The answer is kinda obvious: because the DNA doesn’t encode the fingerprint pattern.
A lot of people misunderstand the DNA. They think that it’s some kind of blueprint that contains the exact 3D model of a creature. The shape of every curve. The coordinates of every polygon.
It’s a well-known fact that the DNA sequence is convertable into a 3DSMAX file, huh…
Here’s a plot twist: this blueprint that you have in mind, it doesn’t exist.
In reality, the DNA encodes for a set of tools to assemble and maintain an organism. The tools are important, but the exact body shape is more a result of different physical and chemical constraints acting upon an embryo.
You might be thinking that the DNA somehow encodes the exact shape of a bone — while in reality, the fine details of its topography are the result of differential mechanical forces acting upon the developing bone.
Why is the bone shape so consistent from human to human then? Well, because the mechanical constraints are consistent from human to human! That’s the #1 reason why the curve of the tibial bone is heritable at all (though having the same assembly tools also helps a lot).
- For some traits, the constraints are rather consistent — making them highly heritable;
- For some traits, the constraints are inconsistent — making them environmentally or developmentally determined;
- For some traits, the constraints are fuzzy and unpredictable — making them borderline random.
As for fingerprint pattern: the exact layout is random-generated, presumably via Turing pattern morphogenesis. There are genes that ensure the development of some fingerprint (as opposed to complete lack of it). But what kind of pattern, the exact layout of it — that’s out of scope of the DNA.
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