Humans, the latest tally suggests, have approximately 21,000 genes in our genome, the set of genetic information in an organism. But do we really need every gene we have? What if we lost three or four? What if we lost 3,000 or 4,000? Could we still function? Humans have variation in their genomes, but the overall size does not vary dramatically among individuals, with the exception of certain genetic disorders like Down’s syndrome, which is caused by an extra copy of chromosome 21 and all the genes that it carries.
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Source: Phys.org