how there are people born with the wrong number of organs?

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There are people born with one, three or four kidneys or two livers and such. What are the reasons for the DNAs ‘ ‘failure’ (if it’s that)?

In: Biology

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Anonymous 0 Comments

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Anonymous 0 Comments

My best educated guess is not a DNA problem per se, but rather some cells got split or mixed up during development.

Embryonic development is fascinating and freaky stuff. I haven’t studied it *very* much, but here’s the low-down: You started as a single cell that divided to become a small ball/clump of cells. At this point any one of those cells can become any cell in your body. But pretty soon, the cells in that ball start separating into 3 major layers: one for your skin and nerves, one for your muscles and blood, and one for most of your internal organs. All sorts of things happen to them: The whole ball folds in on itself to form the tube that will become your mouth/stomach/intestines/anus. Your future-nerve cells move inward from your future-skin cells to form your brain and spinal cord. Sacs form that will become your pleural cavity and abdominal cavity, the fluid-filled bags surrounding your lungs and gut respectively. In short, you have very defined plans for what stuff grows where.

And when scientists mess with that early on, it messes with it later, too. For example, using a syringe to move a few fish embryo cells from the back to the front will cause a dorsal fin to grow on the belly. Moving bird embryo cells from the front limbs to the back will cause wings to grow in place of feet. *As long as they’re already at a certain stage.* Even if these cells are in the wrong spots, at that point they’re set for particular “plans”, and they’ll send signals to surrounding cells according to that plan. Hence growing an entire wing instead of a weird half-wing half-foot.

Back to your original question? My guess is that at some point early on, the clump of liver cells got split somehow. But each of them still has the “liver” plan, they’re sending signals to grow the proper blood vessels and nerves and whatnot, and so they each develop into their own full liver.

This is somewhat based on what we know about [identical twins](https://en.wikipedia.org/wiki/Twin#Monozygotic_(identical)_twins), which happen when a single zygote (fertilized egg) develops into two embryos. In other words, the *entire* early clump of cells splits. It doesn’t seem like too much of a stretch that this can also happen on a smaller level, within a more developed embryo, for just a single organ.