Time travel and procreating with your bloodline (Need answer whenever)

Q1: Imagine “Johnny Timetravel”-- a handsome charmer a quarter of a century in years on earth-- were to head back to when his grandmother was a young, fertile lass of 22. She’s a comely young woman with swivel-y child-birthing hips and ample bosom. Johnny falls madly in love with Grandma and Grandma falls madly in love with Johnny. If Johnny were to perform sexual intercourse on his grandmother shoot a sperm square into her ripe ovum, would their genetic material be considered “too closely matched” to produce offspring devoid of the usual incestuous problems?

How about if Johnny were to head back another 25 years into the past and perform the same sexual hibbidy-dibbidy on his great-grandmother? Would their spawn be healthy and vibrant and able to live a fine, upstanding life without genetic malfunctions?

How about if Johnny were to head back yet another 25 years and do the “it” with his great-great grandmother? How close are they genetically at four generations back?

In short how far back in time would Johnny have to go to impregnate an ancestor safely and soundly without risk of damaging the Timetravel gene pool?

Q2: If you were to look at the genetic material of all 32 of your great-great-great grandparents, would you have equal amounts from each of them, or will some of their genes get pushed out of the way along the way? Is it possible that my great-great-great grandson/daughter will have none of “me” in them anymore?

Since Johnny is the source of the genetic material for the bloodline and he’s his own Grandpa (or more), it doesn’t matter how far back he goes, he’s an ancestor and decendent.

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.

At this point, we need to know what kind of time travel he’s using. (No, I can’t believe I said that, either).

If it’s the “many worlds” type, where changes to the past don’t affect the future, then this isn’t any different from mating with any other close relative that shares 1/4 of your genes, say a first cousin.

On the other hand, if this is the “Back to the Future” type scenario, it’s a lot more interesting. If you’ll recall from that documentary, changes in the past cause the future to “update” in real time, and we get to do a fun game of “count the chromosomes.” Johny’s chromosomes come from four sources: his maternal grandmother (MgM), his maternal grandfather (MgF), and his paternal grandparents (PgM, PgF), ignoring a small amount of mutation and other genetic uniqueness. But the one of the “gF”'s here is Johnny himself.

If it’s the maternal grandmother, this means that Johnny’s mom has the genes of MgM + Johnny, part of which is MgM as well, which means that Johnny himself will get more genes from MgM, which will in turn mean that his mom has more MgM genes, which in turn means… ultimately, Johnny’s mother will end up being a clone of his maternal grandmother, containing effectively no genes not present in MgM. In this case, Johnny is effectively the offspring of his father and his maternal grandmother, genetically.

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.

You don’t have to hypothesize time travel to understand trans-generational crosses, since it happens with domestic animals all the time.

You share on average 1/4 of your genetic material with your grandparents by descent. You also share 1/4 of your genetic material with your first cousins (who share one of your sets of grandparents). So the genetic consequences are the same as having offspring with your first cousins.

You can calculate the degree of genetic similarity (by descent) simply by knowing the number of ancestors in that generation. You share 1/8 of your genetic material with your great-grandparents (same as second cousins), 1/16 with great-great-grandparents (same as third cousins), and so on.

That sound you just heard was my brain esplodin’.

Due to the phenomenon known as crossing over, there is a mixing of genetic material between chromosomes in each generation. Because of this it is possible to end up with some of the genetic material from some ancestors being eliminated by chance. However, I think it would probably take more than 5 generations for this to be likely.

I understand this to be more a question about genetics than about time travel per se.

Excellent hed.

Also, OP was having a suspiciously good time writing the OP.

And “hibbidy-dibbity” is a new one on me, but is a keeper.

I see. And good to know.

Answers on either would be welcome. Genetics was my first thought, but the consequences of time travel schnogging are also interesting…and critically important for the continuation of our species.

Not so. Johnny’s Y chromosome came from his PgF’s father. It was not inherited from his PgM in any way. His Y chromosome (which also does not undergo crossing over with the X) will be preserved intact regardless of the number of generations you go back. (His father’s autosomal genes and X chromosome, however, may ultimately become identical to those of his PgM’s.)

You don’t even need to invoke crossing over. Even if you look at whole chromosomes as units, it’s unlikely that an even mix will be passed on every generation.

I have 46 chromosomes. 23 are from my dad, and 23 are from my mom. When I make a sperm, that sperm could contain 23 of my dad’s chromosomes, 23 of my mom’s chromosomes, or (far more likely), some combination - say, 12 dad’s and 11 mom’s, or 14 mom’s and 9 dad’s.

Because of this, and considering that chromosomes are of different sizes, and tossing in recombination for good measure, you’d expect the mix in each generation to vary. It’ll be, on average, close to 50/50, but it’ll be a bell curve.

I am, of course, ignoring sex chromosomes, but they’re only one pair out of 23.

Yeah, the only case where you can get exact consanguinity numbers (other than identical twins, of course) is parent/child, who are guaranteed to be a 50% match. Any other relation, the numbers you calculate will only be an average, and could be higher or lower.

Wouldnt you share 1/2 of your genetic material with your first cousins since you share 1/2 of the grandparents?

Didn’t Heinlein deal with this? IIRC, Laz Long does the deed with his g-granddaughter, justifying that he and she haven’t enough genetic material in common to make it incest.

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.

Why the assumption that a child of Johnny & Great-Grandma is Johnny’s grandfather rather than a great-uncle / great-aunt? It seems to require much less messing with history to have a one-off fling than to erase one’s great-grandfather.

Nearly everyone in the galaxy is a descendant to some degree of Lazarus Long, but I don’t think he ever slept with any of them as close as three generations. Aside from his two gender-swapped clones or his mother, of course.

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.

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

I think you share 1/4 (on average) of genetic material with your parent’s sibling, and 1/8 with your first cousins.

Of course, really, most human DNA is the same anyway, but you know what I mean.