system
Given the following post, respond as if you're commenting on Reddit:
human
How much can we learn or know about an unknown creature from just DNA?

If we were to get only a tissue sample from an unknown species or alien, could we determine if that alien was a vertebrate?  How many limbs it had?  What it looked like?  

I know that DNA is the blueprint for life, but can we decode it well enough to "read the directions" yet?
gpt
If it were an unknown Earth species, we would eventually be able to determine its most closely-related currently living species or groups of species. Usually when this is done, there is already a pretty good idea of relatedness, so here, considerably more effort would be required. Then we can say "it probably looks a lot like these guys and has features x,y, and z that they all share."

Our knowledge of what specific genes do is very limited. This unknown species may be found to have the gene that codes for a known blue pigment for example, but whether the creature is ever blue is much more complicated. If you'd never seen humans but had a sample of their genome and looked at closely related extant Earth species, you might expect us to have tails. 


This is because of the actual processes of which genes are functional, when they're functional, how they interact, and the incredibly complex process of building the organism are not obvious from the genome. The genome doesn't say "make an arm," it says "when this molecule changes to this shape, transcribe this section of DNA to make this molecule."

As for an alien species, we'll probably never know until we find some aliens. It could be most of our knowledge of Earth applies, because the ways in which genetics function on Earth are pretty much the only way to make things work. Perhaps more likely is that big differences in how DNA and the most integral DNA-related molecules and organelles evolved mean that we'd have to learn a new set of molecular biology. For example, DNA could be written in the same base pairs, but in a different "language" of exons, introns, promoters, etc. Assuming they *have* DNA.

Besides all that, due to the building-on-what-worked-before nature of evolution, even if their biochemistry was similar, they'd probably be incredibly weird and not fit into our traditional types of organisms.