The sum of all the DNA in an organism is called the organism's genome. Studying the genome can reveal quite a bit. Scientists know, for example, that a gene exists for a trait like eye color. But sequence an entire genome, and they see that a lot of genes whose purpose isn't clear. They scratch their heads and say "I wonder what this one does?" or "Wow, the sequence of that gene looks a lot like a particular fish gene whose function we do know." Having a whole genome lets scientists asked questions about genes in a completely different way.
From an evolutionary point of view, the genome is intriguing because it presents evolutionary scientists with a bunch of deep questions to ponder like, if genome sizes are different for different organisms (and some in ways that make no obvious sense), then how did that happen? Or why does so much of the genome seem to be junk (and why is this the case in some species but not others)? Any why are similar genes in different places on the genomes of different species?
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