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
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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