Mice and humans have 20,000 genes, give or take a few. The shocking thing is that 99% of the genes are the same.
In the preface to their 2014 book "Systems Biology of Metabolic and Signaling Networks" the authors say:
"Certainly, life is more complex and far-reaching than our genes, at least by the numbers. This is one of the first lessons gleaned from sequencing the genome of species with diverse lineage and evolutionary paths: the number of genes and core proteomes [proteins] does not correlate with their apparent complexity. For example, the basic proteome of the human genome is not much larger than that of the fly and the worm, but human complexity is."
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https://www.youtube.com/watch?v=SUIjRvv620U