# Time travel and procreating with your bloodline (Need answer whenever) **URL:** \ **Category:** Factual Questions\ **Created:** [January 3, 2015, 7:07pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876 "2015-01-03T19:07:47Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [January 4, 2015, 1:54pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/21 "2015-01-04T13:54:59Z")
> [@dtilque](#): > > In fact, in the first book he appeared in (_Methuselah’s Children_) he refuses to have a relationship with a woman who’s his great-great-grandaughter, I think (possibly another “great” in there). She has no problem with the distant consaguinity, though. One of the entire reasons for the Howard Project was to interbreed long-lived people. The long-term effects were (a) long-lived people and (b) birth defects. Also, lots of redheads. Scientifically, he had cause to be concerned about birth defects. Also, at that point, he’s still within a couple normal lifetimes of having been raised in Missouri in the 20’s. I always read the consanguinity issues as avoiding the real issue, which is that he didn’t want to do it with one of his kids. By the time he’s having sex with his mom, his clone-sisters, his transgender partner of a couple centuries and a couple of human-bodied Pinocchio computer sex-kittens, I think he’s past that little quibble. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [January 4, 2015, 2:05pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/22 "2015-01-04T14:05:21Z")
Since there is crossover, you will share, almost certainly, very close to 50% of each of your parent’s and 25% of each grandparent’s genes. But taking TimeWinder’s scenario, assuming you were your own grandpa, you would end up with 1/4 + 1/16 + 1/64 + … = 1/3 of your grandmother’s genes and 1/7 of your grandmother’s genes if you did it with her instead. ---
**Author:** ![Happy\_Lendervedder](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_lendervedder/32/8488_2.png) [@Happy\_Lendervedder](https://boards.straightdope.com/u/Happy_Lendervedder)\ **Post date:** [January 6, 2015, 10:44pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/23 "2015-01-06T22:44:34Z")
> [@Colibri](#): > > It might be pointed out, however, that crossing over delays the complete elimination of genetic material from some ancestors. By the time you get to the sixth generation back, you have more ancestors (64) than chromosomes (46). If chromosomes were inherited as units, then at least 18 (and probably more) of your ancestors would be completely unrepresented in your genome. Crossing over means that some of their genes will probably still be represented by at least some fragments of your chromosomes. Is there any point where it’s a near-100% certainty that your ancestors/descendants will not have any of your genetic material anymore? Is it possible my dreamy bedroom eyes won’t make it to the 100th generation? ---
**Author:** ![Flander](https://avatars.discourse-cdn.com/v4/letter/f/8c91f0/32.png) [@Flander](https://boards.straightdope.com/u/Flander)\ **Post date:** [January 7, 2015, 2:55am UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/24 "2015-01-07T02:55:15Z")
> [@Smapti](#): > > According to a documentary I once saw on the subject of past nastification, you’ll grow up without a Delta brain wave, which may turn in handy if the Earth is ever invaded by giant brains. Thank you for this. 😃 ---
**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:** [January 7, 2015, 2:58am UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/25 "2015-01-07T02:58:30Z")
If you genuinely have dreamy bedroom eyes, then those will probably persist for quite a few generations, and might even become the default for humanity as a whole. If, on the other hand, your eyes are of merely normal seductive quality, then it’s a crapshoot. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [January 7, 2015, 6:05pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/26 "2015-01-07T18:05:58Z")
> [@Happy\_Lendervedder](#): > > Is there any point where it’s a near-100% certainty that your ancestors/descendants will not have any of your genetic material anymore? Is it possible my dreamy bedroom eyes won’t make it to the 100th generation? The answer will be hugely different if we’re talking about “any” of your genetic material, or specifically about the, oh, handful or so of genes which code for your ocular dreaminess. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 8, 2015, 11:49pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/27 "2015-01-08T23:49:02Z")
> [@TimeWinder](#): > > And that’s the only case that works at all. You didn’t specify which grandmother, but if it’s the Paternal grandmother (PgM), the same reasoning applies, except that now Johnny’s _father_ is a clone of PgM, which means Johnny’s father is female, and Johnny fades out of ever photograph ever taken. I didn’t take the OP’s question as him wanting to become his own predecessor. What if he waited until his natural predecessor was born and did the deed after that; likely an adulterous situation… He would end up with a great-great auntie or uncle that has a bunch of his DNA. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [January 9, 2015, 12:20am UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/28 "2015-01-09T00:20:15Z")
