i don’t get how parents and children are “one generation apart” while children and siblings are “two generations apart” despite having the same ancestors (parents)? thank you
In: Biology
You get half your dna from each parent.
So between you and one parent you have 50% in common (ignoring the stuff that was the same for both of them).
Your sibling also gets half from each of your parents, but critically a different half.
Imagine a deck of cards and you deal out half of them to you. Then you get another full deck of cards and deal out half to your sibling. Some of the cards you will have both got, some of them only one of you will have gotten. It turns out that roughly half of each of your cards are in common, or 25% of all the cards.
Trying to figure out whether you can marry your second cousin, eh? 😉
Joking aside, the easiest way to think about it is to look at consanguinity as a tree that follows *inherited* relationships from parent-to-child.
You inherit from your parents so they’re one step **up** the tree from you. Your children inherit from you, so they’re one step *down* the tree from you.
But your sibling is not a direct ancestor or inheritor *from you*. Genetically, *you* don’t inherit anything *from* or pass along anything *to* your brother or sister. So their degree of consanguinity is further away.
To find out how far away, ask yourself how many steps *up* the tree or *down* the tree — or *up and then back down* does it take to get from you to that other person.
A grandparent? They’re two steps up the tree from you: up to your parent then up to your grandparent. Two degrees of consanguinity.
A sibling? They’re one step up the tree and then one step down: up to your parent then down to your parent’s *other* child (your sibling). Two degrees of consanguinity.
A first cousin? They’re two steps up the tree and then two steps down: up to your parent, then up to your grandparent, then down to your grandparent’s other child (your uncle/aunt), and then down to your uncle or aunt’s child (your first cousin). Four steps, four degrees of consanguinity.
And so on.
[This is a good diagram](https://upload.wikimedia.org/wikipedia/commons/0/0d/Table_of_Consanguinity_showing_degrees_of_relationship.svg) that might help make that “tree” more clear.
EDIT: BTW, calling those steps up and down the tree “generations” is probably the not the right term. The better term is “degrees of consanguinity.” A generation is something different.
If you look at that chart I linked above and read it from left-to-right (instead of up and down the tree like you do for consanguinity), you’ll get a better idea of a “generation.” E.g., you, your siblings, your first cousins, your second cousins, and your third cousins are all of the same “generation” because the degree of consanguinity between *each* of you *and your common ancestor* — in this case shared great-great grandparents — is four degrees of consanguinity.
Since *each* of you have the same degree of consanguinity *to a shared ancestor*, you are of the same generation.
*But*, even though you’re of the same generation (have the same degrees of consanguinity to the shared ancestor), your degrees of consanguinity *with each other* will be wildly different. Your degree of consanguinity with your sibling is 2, but your degree of consanguinity with a third cousin (with whom your closest common ancestor is that great-great grandparent) is 8.
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