How identical Are Identical Twins

Mrs. Chili writes:

Tell me the deal with identical twins. I’m married to one, and I can tell you for sure that they LOOK different – they sound different on the phone, and they behave differently (well, most of the time). What, exactly, is identical about them?

I mean, I get that they have the same blood type and that they’re DNA matches to one another, but are their fingerprints the same? Why does Bruder Chili need glasses and Mr. Chili doesn’t? Could Bruder Chili make a scientific claim to be the father of Mr. Chili’s children? Would Bruder Chili’s kids be more than cousins, genetically, to Mr. Chili’s kids? Would they, essentially, share the same genetic father?

First of all, as you have observed, ‘Identical’ is a misnomer. They really are different. How different? well that depends. Biologically, the way identical twins come about is that a early embryo splits in two. Both halves then develop into babies. They are genetically identical, but that does not mean that their bodies are identical. As embryos (and babies, children and adults) grow and develop, there are tone of environmental factors that effect the development. These factors cause ‘drift’ (for lack of a better word), so the two ‘identical’ twins slowly become less and less identical.

Fingerprints: nope, they do not have the same fingerprints. The paterns on the surface of the skin are formed during embryonic development, and there are a ton of factors (again) that effect the patterns. Some are genetic, some have to do with the way individual cells move during development, and some (may) have to do with really minute details of position, exposure to amniotic fluid, and what the fingertip is touching, pressing against, or squeezed by at a particular time. In other words, environmental factors (note that during development, environmental factors can be both internal and external).

Eyeglasses? depends on why they need glasses, and their personal responses to those needs. Many people ‘need’ glasses, but refuse to wear them, and get along just fine. There is some evidence that wearing glasses can cause a ‘crutch effect’ – your body gets used tothe crutch, and gets lazier, making the need for the crutch more pronounced. Once again, environmental factors can play a big role in the need for glasses.

Now for the fun one: Can one twin make a ‘scientific’ claim to be the parent of the other twins child? If you are defining ‘scientific claim’ to mean a claim based purely on genetics, the answer would be yes. Remember that both twins are genetically identical. I’m not sure if that counts as a scientific claim, but as a genetic claim, it would be valid. I can’t think of (and my searches turned up nothing) any way, based on genetics to discriminate between a pair of twins in a paternity question. I wonder if this one ever came up in court somewhere….

 So, in summary, identical twins are genetically identical, but that is about it. What is interesting is that when the division of the embryo happens, it is possible for the twins to be different right from the start. If the embryo divides in a way that puts more cells in one of the twins than the other, they are not ever physically identical. An embryo starts out as a single cell, and divides. For the first few divisons, the number of cells simply doubles 1 to 2 to 4 to 8 to 16 and so on. At this point, the cells in the embryo are identical (pluripotent stem cells – they can mature into any cell type inthe human body). Eventually, as the cells begin differentiating (turning into other cell types), they begin to divide at differnt rates, and the exponential cell multiplication slowly fades away. Because the early cells are the pluripotent stem cells, if a 4 or 8 cell embryo splits apart into multiple embryos (2, 3, 4, 5, 6, 7, or 8), all of the new embryos are viable. This is how triplets, quads, etc. happen.  Of course, as you get more and more embryos, other complicating factors enter the picture – like there is only so much room, nutrition etc avaliable, and these environmental factors can lpay a huge role in the survivability of the embryos.

As long as an embryo splits in two while all of the cells are pluripotent stem cells (undifferentiated), both of the new embryos will be viable. If an embryo splits after the cells begin to differentiate, it is less likely that both (or either) embryo will survive.