system
Given the following post, respond as if you're commenting on Reddit:
human
Biological symmetry and DNA: Are repeated body parts coded for once or multiple times?

In the case of a starfish, does the DNA essentially say, "Here's how to build an arm, now repeat 5 times?" And if so, are mutations affecting only one arm extremely rare?
gpt
You're right to think that this would be redundant. Body parts (and at much smaller levels, tissues, cells, and molecules of proteins) are not individually coded for in this way. 

Rather, the genome encodes developmental genes that pattern the body from the earliest stages of development. There are many mechanisms, but in general, these proteins will be expressed in *gradients*. That is, there are more of them in some places, and less of them as you get farther away from their source. In this way, cells can tell how far away they are from specific points, and alter their gene expression accordingly. 

A protein that is only expressed along the midline will help nearby cells (which will receive a lot of stimulation from it) start along the pathway to becoming vertebrae, whereas cells that don't sense so much of it will instead activate other genes, perhaps turning into limb buds. The left and the right sides will *both* be similarly far away, and so similar developmental processes will create limbs on both sides. 

So, it's not about telling cells they should start making a left arm, but rather letting those cells know they're in the right place with the right potential to form *an* arm. There will ideally be other cells receiving the same message on the right side. This is why it is much more common for a genetic defect to affect both sides similarly: a defective morphogen won't work on either side. But accidents of development can occur due to chance environmental causes, and this is what's more likely to be responsible for a defect on just one side of the body.