Human DNA is roughly 2 billion triplets. That is comparable in information density to two thousand beach-read novels of 150,000 words each. Cell division keeps one set of DNA, reads it, and produces a second set for the new cell. Along the way a few dozen triplets get mis-copied.
How bad is that? Spend a lifetime reading the second set of novels, and it should take at least that long. Try to spot the several dozen typos spread across two thousand books.
But wait, there’s more! From your first cell, the fertilized egg, you have become something like a trillion cells—very very rough number, but used for illustrative purposes only. The direct route to a trillion cells requires forty cell divisions. So now, a) no two of your cells have perfectly identical DNA, and b) your two thousand novels in the final half-trillion cells have a few thousand typos each.
Life goes on, and there are miracles in that—and all the fuzziness in cell division also makes evolution unstoppable.
Truth in advertising: the special division that produces two eggs swaps major pieces across chromosomes then doesn’t copy – each resulting cell has a random half of the parent DNA.
All other cell divisions produce two whole cells, including the precursors to eggs and sperm. DNA copying errors not only introduce mild fuzziness, they can actually send one too many chromosomes to one cell and one too few to the other. This is how, for instance, Down Syndrome occurs – TWO chromosome 22’s in egg or sperm, THREE in the child. Other chromosome doublings, and (guessing here, but duh!) one too few of anything means the zygote gets aborted early. XXY and XYY babies, even XXYY in very rare cases, happen the same way. Life is sooooo much more complex than we realize.
Ditto, individual genes can double up, and move. The gene for skin pigmentation SWARMS over virtually every chromosome in individuals from most of Africa. Those whose skin is “blue-black” have the greatest number, while the gene is virtually absent in sunburn-prone Nordic folk. This is why Southerners found it within their imaginative reach to tell just how many African vs how man European great-great-grandparents a child might have, based on how many of those melanin-producing genes came through the process of the half-Dad – half-Mom DNA mixmaster.
Mark Twain even put that circumstance into his novel Pudd’nhead Wilson, in which a mostly-European “black” nanny swapped her child for her mistress’s child, and raised the all-European boy as “hers” and black, with her own son raised “white.” And this was just at the end of the 19th Century, when his fame allowed him to get away without being lynched for such an infamy. Put me on record as despairing over the humanity of all deluded nincompoops who consider the child of two humans to be less-human based on their great-great-grandparents.
Oh, and Another Thing 