Feedback & Requests Roleplaying Gaming Discussion Entertainment & Media
Politics & World Pointless Worldbuilding Creative Forum
The Sports Center Science, Math, & Technology The Nostalgia Forum Sexuality
Hot Takes Complaints Hobbies & Interests Pro Wrestling
File a Complaint: add_comment New

Science, Math, & Technology


STEM_fields++;

Consanguinity

Posted 3 Weeks ago by chiarizio

The official definition of consanguineous marriage between humans is whenever the couple share 3.125% (1/32) or more of their autosomal variable genes.
So full second-cousins would be consanguineous, but half-second-cousins would not be; neither would second-cousins once removed.

What kind of clan-based marriage proscriptions could prevent consanguineous marriages?

Suppose a society has dual unilineal descent groups; patriclans and matriclans.
If the proscription is that neither parent of either party can belong to either clan some grandparent of the other party belongs to, that would mean no grandparent of either party could be a full sibling of any grandparent of the other party.

Neither grandfather of one party could have the same father as any grandparent of the other party; for if they did, then whichever parent of that first party who was the child of that grandfather, would be in the same patriclan as whichever grandparent of the other party.

Likewise, neither grandmother of one party could have the same mother as any grandparent of the other party; for if they did, then whichever parent of that first party who was the child of that grandmother, would be in the same matriclan as whichever grandparent of the other party.

However, either grandfather of either party could be a uterine half-brother (share the same mother) of either grandfather of the other party. And, either grandmother of either party could be the agnate half-sister (share the same father) of either grandmother of the other party.
So there are eight ways the parties could be half-second-cousins.

And four of these could be true at the same time. The parties could be quadruple half-second-cousins, sharing 6.25% (1/16) of their variable autosomal genes.

….

Things would be improved if we also used the Mundugumor geun or “rope”, in addition to patriclans and matriclans.
If we do, we find that either party’s father’s mother’s father could not also be the father of either grandmother of the other party.
For if they did, the first party’s father would be in the same “rope” as some grandmother of the other party.
Likewise, either party’s mother’s father’s mother could not also be the mother of any grandfather of the other party. If she was, then the first party’s mother would be in the same “rope” as some grandfather of the other party.

Now there are only two ways the parties can be each other’s half-second-cousin.
Each party’s father’s father could be a uterine half-brother of the other party’s father’s father.
And/or, each party’s mother’s mother could be an agnate half-sister of the other party’s mother’s mother.

But so far, these can both be true at once, making the two parties share 3.125% of their autosomal genes.

So let’s just add a restriction that the two parties cannot share more than one great-grandparent.

They can have the same FFM as long as they don’t have the same MMF; or they can have the same MMF as long as they don’t have the same FFM.

Now the average fraction of their autosomal variable genes that they have in common, is 1.5625% = 1/64.

There are 2 Replies


Someone’s parent’s parent’s parent’s child’s child’s child would be 6 degrees away by the Roman civil count.
If the unique shortest path between two people is six degrees long, and there’s no other path that long or shorter, then they average about 1/64 of their variable autosomal genes derived from a recent common source.
But if there’s a two-way tie between two shortest paths, and they’re both sixth degree, their index of consanguinity will be 2/64 = 1/32 = 3.125% instead.
If it’s a three-way tie, it’ll be 3/64 = 4.6875% ; and if it’s a 4-way tie it’ll be 4/64 = 1/16 = 6.25%.

The system discussed in the previous post is meant to insure that the couple are no more closely related than half-second-cousins.
The reason for the rule that they can’t have both the same FFM and the same MMF, is to keep them from being double-half-second-cousins, so their coefficient of relatedness will be at most 1.5625%; or, at any rate, that it will be less than 3.125%.

….

Note that it allows them to have the same FFM, and their MMFs could be each other’s full brothers;
Or they could have the same MMF, and their FFMs could be each other’s full sisters.
In either of those cases, their coefficient of relatedness might be 1/64 + 1/128 = 3/128 = 2.34375%.
But to reach that figure we’d need to count two 8th-degree paths as well as one 6th-degree path.
I think it might be better to ignore relatives 7 or more degrees away.

3 Weeks ago
chiarizio

The baseline risk for two unrelated parents to both contribute the same harmful recessive gene to their child seems to be about 2.5%, or about one in forty.

If a couple are first-cousins in only one way, and don’t also share some other blood-relationship; and come from ancestors who haven’t practiced cousin-mating within living memory; then the odds they’ll experience such a tragedy are about doubled, to around 5%, or about one in twenty.
If they don’t share a common blood-relative with a genetic disease, they probably shouldn’t worry about it; except that, if it does happen, they should probably worry about it happening a second time. But “worry about it” means they should get genetic counseling; I’m not an expert.

OTOH if the couple come from ancestors who have practiced cousin-marriage in living memory — especially if they’ve done it several times — then the odds that couple will experience such a tragedy, is roughly triple the background rate; about 7.5%, or about one in thirteen.
That’s probably high enough to make such a marriage inadvisable.
Especially if they share a common blood-relative who has a genetic disease.
In that case, my inexpert opinion is that they should not breed shared offspring. They should not marry; or they should divorce; or they should only adopt; or something. I’m not an expert but I have an opinion anyway, and I’m going to apply it to my conworlds.

1 Week ago
chiarizio

Semi-permanent Lockdown

Accounts are required to post for the forseeable future. Contact me through discord for account issues or registration: Riven#7868