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Both the eyes are not identical in geometrical shape and size i.e. physical appearances of both the eyes of a person are different from each other. The same difference is observable in both the hands, ears.

I don't know which causes the difference in geometrical shape and size of the twin organs in the human body.

Any explanation will be greatly appreciated.

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    $\begingroup$ Does it help to think about it the other direction - why would they be the same? $\endgroup$
    – Bryan Krause
    Jan 27 at 16:21
  • $\begingroup$ Well, both the eyes belong to the same person and develop under apparently similar conditions but they are geometrically different. I really don't know where this geometrical difference in twin organs comes from. $\endgroup$ Jan 27 at 16:26
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    $\begingroup$ "Similar", not identical. $\endgroup$
    – Bryan Krause
    Jan 27 at 16:37
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    $\begingroup$ "which conditions must be identical" - All of them. Which is pretty much physically impossible. If DNA influences geometry, you'd expect less variance within an individual than between individuals. I'd be pretty confident that if you go looking for this, you'll find it for the cases you describe. $\endgroup$
    – Bryan Krause
    Jan 27 at 16:48
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    $\begingroup$ Maybe a nitpick, but the paired limbs/organs generally aren't identical, they're mirror images. Which might have some influence on development: perhaps the most obvious is human handedness, with the preferred hand being used more. $\endgroup$
    – jamesqf
    Jan 27 at 18:55
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The topic you're looking into is fluctuating asymmetry, a broad field of study. Among the free reviews this one is a bit old but well on topic. The general idea is that there is "developmental noise", which is opposed by mechanisms for developmental stability. The review covers a variety of conclusions people try to read in to these studies, often about the stresses on the organism or the mechanisms by which development is "canalized" to reach a specific goal.

The nature of the noise is usually presumed rather than proven, since many different factors are presumably adding up to make small differences. We may assume (though I'm not always sure it is true) that mRNA, protein, and signalling molecules diffuse to reach their targets, which is to say they are subject to Brownian motion with random variations in timing. The need for multiple factors to get together for a complex process like transcription or translation means that the number of copies of a protein produced under a given circumstance will vary. An example of this in hypertrophic cardiomyopathy has been proposed to contribute to disease progression, due to random bursts of transcription in individual heart cells affected by the disease.

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Body development is a complex process in which the interaction of a huge amount of molecules and the environment give rise to the final product.

In such a complex system is not surprising that some chaotic dynamics exists, and if so, small perturbations during the process give rise to not so small difference in the final product.

What is surprising is that, even with such a big complexity, the final product (like the two eyes) is a stable state. So I think the main question is not what make them different, but how the development process achieves to generate symmetry.

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The other answers talked about how asymmetry happens. Here I say my idea on why asymmetry could be beneficial. Focusing on left and right brain lobes, you see that the difference between these lobes give new functionalities to brain and these new abilities could be beneficial during evolution of the taxa. I think asymmetry which looks like stochastic, maybe in thousands and millions of years, little by little (and some times suddenly) make new (useful and harmful) inheritable functions.

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