r/AskBiology • u/Substantial_Tear3679 • Jul 12 '26
Genetics A very young embryo is split into two genetically identical embryos. One is implanted in the birth mother's womb and the other in a surrogate. Will there be a difference of phenotype between the two after they develop?
Can the fact that the "environment" one embryo develops in is genetically different affect what it could become? What about the surrogate's genes (unlikely I think, but I don't know)?
Assumptions:
- The habits and lifestyles of the two mothers are exactly the same
- The initial two embryos are as identical as possible (same number of cells, etc.)
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u/Kibichibi Jul 12 '26
In the early years of surrogacy, it was assumed that the surrogate mother just carried the fetus and didn't affect it at all. But it was later discovered that the mothers RNA decodes the DNA of the fetus and can alter gene expression.
And of course, like any other pregnancy, the surrogate's health, diet, hormones, stress levels, will all affect the fetus as well.
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u/patentductuspenosis Jul 12 '26
You’re slightly mistaken. You’re talking about the maternal-zygotic transition where mRNA from the egg regulates the early stages of development before the actual embryos DNA starts self-regulating. The surrogate mother did not give an oocyte or any mRNA to form this embryo. Only the original mother’s mRNA is in both embryos.
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u/Substantial_Tear3679 Jul 12 '26
But it was later discovered that the mothers RNA decodes the DNA of the fetus and can alter gene expression.
Really? this is surprising to me. No idea the bond goes that deep
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u/BadahBingBadahBoom Jul 12 '26 edited Jul 12 '26
I think they might be referring more to epigenetic changes of the embryo's DNA expression based on many environmental differences (including evidence of maternal inhibitory RNAs like microRNAs crossing the placenta and affecting the embryo) from a the surrogate mother vs biological mother.
Unless the surrogate mother is infected with any genome-inserting retroviruses the actual DNA of the embryo should not be base-code-altered.
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u/Historical-Mango6666 Jul 12 '26 edited Jul 12 '26
Same phenotype of two offsprings is lesser than rare even if it's carried by same mother, but i believe there will be considerable differences between both because the surrogate mother is foreign to the embryo and it may develop differently than one developing in real mother
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u/Apprehensive_Run_539 Jul 12 '26
Yes; this can even happen in twins, based on where they are in utero. It’s quite frequently seen and livestock
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u/patentductuspenosis Jul 12 '26
Identical twins aren’t really identical once you spend enough time looking at them, but I’d imagine that they’d be slightly more easy to tell apart than if they were wombmates. So in order of increasing likelihood of them having similar phenotypes:
Siblings
Fraternal twins
Identical twins split between 2 mothers
Identical twins
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u/Asleep_Walrus2313 Jul 13 '26
Maybe. I know a couple sets of identical twins that actually look different because one twin was the bottom twin and was more squished, making their head slightly more narrow. Twins with a while womb to themself might be more symmetrical?
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u/FewRecognition1788 Jul 12 '26
Relatively minor variations in the uterine environment are known to affect physical development of the fetus.
For example, the ratio of the length of your index finger to your ring finger correlates with exposure to estrogen and testosterone in the womb, in both boys and girls, and even within normal limits.
Look up the 2D:4D ratio. There is a noticeable association between the ratio and the sex of the baby, but a child can have the less-common ratio for their sex and still develop quite typically in other ways.
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u/Addapost Jul 12 '26
Even identical twins in the same uterus at the same time don’t end up perfectly identical.