# Could two siblings share no DNA whatsoever? Is so, could they safely have children? **URL:** \ **Category:** Factual Questions\ **Created:** [December 1, 2016, 3:00pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532 "2016-12-01T15:00:44Z")\ **Posts on this page:** 7\ **Page:** 4
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [December 3, 2016, 8:41pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/61 "2016-12-03T20:41:47Z")
> [@Annie-Xmas](#): > > Since you get half your DNA from each parent, could two siblings have no DNA connection whatsoever? And how little DNA would they have to share to safely have children? Regarding “safety” I can’t recall the exact thread but a bio-savvy doper indicated in a previous thread regarding incest or inbreeding that the real world risk of serious genetic anomalies with biological sibling incest is certainly higher with bio-siblings, but overall the chances are the large majority of the kids from this as one off scenario would be relatively normal unless some serious recessives lined up. Where you see real problems crop up is across multiple generations of closely related first cousin type breeding with a restricted population like the Amish or some restricted/limited gene pool communities like some hill people. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 4, 2016, 4:33am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/62 "2016-12-04T04:33:06Z")
Oh, and since the question of fraternal twins vs. siblings came up, one reason for preferring to compare identical twins to fraternal twins, rather than just to ordinary siblings, is that some conditions are caused by the intra-uterine environment (exposure to hormones pre-birth, for instance), and looking at twins controls for that. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [December 4, 2016, 1:11pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/63 "2016-12-04T13:11:57Z")
> [@astro](#): > > Regarding “safety” I can’t recall the exact thread but a bio-savvy doper indicated in a previous thread regarding incest or inbreeding that the real world risk of serious genetic anomalies with biological sibling incest is certainly higher with bio-siblings, but overall the chances are the large majority of the kids from this as one off scenario would be relatively normal unless some serious recessives lined up. Where you see real problems crop up is across multiple generations of closely related first cousin type breeding with a restricted population like the Amish or some restricted/limited gene pool communities like some hill people. Maybe **The Scrivener** ’s post [here](http://boards.straightdope.com/sdmb/showpost.php?p=9027411&postcount=25)? Yeah, “small” in absolute terms. Large in relative\* change. And that’s just for one off occurrences of cousin inbreeding. For sibling inbreeding, it’s got to be at least double that. “Don’t try this at home.” - NPI ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 4, 2016, 7:17pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/64 "2016-12-04T19:17:57Z")
> [@astro](#): > > …Where you see real problems crop up is across multiple generations of closely related first cousin type breeding with a restricted population like the Amish or some restricted/limited gene pool communities like some hill people. Just to clarify this: it’s a common misconception that the increasing problem in such communities is due in some sense to deteriorating “genetic quality”. In fact, the cumulative effect is only the increasing probability of consanguinity. If an individual from such a population enters outbred marriage with an unrelated individual, there should be no problem. ---
**Author:** ![Lamia](https://avatars.discourse-cdn.com/v4/letter/l/8e7dd6/32.png) [@Lamia](https://boards.straightdope.com/u/Lamia)\ **Post date:** [December 4, 2016, 7:31pm UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/65 "2016-12-04T19:31:02Z")
> [@md2000](#): > > As I read once, the reason for sex tests in the Olympics, before the Soviets and their amazing potions, was due to a developmental anomaly. If something interrupts the development process at just the right point, an XY fetus could fail to complete the emergence of male genitalia; the resultant child would appear to be female externally, but the vagina would be very short, no uterus, and the undescended testes would be in the position the ovaries should be and not produce male levels of testosterone. They would grow up to all appearances female, except infertile. **They would have a tendency to better physical performance with slightly more testosterone than a typical female, hence over-represented in competitive sports.** (Emphasis mine.) Bit of a hijack, but I don’t think the bolded portion is correct. You’re describing women with androgen insensitivity syndrome (AIS), who are genetically XY but do not develop male bodies because (as the name suggests) their testosterone receptors are insensitive to the hormone. These women do have higher testosterone levels than is typical, but as I understand it, it’s basically just floating around doing nothing. A quick Google indicates that the International Olympic Commission (IOC) does not consider women with AIS to have an advantage over other women when it comes to sports (quote is from a [2011 article in Nature](http://www.nature.com/news/2011/110414/full/news.2011.237.html)): > [@](#): > > The IOC and IAAF concluded that, because such women [with AIS] are resistant to androgens, they gain no competitive advantage from their high testosterone levels and have exempted them from the ban. > > “If you have AIS, you should still be able to compete,” says Malcolm Collins, a medical biochemist specializing in sports medicine at the University of Cape Town, who was not involved in drawing up the guidelines. I have heard that women with AIS tend to be tall so that probably gives them an advantage in some sports, but of course there are many tall women who are genetically XX. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [December 5, 2016, 5:33am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/66 "2016-12-05T05:33:54Z")
There was also some reference to testosterone being converted to equivalent female hormones that I ran across? (IANABiologist) It also mentioned that this conversion was responsible for younger adolescent males sometimes starting developing breasts early in puberty. (Which went away as things sorted themselves out) ---
**Author:** ![JoseB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/joseb/32/1530_2.png) [@JoseB](https://boards.straightdope.com/u/JoseB)\ **Post date:** [December 5, 2016, 7:51am UTC](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532/67 "2016-12-05T07:51:44Z")
> [@md2000](#): > > Just thought I’d throw in the comment about the most recent notorious case of interbreeding, Mr. Fritzl of Austria \<…\> Interestingly enough, the most notorious case of deleterious inbreeding ever, King Charles II of Spain, is somewhat related to Austria… He was from the house of Hapsburg, and I read somewhere that due to many generations of cousin-cousin and uncle-niece marriages his genome was actually **more** inbred than the genome of the offspring of two siblings (!!). The poor guy was beyond pitiful, it seems, if contemporary diplomatic reports are to be believed. Even his official portraits, extra-flattering as they are, cannot hide his deformities. [Previous page](https://boards.straightdope.com/t/could-two-siblings-share-no-dna-whatsoever-is-so-could-they-safely-have-children/773532.md?page=3)