system
Given the following post, respond as if you're commenting on Reddit:
human
What does '98% genetically identical' mean? If something were 100% genetically identical to humans would it also be a human or would it just act and work like we do?
gpt
There's a pretty huge issue in the common nomenclature of "genetically identical" or how "related" things are. There are *many* ways to measure relatedness, and not all of them are the same. I'll explain it right here because it's awesome, if you want a direct answer then just skip to the bottom.

When you're talking about relatedness between species, generally the entirety of the genome is being analyzed. So you have billions upon billions of little bases on the DNA, letters in code of ATCG, and relatedness is simply "number of bases in common divided by total number of bases." This has huge issues, however, because genes are always changing. They don't stay put, and it can be difficult (and subjective) to decide what the relatedness is in situations where genes have moved, changed, or duplicated. What if a chimpanzee has ATCGATCG in a certain part of the genome but a human only has ATCG? Does that the species 50% related or 100% related? What if the chimpanzee has AAAAATCG while the human has only ATCG? They could be considered closely related if you realize that the ATCG at the end is the same thing and "realign" the genome, or they could be considered poorly related if you don't realign. 

Those are issues with relatedness between species, which is what your question is getting it. There is also relatedness *within* species. This is an even hairier issue. You've heard people say that you're 50% related to your sister, right? How could you be only 50% related to your sister and also be 98% related to a chimpanzee? That's because when we talk about relatedness within species, we're usually looking at relatedness along variable genes, not just overall. So your mom and dad might have different genes for skin tone, hair color, height, and whatever else: half of these will be inherited by each child. Each child is halfway related to each of their parents and, thus, halfway related to each other, but *only for variable genes.* You will certainly be 100% for the vast majority of the genome, because lots of that stuff doesn't change. Many crucial genes, like those which cause cells to form and hearts to beat, often will have no variation (because any change would be fatal). Relatedness between species takes this into account and gets very high values while relatedness within species only looks at a small chunk of genes, which are the variable ones. 

So, now you should know that your question is most definitely aimed at the broader, species-level designation of relatedness. 98% genetically identical means that 98% of DNA bases (A, T, C, or G) are shared are shared between the two species. If something were 100% genetically identical to humans, that would mean that 100% of the genes found in its body are also found in the human population. It would necessarily be human if that were the case; it would contain no non-human information in its genome.