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.

14 Responses to “How identical Are Identical Twins”

  1. mrschili Says:

    So, if my brother-in-law has children, will those kids and my kids be, genetically, half-siblings?

  2. sphyrnatude Says:

    yes. Because they share (genetically) a parent, from a genetic standpoint they would be half siblings. The same as if someone had a kid with one partner, then had a second kid with another partner. The kids would have the same “genetic distance” from each other as kids from two couple that share a twin.

  3. Organic Mama Says:

    My twin and I have children that while have some very similar features, are widely different, based solely on our er, sperm doners. Our youngest daughters have similar noses, a very similar chins and there the sameness ends. Genes play so differently in our half-sibling children; it’s really fascinating.

  4. sphyrnatude Says:

    genetics is a funny subject. Kids that are distantly related can look very similar, while siblings can look very different. It doesn’t just happen when you have twins in the mix……

  5. Rachel Says:

    Can identical twins both reproduce?

  6. sphyrnatude Says:

    Rachel: As long as there isn’t some other problem (not related to being identical twins), ther eis no reason ientical twins can not both reproduce.
    The cells that make the reproductive cells (sperm and ova precursors) develop well after an identical twin would have to divide, so both twins would have the required cells.

    an interesting side note: There seems to be seom genetic dispostition for twins – if mom or dad have twins, the kids are a bit more likely to have them too.

    -Dr. Science

  7. Organic Mama Says:

    Ok, can you explain that. I understand the predisposition for releasing more than one egg at a time (causing fraternal twins) can be inherited (and apparently often skips a generation), but I thought identicals were freaks of nature (I’m one, so I should know!) with no inheritable tendency. Am I wrong?

  8. sphyrnatude Says:

    well, as far as I know, there is no well-defined mechanism for the increased probability of next-generation identical twins. everything that I’ve found indicates that whatever caused the initial split that caused the “first” set of identical twins is probably something tht has at least soem minor connection in genetics.

    without getting into a ton of cell physiology, one of the more plausible ideas that I ahve heard is that there is a specific set of genes that play a role incell adhesion (making the cells stick together instead of just floating apart). Anyway, the theory is that there is a different set of these cell adhesion factors in very early development, and maybe folks with a tendancy towards identical twins have a “weaker” set of adhesion factors.

    This isn’t as far-fetched as it sounds – as cells develop into different cell types, the adhesion factors change a lot – if they didn’t, all the cells in your body would stick together, and you’d be one solid mass of unmoving cells – imagine a 100+ pound pot roast…. Anyway, if there was some minor variation in the genes that control the formation ofhtese early cell adhesion factors, and a “weaker” adhesion resulted, it would be more likely that the early embryo would split, producing identical twins. If the weakened adhesion was a result of geneetics (instead of some other environmental factor), it could be passed on to the kids…

    -Dr. Science

  9. Says:

    Oh my gosh! I always feel quite odd, because my mother was married before my dad and had two kids, and my dad is an identical twin, and his brother has two kids. I always felt like I was just as much like my cousins (who I don’t see very often) as I was like my half siblings! This is uber weird and explains a lot.

  10. oni gbedex odunayo Says:

    it’s educative. i’m just knowing now

  11. erika Says:

    im an identical twin to another girl and i feel what she feels is this normal or not (if she hits her knee i can feel it at the same time): ( or 🙂

  12. sphyrnatude Says:

    Erika, this would be a very unusual thing to have happen. There are many anecdotal (meaning unproved) stories of things like this between identical twins, but when they are tested in controlled circumstances, they have never been able to be demonstrated to be real.
    Here is a simple test you can try. You’ll need a couple of friends, and 2 separate rooms where you can’t see or hear people between the two rooms, and two watches that are synchronized. Put your twin and a friend in one room, and you and another friend go into the other room. Make sure you can’t see or hear each other (try hard). Once you’ve made sure you can’t, the friend in the other room should do something to your twin (something not harmful, but that you would expect to be able to feel yourself). They should write down EXACTLY what they did, in as much detail as possible, and record the time. They should wait a few minutes and do something else. Do this for a few rounds. You shouldn’t know what she is going to do.
    If what you have explained is real, you should be able to tell your friend (the one in the room with you) what is being done to your twin. Have your friend write it down (with lots of details and the time).
    When the friend with your twin is done, they should wait a few minutes more, then come get you.
    Compare the notes that the friends made. If they match exactly, you’re on to something. If they don’t, don’t worry. Many twins experience this type of thing – it simply demonstrates a very strong empathy between you.

  13. Julie Newbold Says:

    Yes… This HAS come up in court before.

    • sphyrnatude Says:

      So it finally did come up in court (the original article was written years ago, and at that time, it hadn’t).
      It is interesting to note that the court completely ignored the scientific issue, and simply opted to accept the mothers opinion of who the father of the child was. Science can’t answer every question…..

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