If we’re not talking about the problems of becoming your own literal grandpa, the match between you and your grandmother are no closer than the one between first cousins, which is legally/socially approved in many localities. There may be a slight increase in the incidence of the expression of hereditary diseases, but not strikingly so. The real effects of inbreeding come when you have repeated pedigree collapse. If your parents and aunt and uncle were also first cousins, marrying your first cousin means your offspring only have 4 great-grandparents instead of the expected 8–which is as close as a sibling mating. ---
**Author:** ![sbunny8](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@sbunny8](https://boards.straightdope.com/u/sbunny8)\ **Post date:** [January 9, 2015, 2:49pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/29 "2015-01-09T14:49:39Z")
I forget which Heinlein book it was, but in one of the later stories Lazarus Long ends up with two “daughters” Lazi and Lupi who are genetically identical to him except that they don’t have a Y chromosome. The two young women convince him to have a threesome with them by saying it’s basically masturbation (hah!) but also they point out that inbreeding is only bad if it reinforces bad recessive genes and the evidence suggests that Lazarus doesn’t have any of those to reinforce. Then he goes back in time and has sex with his own mother. RAH was a weird person. It’s worth noting that, in large populations of cats and dogs, this sort of thing happens regularly. Animals end up mating with their own siblings or parents no more and no less frequently than anyone else in the colony, and that generally doesn’t produce terrible effects. In a pride of lions, the alpha male will have no reluctance to mate with his own daughters. But when humans do this sort of thing, it’s usually not accidental, it’s done on purpose. For example, trying to keep the family tree of the royal family from including any peasants. THAT’S when you run into trouble. ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 9, 2015, 3:27pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/30 "2015-01-09T15:27:19Z")
> [@Lemur866](#): > > The real effects of inbreeding come when you have repeated pedigree collapse. If your parents and aunt and uncle were also first cousins, marrying your first cousin means your offspring only have 4 great-grandparents instead of the expected 8–which is as close as a sibling mating. This could also happen if two siblings from one family marry two siblings from another family. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [January 9, 2015, 5:05pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/31 "2015-01-09T17:05:34Z")
> [@Happy\_Lendervedder](#): > > Is there any point where it’s a near-100% certainty that your ancestors/descendants will not have any of your genetic material anymore? Well, remember your 100th generation descendant has to get their eye genes from somewhere. So there’s certainly a positive chance they’ll get one from you. In fact, assuming you do end up having a 100th generation descendant, the odds of them getting a particular gene from you have to be at least one in 7 billion or so (based on the current world population). Some quick analysis and very reasonable assumptions could probably get the odds down a lot farther. That’s leaving aside whether the gene mutates between you and them. Up to you whether that would be the “same” “gene” or not, if it’s a direct copy from yours but with a copying error along the line. Now, if your bedroom eyes are the result of the interaction of multiple genes, then things start getting much more ugly, since you’d need the 100th generation-vedder to get all of the appropriate genes, exponentially worse odds the more genes are involved. And of course, you’d need to also cut the odds for the chance of a mutation that changes things bedroom-wise. [And of course, this is all based on the assumption that humans are going to continue biological mating with random meiosis, and no genetic engineering. Not something I’d really expect 100 generations from now, but we can only go with what we know] ---
**Author:** ![vomit\_comet](https://avatars.discourse-cdn.com/v4/letter/v/73ab20/32.png) [@vomit\_comet](https://boards.straightdope.com/u/vomit_comet)\ **Post date:** [January 9, 2015, 5:44pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/32 "2015-01-09T17:44:37Z")
> [@Quercus](#): > > [And of course, this is all based on the assumption that humans are going to continue biological mating with random meiosis, and no genetic engineering. Not something I’d really expect 100 generations from now, but we can only go with what we know] People like Kurzweil want to mate with computers now, and Gates wants to kill 95% of the population, so, nothing will exist in 100 generations from now except solar powered computers with downloaded human brains. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [January 9, 2015, 6:11pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/33 "2015-01-09T18:11:20Z")
