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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Twiblings, four-parent babies, and other interesting consequences of advancements in reproductive technology, published by GeneSmith on May 20, 2023 on LessWrong.
Two weeks ago I finally published my year-long research project into how to have polygenically screened children. In that post, I explained all the practicalities of embryo screening and how parents can use it right now to make their children smarter, happier, and less prone to disease.
This week I'm going to talk about some of the more off-the-wall methods that have been proposed for increasing genetic gain across a variety of traits, along with their implications for family structure, society, and the human experience.
What are Twiblings?
A twibling is somewhere between a sibling and a twin. If identical twins share 100% of their DNA and siblings share about 50%, twiblings share 75%.
To the best of my knowledge, twiblings don’t exist in nature. To get one, you’d need two different genetically identical eggs to be fertilized by different sperm. But why would we create twiblings?
Anyone who has gone through IVF knows that the main limiting factor in all cycles is the number of eggs the female partner can produce. In an average menstrual cycle, a woman will produce exactly one egg. In an average ejaculation, a man produces about 100 million sperm. Using a specialized set of medications, we can increase the number of eggs harvested per menstrual cycle to between 5 and 100 (there's a lot of variance depending on maternal age and other factors). But even 100 is a lot less than 100 million.
Older women in particular have a very hard time producing eggs. Women over the age of 42 often produce 5 or fewer per retrieval, and many of those have chromosomal issues that prevent them from turning into a baby. So the few eggs without such issues are very precious.
It sure would be nice if there was a way to duplicate the few viable eggs these women can produce.
As it turns out, this is possible! And we’ve already done it in the lab. You can make a haploid stem cell line by tricking an egg into thinking it’s an embryo; you let a sperm fertilize it, then you pull out the sperm’s pronucleus before it fuses with that of the egg.
Success! The egg now thinks it’s an embryo and will start dividing.
So now you have a bunch of haploid embryonic stem cells, each of which is fairly epigenetically similar to an egg.But there’s still one tricky step left; how do you derive an egg from these haploid embryonic stem cells such that it can be fertilized again?
The most obvious answer is to use nuclear transfer; pull the haploid nucleus out of these embryonic stem cells and stick it in another egg. So you do this for a bunch of the haploid embryonic stem cells using a bunch of other eggs and now you have many eggs from the same mother!
Of course this necessitates HAVING other eggs, which we already established are in short supply. But thankfully, those other eggs don't need to come from the same woman. You can get donor eggs without too much trouble. And if you don’t care much about the donors' DNA you can get them for quite a bit less money.
But if you can clear that hurdle you now have a very interesting situation: you’ll have multiple genetically identical eggs, each of which can be fertilized by a different sperm.
If the mother decides to have multiple children, they will be more genetically similar than any siblings, but less similar than identical twins. Twiblings!
Twins of different ages
At the 2, 4 or 8 cell stage, the cells in an embryo can turn into any tissue in the body or placenta. This leads to a natural question: if we split the embryo in half at that stage, will it form two embryos?
It would! Or at least it did in other mammals we’ve studied. For some reason, no one seems to have tried this in humans yet. I don’t fully und...
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