4 ms·

You do not necessarily have 4 grandparents. Try to think of ways in which you can have fewer than 4 (hint: it's a little bit impolite to talk about). Neither do

by xamuel 3y ago

You do not necessarily have 4 grandparents. Try to think of ways in which you can have fewer than 4 (hint: it's a little bit impolite to talk about). Neither do you necessarily have 8 great-grandparents, for the same reason.

kaycebasques 3y ago
We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? Trying to wrap my mind around how that changes the calculation... it would drop the number from 2x every generation down to what?
- lolinder 3y ago
  We only call it incest when the nearest common ancestor is a parent or (depending on the jurisdiction) a grandparent. If your nearest common ancestor is a great-great-great-great-grandparent, that's not incest (it would most likely not even be known at the time of marriage), but it still leads to large amounts of duplication in your family tree. Every family tree will have many instances of nth-cousin marriages simply because people tend to marry people who live close to them. It's very hard to calculate what the actual number would be because it depends a lot on where your ancestors lived and how much they migrated over time.
  - kaycebasques 3y ago
    "Inbreeding" was the word I was looking for, methinks
- thaumasiotes 3y ago
  > We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? That really depends what you're imagining when you say "incest". In literal terms, yes, we're talking about mating between two people who share common ancestors. Those do not need to be particularly recent. In genetics there is the concept of the "coefficient of inbreeding" which tracks how much of an individual's genome is identical-by-descent across both chromosomes. (That is, if you inherit a particular stretch of DNA from your mother, and you inherit the same stretch of DNA from your father, and the reason they are the same is that your father and your mother both inherited that DNA from the same person, then that stretch of DNA contributes to your coefficient of inbreeding.) If you're imagining brother-sister marriages, that is a much higher level of inbreeding than would be common in human history. (Expected coefficient of inbreeding in the offspring: 25% assuming zero inbreeding in the parents, perhaps because they are themselves the offspring of a cross between an African Pygmy and an Australian Aborigine.) But it is true that different populations have more or less inbreeding than others; we can say that Arabs tend to be more inbred than the French. This is not necessarily because they tolerate incest at closer degrees of kinship than the French. It is mostly because they have a greater quantity of still-relatively-remote incest. (Not the greatest example; Western Europe hugely stigmatized cousin marriage under the influence of the Catholic Church, so in reality Arabs do tolerate closer degrees of kinship in a marriage than the French do. But the concept is sound - inbreeding will "build up" in a population over time.)
- chromatin 3y ago
  the word is "Consanguinity" https://en.wikipedia.org/wiki/Consanguinity https://en.wikipedia.org/wiki/Consanguinity

by xamuel 3y ago

kaycebasques 3y ago

We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? Trying to wrap my mind around how that changes the calculation... it would drop the number from 2x every generation down to what?

lolinder 3y ago
We only call it incest when the nearest common ancestor is a parent or (depending on the jurisdiction) a grandparent. If your nearest common ancestor is a great-great-great-great-grandparent, that's not incest (it would most likely not even be known at the time of marriage), but it still leads to large amounts of duplication in your family tree. Every family tree will have many instances of nth-cousin marriages simply because people tend to marry people who live close to them. It's very hard to calculate what the actual number would be because it depends a lot on where your ancestors lived and how much they migrated over time.
- kaycebasques 3y ago
  "Inbreeding" was the word I was looking for, methinks
thaumasiotes 3y ago
> We're talking incest? And the theory (or fact) is that it was a very widespread practice throughout human history? That really depends what you're imagining when you say "incest". In literal terms, yes, we're talking about mating between two people who share common ancestors. Those do not need to be particularly recent. In genetics there is the concept of the "coefficient of inbreeding" which tracks how much of an individual's genome is identical-by-descent across both chromosomes. (That is, if you inherit a particular stretch of DNA from your mother, and you inherit the same stretch of DNA from your father, and the reason they are the same is that your father and your mother both inherited that DNA from the same person, then that stretch of DNA contributes to your coefficient of inbreeding.) If you're imagining brother-sister marriages, that is a much higher level of inbreeding than would be common in human history. (Expected coefficient of inbreeding in the offspring: 25% assuming zero inbreeding in the parents, perhaps because they are themselves the offspring of a cross between an African Pygmy and an Australian Aborigine.) But it is true that different populations have more or less inbreeding than others; we can say that Arabs tend to be more inbred than the French. This is not necessarily because they tolerate incest at closer degrees of kinship than the French. It is mostly because they have a greater quantity of still-relatively-remote incest. (Not the greatest example; Western Europe hugely stigmatized cousin marriage under the influence of the Catholic Church, so in reality Arabs do tolerate closer degrees of kinship in a marriage than the French do. But the concept is sound - inbreeding will "build up" in a population over time.)
chromatin 3y ago
the word is "Consanguinity" https://en.wikipedia.org/wiki/Consanguinity https://en.wikipedia.org/wiki/Consanguinity