• Question: Why are identical twins identical?

    Asked by elkerobinson to Charlie, Eoin, Jemma, Julian, Steve on 22 Mar 2011.
    • Photo: Stephen Moss

      Stephen Moss answered on 22 Mar 2011:

      Hi Elke
      Good question. We all start life as an egg, which gets fertilised by a sperm, which then forms a tiny ball of cells that just goes on dividing. Occasionally the first two cells, instead of dividing and growing into a single embryo, separate and start from scratch again. Then we end up with twins, and they’re identical because they have exactly the same DNA.

    • Photo: Charlie Ryan

      Charlie Ryan answered on 22 Mar 2011:

      hi elkerobinson!
      I’m no biologist, but isn’t it something to do with them both being conceived from the same egg, which then splits at an early stage into two identical embryos!!??

    • Photo: Julian Rayner

      Julian Rayner answered on 22 Mar 2011:

      Hi elkerobinson. Identical twins are formed when a single fertilised egg splits in two in the very early stages of development. That means that both developing embryos have exactly the same DNA. Non-identical twins, by contrast, have completely different DNA – they form when two eggs are fertilised and both develop into babies.

      Identical twins are hugely interesting to geneticists – if you want to know whether a trait like heat disease or diabetes is due to a genetic mutation, study twins – if they have an exactly equal chance of getting the same trait, then it is all down to genetics, because they have exactly the same DNA. If there are differences between them, then the environment (what they eat/where they live, etc) must also be playing a role as well as genetics.

    • Photo: Eoin Lettice

      Eoin Lettice answered on 22 Mar 2011:

      Hi elkerobinson,
      Good question!
      When a human pregnancy happens, one female egg is fertilised by one male sperm. That way, the genetic material (DNA) from both the mother and father is combined to produce a new offspring which has some of the characteristics of each parent.
      In some rare cases (3 in every 1000 births), the fertilised egg (the zygote) splits in two to form two genetically identical embryos which grow and develop in to two genetically identical twins, just like me and my brother!

      Eoin

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