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Identical twins are genetic carbon copies

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The DNA sequences of their genes are exactly the same

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Yet physically, identical twins become increasingly different over time...

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why is this so?

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Genes provide the instructions

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for the development and maintenance of the body.

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Yet a second set of instructions known as the epigenome

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interacts with DNA

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to activate or suppress

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the expression of particular genes.

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Certain chemicals

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known as <i>epigenetic tags</i>

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turn genes OFF

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or ON

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without changing the underlying genetic code.

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During fertilization

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one set of chromosomes from Dad

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and one set of chromosomes from Mum

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are combined to form an embryo.

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Epigenetic tags are erased

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from Mum and Dad's chromosomes

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during the first days after fertilization.

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However, on some genes the tags remain

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these are known as IMPRINTED genes.

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Identical twins result

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when a single embryo splits in two.

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Each embryo has the same genome

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and the same epigenome.

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As the embryos develop

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cells differentiate into distinct types

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such as bone, muscle and skin.

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As each type specialises epigenetic tags

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activate and silence specific genes

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leading the cells to specialise further.

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Each cell type gradually takes on

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a unique epigenetic profile.

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Because the twins share the same environment

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their epigenomes are very similar at birth

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and through the early years of life.

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As the twins age,

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their environments begins to differ.

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Their genomes remain the same

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but signals from the environment

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act on the twins epigenomes

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to activate and silence

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different genes.

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Diet, for example

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is an environmental factor

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that can have an effect

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on the epigenome.

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Differences in physical activity

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can also cause epigenetic differences

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between the twins.

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Exposure to toxins

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can influence the epigenome.

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Stress also plays a role

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in shaping the epigenome.

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By the time the twins

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are well into adulhood

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their epigenomes, thus gene expression patterns,

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are very different...

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... making each twin unique!