twins does appear to run in families, and it is also much more likely if you are using fertility treatments.
Identical twins, though, are different. With identical twins, there is a key difference in the conception process.
(That’s right: Conception is a process. Although this comes as a surprise to many, it is not a surprise to those of us who are identical twins. That’s because it is literally a part of our DNA. If conception were a single moment in time, then identical twins would not exist.)
It starts the same way you’ve always been taught: with the egg and the sperm. The egg is fertilized and it starts to divide. It does this for a while, always doubling the number of cells.
The best we can tell, most of the time, the number of cells keeps increasing and increasing and increasing, gradually becoming more complicated along the way as cells start to specialize and become things like brain cells and nerve cells and skin cells.
But occasionally, something different happens.
You start out with one fertilized egg cell. It divides into two identical cells. They divide into four. They divide into eight.
And then something weird and wonderful happens: instead of continuing the progression and dividing into sixteen identical cells, the group of eight cells divides itself into two totally separate groups. Each resulting group has four identical cells. Then each group of four cells divides into eight, and then into sixteen, and so on, and so on, and so on*.
If everything works out, you end up with two healthy newborn babies. They have identical DNA (but different fingerprints). They will almost certainly look very much alike: two sisters or two brothers who share, not only a birthdate, but also a genome.
Of course, “identical” is an imprecise word. Every time a cell divides, there is a small chance that a random mutation will be introduced to its DNA. So even at birth, identical twins can be slightly different. And then, of course, the two babies have two separate (although probably adjacent) childhoods and two sets of experiences, and over time, this can result in them looking very different. But they will always be identical twins, even if one of them cuts her hair and the other dyes it.
There have been identical twins born with noticeably different skin colors. There are even “mirror image” twins, who look like each other’s mirrored reflections. (This happens when that little clump of cells splits late in the process. If it splits really late, you can get conjoined twins.) In extreme cases, one twin will be left-handed and the other right-handed. It really extreme cases, one twin can actually have his or her internal organs mirrored.
But all of these variations on identical twins are identical twins, even if they don’t look identical to you. It’s their DNA that makes them identical—not their appearance.
Scientists do not know exactly what causes identical twinning. It is not thought to be hereditary or affected by fertility treatments. It may be purely random.
If you have triplets or quadruplets or so on, then most of the time they will be fraternal. However, it is possible that some of them may be identical. So, for example, while identical triplets** are very, very rare, it is not unheard of to have two babies who are identical and one who is not. All three of them are still “triplets”. (The two who are identical are also “twins”.)
Today, it is common for possibly identical twins to be tested at birth, but that has not always been the case, and some families opt out of the procedure. Instead, they watch the twins as they grow up and see if they hit important milestones (teething, walking, etc.) at about the same time. This is a pretty good indicator that they are identical, but not foolproof. (Hint: Do not suggest to twins that have not been tested that we might not actually be identical. Being identical is part of who we are. We don’t like it when you question it.)
Next: Other weird and awesome stuff: Or, why you might be a twin (sort of) and not know it.
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