This article is about a pair of twin sisters who married a pair of twin brothers, and are both pregnant.
The article claims, “The sisters’ children won’t just be cousins, but will be full genetic siblings and quaternary multiples.”
Firstly, are they really “genetic siblings”? How is that defined, exactly?
Second, what the heck is a “quaternary multiple”? I could not find a definition of that phrase anywhere on the internet.
“Genetic sibling” is not a technical term, it does not mean anything. They are clearly not siblings, since by definition siblings have the same parents. It’s no more correct to call these two “genetic siblings” than it is to call identical twins “genetic the-same-persons”.
They are cousins. They could be referred to as “double cousins”, that refers to the more general case where the parents of cousins are two pairs of (not necessarily identical) siblings.
What is true is that since their parents are genetically identical, they have the same genetic similarity (r=50%) as siblings.
“Quaternary multiple”? I have no idea.
What it means is that they will have the same degree of genetic relationship as full siblings. Two siblings share 1/2 of their genes by descent from their parents. Ordinary cousins will share 1/4 of their genes by descent. Because the parents of these cousins are genetically the same, they will also be related by 1/2.
I have never heard of a “quaternary multiple.” I think they must have just made that up.
I’m looking forward to that Maury show: “DNA tests confirm you are BOTH the father!”
All of the kids get their genes from the same pool, their fathers have the same genes as each other as do their mothers. Each of their kids could have been born to the other parents or from a different combination of their parents. They are not legally siblings, but there is no way to determine their parentage through their genes.
Not by the usual straightforward fingerprinting. And not trying to be pedantic but just as a matter of interest - there are always some differences that arise as de novo mutations even between identical twins, notably copy number variation. Much more extensive sequencing is required, but sequencing an entire genome only costs a few thousand dollars these days. If enough DNA is available to do more extensive sequencing, and there is some pressing need to find out, it will almost certainly be possible.
…on average? Couldn’t they end up with more or less than exactly 50% of their genes in common?
Yes, it’s an average. Because of crossing over between chromosomes, there is a range of values between siblings of between 38-61%.
Double first cousins. My Daddy has them.
His dad and uncle married a pair of sisters.
The kids are double first cousins.
Next, if you want, I’ll tell you about my brother who isn’t my brother.
Do tell, Beckdawrek!
In the UK, there is no legal impediment to stop first cousins from marrying. I believe it is more common among Asian immigrants. I wonder whether double firsts would run the same risks as siblings in respect of children.
Only if their parents were identical twins.
Double first cousins will share more genes in common than non-double-first cousins - that’s genes both good and bad. Assuming no identical twins involved, siblings will always be more closely related than cousins.
While Charles II of Spain is often held up as an example of why too many first-cousin marriages are bad his full sister Margaret Theresa is proof that an incestuous background does not always impose calamity. But the Spanish Hapsburgs had an unusually straight family tree pole with startlingly few branches. Outside of royalty, such inbreeding in humans is very rare. Actually, inbreeding to that degree is pretty rare even among royalty. As the Hapsburgs demonstrated (as did a few Ancient Egyptian dynasties) after a few generations fertility drops off significantly. There are conceptions but few live births or long-surviving infants and if the family persists in that form of mating eventually you get no offspring at all.
This could be a trial balloon for a reboot of the Patty Duke Show.
Correction: Ordinary double cousins (where the parents are two sets of siblings, but not identical twins) will share 1/4 of their genes by descent. Ordinary single cousins (by far the more common case) share 1/8.
And if I understand the statistics correctly, crossover actually narrows the range of variation, by increasing the effective number of “coin flips”. If every chromosome were discrete and indivisible, then you’d have 46 coin flips to determine the offspring of a couple, and so expect to have something like 23 ± 7 of them shared between siblings, but if you have on average one crossover per chromosome, then that’s more like 92 coin flips, and so you’d expect 46 ± 10 of them shared between siblings.
Thanks for correcting my slip. Grandparents/grandchildren, aunts or uncles/nieces or nephews, and half-siblings are also related (on average) by 1/4. Ordinary cousins are related by 1/8.
Once you get down to second cousin it’s just 3%, and such marriages are often permitted.
I hope the Moms deliver the kids in separate hospitals. If any mix-up occurred taking the newborns home there’d be no way1 to straighten it out.
Or would it be a mixup at all? If identical babies plop out and nobody is around to see them separately, are they one person? Enquiring minds want to know!
For extra double(-mint2) fun, consider if these pregnancies also turn out to be identical twins. Fast forward 16 years to the Prom and all sorts of things could go wrong, go wrong, go wrong …
Modulo @Riemann’s comment just below yours that Modern Science! has an answer.
Those of us of a certain age remember those ads all to well.
Actually there is, if you took a blood sample or fingerprints of the infants at birth and kept track of which mother each was born to. The infants are only going to be as similar as fraternal twins, not identical twins. There will only share on average 50% of their genes by descent. And the fingerprints of even identical twins differ from one another, due to differences in development in the womb. The infants could also differ in features such as birthmarks or other physical features.
Actually, I’ve heard the term “genetic sibling” used for this situation before, and even “genetic half-sibling” for the children of identical twins married to unrelated people. The context was matching bone marrow donors. I don’t know what would make it a “medical term,” but it wasn’t invented by the author of the article referenced in the OP.
I think “quaternary multiple” is a $10 term to express something like “second generation multiple.” Since the babies are going to be born around the same time, and grow up in one household, and one assumes, have the same peer group, they will be socially like fraternal twins, ie multiples, more than cousins (I sort of get this idea, because I was practically raised by my aunt-- I was in her house more than my parents’, I think; I have a cousin very close to my age, and we were constantly asked if we were twins-- we look quite a bit alike). But they weren’t gestated together, hence the qualification of the word “multiple.”
I had a baby in 2006, and the precautions against both mixing up babies, and allowing someone to take a baby off the floor were just crazy. They can only be more involved now.
I doubt they will fingerprint the babies at birth, but they will take blood samples, and it’s possible they will have different blood types. They will weigh them, and yes, babies’ weights change, but the bigger one will probably remain the bigger one.
They will also immediately put bracelets on them, and anklets that have an electronic connection with bracelets on the mother. The ones my son and I had played a tune (briefly) whenever he got within a couple of feet of me. If another baby with a non-matching bracelet had gotten close to be, they would have buzzed.
Then, cord blood is often preserved. And in this case, it might be preserved and labeled “just in case.”
While the babies cannot be ID’d by being matched to their parents, they can be ID’d by being matched to their own DNA.
The author of the article is quoting a tweet by the parents. Informally, it’s obvious what they meant, probably based on a mangled understanding of genetic terminology. But it’s not correct, and it’s certainly not the terminology a geneticist would use. Your degree of genetic identity by descent does not define your familial relationship to someone. Even without this highly unusual familial situation with two pairs of identical twins, it’s entirely possible for cousins to have 50% i.b.d. through inbreeding. This would not magically make such cousins into siblings, they are still inbred cousins. Also, of course, there is no one-to-one mapping between familial relationship and degree of i.b.d.
Where I’ve seen the term used colloquially before, in reference to actual siblings, it’s a less common synonym for “biological siblings”, i.e. to distinguish from adoptive siblings.