# Are cousins with twin parents genetic siblings? **URL:** \ **Category:** Factual Questions\ **Created:** [August 15, 2020, 2:07am UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146 "2020-08-15T02:07:53Z")\ **Posts on this page:** 8\ **Page:** 3
**Author:** ![brossa](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/brossa/32/3040_2.png) [@brossa](https://boards.straightdope.com/u/brossa)\ **Post date:** [August 16, 2020, 1:35pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/41 "2020-08-16T13:35:57Z")
> [@Temporary\_Name](#): > > I recently heard a podcast where a former Mormon claimed that along the Arizona Utah border where polygamist marriages are common and inbreeding is very common, that the women often give birth to what he described as a cell mass of protoplasm. (If I am remembering his terminology correctly.) “Cell mass of protoplasm” might be a garbled understanding of _molar pregnancy_, or gestational trophblastic disease. The risk of GTD is higher among mothers below 20 and over 35 years of age, and with paternal age over 45. It’s possible that a given community would have more pregnant women and fathers in these age groups than the general population would. There are also reports of familial increased risk for GTD. A cursory Google search didn’t turn up any particular association with LDS and GTD, though. “Cell mass of protoplasm” could also just be garbled bullshit, too. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 16, 2020, 2:25pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/42 "2020-08-16T14:25:59Z")
> [@lobotomyboy63](#): > > One egg gets fertilized and splits, giving twins. Split again, quads. But how to get to triplets? The identical triplets article suggests that sometimes they began as identical quadruplets and one was absorbed. Not really. How about “One egg gets fertilized and splits, giving twins. Of the two now completely independent growing cells, one splits and one does not. Now there are 3. The rest of the development process continues per normal.” If that’s how it works we’d naively expect that the prevalence of identical twins vs singletons would be X%, triplets vs twins would be about X2, and quads to triplets would be about X3. Per [wiki](https://en.wikipedia.org/wiki/Twin#Monozygotic_(identical)_twins) X for identical twins vs singles is about 0.3% Less of course all the partial or total miscarriages involved in any newly fertilized egg. Which of course become more likely as the number of developing blastocysts then embryos compete. See this article on [multiple births](https://en.wikipedia.org/wiki/Multiple_birth#Triplets) for more supporting my contention. Which also admits that identical multiples are so rare that the error bars are much larger than the statistics. ---
**Author:** ![Temporary\_Name](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Temporary\_Name](https://boards.straightdope.com/u/Temporary_Name)\ **Post date:** [August 16, 2020, 2:59pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/43 "2020-08-16T14:59:06Z")
A couple of clarifications to start- first the LDS church proper does not endorse nor allow polygamy; they abandoned the practice over a hundred years ago. But there are splitter groups that still practice those ways and they were in the news within the last few years and do seem to cling to the border between the two states to avoid investigation by either. Second, he definitely meant mental disabilities (and I think he mentioned specifically those along the autism spectrum but I couldn’t remember exactly so phrased it more generally so I wouldn’t have to slog through an hour and eight minute podcast.) After having heard news reports (and a few rumors about “those Mormons” from proper LDS friends), then hearing this podcast—I have no trouble believing the communities being discussed have both the inbreeding problem, and the age challenges mentioned by **brossa**. I will gladly link to the podcast if anyone would care to hear the source material. In fact, I will describe it so it may be easily found: it is Seth Andrew’s The Thinking Atheist Podcast #112; The Mormons from June of 2013. (And it is over an hour and a half long.) ---
**Author:** ![Temporary\_Name](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Temporary\_Name](https://boards.straightdope.com/u/Temporary_Name)\ **Post date:** [August 16, 2020, 3:48pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/44 "2020-08-16T15:48:21Z")
The part I was making reference to starts around 45 minutes in. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [August 16, 2020, 6:45pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/45 "2020-08-16T18:45:45Z")
> [@doreen](#): > > That’s true- but that’s not the sort of disability they’re hoping for. SSI isn’t only for people with physical disabilities - people with intellectual disabilities also qualify. True - but fumarase deficiency a.k.a. “Polygamist Downs” which has spiked in FLDS poly communities isn’t _just_ intellectual disability. Some folks might be hoping for just “funny in the head” disabilities but it’s all a roll of the dice. > [@Temporary\_Name](#): > > A couple of clarifications to start- first the LDS church proper does not endorse nor allow polygamy; they abandoned the practice over a hundred years ago. But there are splitter groups that still practice those ways and they were in the news within the last few years and do seem to cling to the border between the two states to avoid investigation by either. Yes. That is why I am careful to abbreviate them as “ **F** LDS”, the Fundamentalist Latter Day Saints which are definitely not mainstream Mormons. And thank you VERY much for stating where in the podcast what you refer to is located. I wish more people would do that. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 16, 2020, 7:40pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/46 "2020-08-16T19:40:16Z")
**Moderator Instructions** I think all the discussion of Mormons and polygamy is getting off track with respect to the OP. While inbreeding is of interest, let’s try to stick more closely to the subject of the OP. Colibri General Questions Moderator ---
**Author:** ![lobotomyboy63](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobotomyboy63/32/2900_2.png) [@lobotomyboy63](https://boards.straightdope.com/u/lobotomyboy63)\ **Post date:** [August 16, 2020, 9:49pm UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/47 "2020-08-16T21:49:32Z")
> [@LSLGuy](#): > > Not really. > > How about “One egg gets fertilized and splits, giving twins. Of the two now completely independent growing cells, one splits and one does not. Now there are 3. The rest of the development process continues per normal.” Yes, that too. The article says: > [@](#): > > It is rare for triplets to be completely monozygotic, meaning that all three children formed from a single egg that split three ways or splits into two and then one of the two splits again, ultimately resulting in three embryos with the same general DNA characteristics. Monozygotic triplets are always of the same gender, either all boys or all girls. Some monozygotic triplets are actually [quadruplets](https://www.verywellfamily.com/quadruplets-quads-2447324) where one embryo has [vanished](https://www.verywellfamily.com/vanishing-twin-syndrome-2446714) or been reabsorbed.1 ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [August 17, 2020, 5:23am UTC](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146/48 "2020-08-17T05:23:47Z")
Putting infants’ footprints on birth certificates started well before the 1960’s. [Previous page](https://boards.straightdope.com/t/are-cousins-with-twin-parents-genetic-siblings/918146.md?page=2)