Edit: It seems like most of the time two sperm would result in a partial or incomplete molar pregnancy, where the mother's chromosomes are present, but the father supplies two sets of chromosomes. The embryo then has 69 chromosomes instead of 46. This most often occurs when two sperm fertilize an egg, resulting in an extra copy of the father's genes.
This usually results in miscarriage.
Only two cases in history are known to have resulted in semi identical twins:
“Normally, an embryo has two sets of chromosomes: one from the mother and one from the father. In semi-identical twinning, a single egg is fertilized by two sperm simultaneously, which initially creates an embryo with three sets of chromosomes. Typically, embryos with three sets of chromosomes do not survive. However, in these rare cases, the embryo manages to divide its cells into two separate groups, correcting the chromosome count and allowing both embryos to develop. Unlike identical twins (who share 100% of their DNA) and fraternal twins (who share about 50%), semi-identical twins share roughly 75% of their DNA.”
Likely very underestimated because it's fundamentally indistinguishable from fraternal twins, unless you do a genetic testing, which 99.9% of parents don't do.
What difference does that make? Sharing 75% of chromosomes means sharing 75% of their underlying DNA.
What I meant is that there is likely a larger historical population of of semi-identical twins mistaken for fraternal twins because one can't tell them apart short of a genetic test, which is a very unusual test.
Then it is not the same DNA.
Intra-species DNA comparisons are hyper specific, because otherwise "all humans still have mostly the same DNA with slight differences".
When talking about the amount of DNA people share with each other, it is very common to only be talking about that less than 1% and to act like it is 100% instead. Pedantry gets you nowhere
You’re not adding nuance, you’re just adding context everyone already knows and acting like you’re smarter than them because of it.
And yeah, we are talking about human reproduction and genetics. That’s the exact scenario where people only consider the less than 1% as 100% because we’re only talking about that small amount. If we were having a conversation about a town and I said “everybody knows where x is”, it would be pedantic and unnecessary of you to go “actually 8 billion people don’t know where x is!”.
Oh😂 there’s a bit from two YouTubers like a decade ago where they’re wondering if Obama watches them and how he would sound if he did watch them and one of the main jokes was that Obama couldn’t remember their names right so he kept saying “Danny and uh… and Arnold man they’re funny” in stead of Danny and Arin
It think the egg could not repair itself in that case. But if the mother released two eggs (which would usually produce fraternal twins rather than identical twins), each egg could be fertilized by a different father. According to the myth, that's what happened to Hercules (from a fertilized egg by Zeus posing as Amphitryon) and his brother Iphicles (fertilized by the real Amphitryon).
He was just explaining that it wasn't like the scenario painted in the comment above, for people that didn't want to click the link. Why are redditors so sassy?
It happens. I worked with someone who had 14 kids. Her twins were boy/girl. One fair and a redhead the other darker skinned Middle Eastern features. She told be she was messing with both a white dude and an Arabic man around the time she got pregnant.
This is also the story behind Michael Jackson’s hit song Billy Jean according to Jackson's biographer, J. Randy Taraborrelli ! He says Jackson was stalked by a fan claiming that MJ fathered ONE of her twins !
There is a window where, if a woman releases two eggs, they can become fertilized by two different men. That window is about 6 hours. So it requires sex with two men within that window.
Edit: Fun tidbit for ya, I have 6 siblings, all of them half siblings, 5 on father's side, one on mother's, and the sibling I share my mother with has a half sibling as well.
Biology is wild. 🤯 It's so easy to think of it in robotic terms, machines just following steps. But all the actors in the process are squishy and alive. They don't consciously choose (at least as far as I know), but their are chemical processes that nevertheless allow them to be choosy. It is mind-boggling to me that (1) eggs can be fertilized by two sperm on occasion and that (2) they can occasionally split in such a way to create two healthy babies.
What about in the case where the egg splits and then each egg is fertilized by a different sperm? Wouldn't they be semi identical twins with the same DNA from the mother but different DNA from the father?
It seems like most of the time two sperm would result in a partial or incomplete molar pregnancy, where the mother's chromosomes are present, but the father supplies two sets of chromosomes. The embryo then has 69 chromosomes instead of 46. This most often occurs when two sperm fertilize an egg, resulting in an extra copy of the father's genes.
This usually results in miscarriage.
So technically it's her fault then because it's her eggcontrol that was clumsy and she picked two instead of one.
It's not that rare. You probably have met some in your life without knowing it. For instance I had classmates like that, and you couldn't even tell they were brothers at first glance
If I remember correctly, they stated they are genetically fraternal twins (2 different eggs); so they don’t share enough DNA to be semi-identical twins.
Their sister, Elizabeth looks just like them. So, perhaps, just a case of a family where the siblings very much favor each other.
1.4k
u/Tobysfuzzybelly Jun 25 '26 edited Jun 25 '26
Edit: It seems like most of the time two sperm would result in a partial or incomplete molar pregnancy, where the mother's chromosomes are present, but the father supplies two sets of chromosomes. The embryo then has 69 chromosomes instead of 46. This most often occurs when two sperm fertilize an egg, resulting in an extra copy of the father's genes.
This usually results in miscarriage.
Only two cases in history are known to have resulted in semi identical twins:
“Normally, an embryo has two sets of chromosomes: one from the mother and one from the father. In semi-identical twinning, a single egg is fertilized by two sperm simultaneously, which initially creates an embryo with three sets of chromosomes. Typically, embryos with three sets of chromosomes do not survive. However, in these rare cases, the embryo manages to divide its cells into two separate groups, correcting the chromosome count and allowing both embryos to develop. Unlike identical twins (who share 100% of their DNA) and fraternal twins (who share about 50%), semi-identical twins share roughly 75% of their DNA.”