> [@Quercus](#): > > Well, remember your 100th generation descendant has to get their eye genes from somewhere. So there’s certainly a positive chance they’ll get one from you. Also note that, if you have any 100th generation descendants, you’ll likely be an ancestor of lots of people if not most people, and you’ll likely show up in multiple ancestor slots for a particular person. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [January 9, 2015, 7:06pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/34 "2015-01-09T19:06:55Z")
> [@dtilque](#): > > In fact, in the first book he appeared in (_Methuselah’s Children_) he refuses to have a relationship with a woman who’s his great-great-grandaughter, I think (possibly another “great” in there). She has no problem with the distant consaguinity, though. > > When I first saw the thread subject, I thought of a different Heinlein story, but perhaps we should leave that until this thread is [zombified](http://en.wikipedia.org/wiki/All_You_Zombies) Don’t forget “Time For The Stars” where IIRC the implication is that after getting back, he will carry on with his (greatgreat?)granddaughter. > [@Colibri](#): > > No, because the 1/4 that is passed on from each grandparent is independent of the 1/4 that is passed on from the other grandparent in each family. Two siblings get a random selection of the genetic material of their parents, so they will be 50% match typically. However, each child of those siblings will only get half (typically) of the genetic material; but their choices too are independent. Child AA of Parent A may get say, 25% of what came from the grandparents and matches parent and uncle/aunt B. Child BB will get a different set of 25% from that matching 50%, so they will in fact only (typically) match AA to BB about 12.5%. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [January 9, 2015, 7:39pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/35 "2015-01-09T19:39:16Z")
> [@md2000](#): > > Don’t forget “Time For The Stars” where IIRC the implication is that after getting back, he will carry on with his (greatgreat?)granddaughter. His great-grandniece, the great-granddaughter of his brother. Although since his brother was his identical twin, genetically she’d be the same consanguinity as his great-granddaughter. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [January 9, 2015, 8:02pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/36 "2015-01-09T20:02:12Z")
> [@Lemur866](#): > > Also note that, if you have any 100th generation descendants, you’ll likely be an ancestor of lots of people if not most people, and you’ll likely show up in multiple ancestor slots for a particular person. Exactly. If that 100th-gen descendant had completely unique ancestors in this generation, that would be 2^100 ancestors today, but there are only about 2^32 people alive today. So even if every single person alive today had descendants which interbred such that every signle person alive today was an ancestor of that 100th-gen-vedder, then Happy would still on average contribute 1/7 billionths of their genes. ---
**Author:** ![msmith537](https://avatars.discourse-cdn.com/v4/letter/m/d9b06d/32.png) [@msmith537](https://boards.straightdope.com/u/msmith537)\ **Post date:** [January 9, 2015, 8:07pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/37 "2015-01-09T20:07:58Z")
> [@Smapti](#): > > According to a documentary I once saw on the subject of past nastification, you’ll grow up without a Delta brain wave, which may turn in handy if the Earth is ever invaded by giant brains. Oh, a lesson in not changing history from Mr. “I’m My Own Grandfather”! ---
**Author:** ![Happy\_Lendervedder](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_lendervedder/32/8488_2.png) [@Happy\_Lendervedder](https://boards.straightdope.com/u/Happy_Lendervedder)\ **Post date:** [January 9, 2015, 8:28pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/38 "2015-01-09T20:28:16Z")
> [@Quercus](#): > > Exactly. If that 100th-gen descendant had completely unique ancestors in this generation, that would be 2^100 ancestors today, but there are only about 2^32 people alive today. > > So even if every single person alive today had descendants which interbred such that every signle person alive today was an ancestor of that 100th-gen-vedder, then Happy would still on average contribute 1/7 billionths of their genes. The Lendervedder blood lives on! ---
**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:** [January 9, 2015, 9:22pm UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/39 "2015-01-09T21:22:39Z")
Also consider that, 100 generations hence, you’ll probably have a lot of descendants. The odds of any particular one having any particular gene from you is slim, but the odds of any particular gene of yours living on in _any_ of them are pretty good. ---
**Author:** ![Siam\_Sam](https://avatars.discourse-cdn.com/v4/letter/s/d78d45/32.png) [@Siam\_Sam](https://boards.straightdope.com/u/Siam_Sam)\ **Post date:** [January 10, 2015, 2:13am UTC](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876/40 "2015-01-10T02:13:26Z")
I don’t think you need a time machine to be your own grandfather. You just need to live in Kentucky. [Previous page](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876.md?page=1) [Next page](https://boards.straightdope.com/t/time-travel-and-procreating-with-your-bloodline-need-answer-whenever/708876.md?page=3)