Another Doper and I were talking offboard and the topic of genetics came up. I mentioned offhand that I had read somwhere that I had read that siblings are more closely related to one another than they are to their parents. He disagreed with me and insists that siblings are related to each other as much as they are to their parents. Neither of us have cites.
Can anyone clear this up for us?
I think I have been told that because siblings share the same genet make up (half of each parent ), they are just like each other but only half like either parent, thdy are more closely related
Spelling and grammer subject to change without notice
On average, siblings share have 50% of their genes identical, exactly the same as with either parent.
Almost all of the genes of two humans are identical. 99+%. So let’s skip over that and focus only of those that differ.
Take 100 genes that two parents do not share. Parent A has genes A1, A2, A3, … A1000. Parent B has genes B1, B2, B3, … B1000.
Child C gets genes A1, B2, B3, A4, … B1000. Child D gets genes A1, B2, A3, A4, … A1000.
Each child averages about 50% of those genes with a given parent. (But still over 99% total.) Therefore, for each parent, each sibling will share 25% (.5 times .5) of that parent’s subset of genes. Since they have two parents in common, multiply 25% times 2 to get 50%. Again, this is only for the genes in the parents that are different.
(There is a lot of complicated stuff about gene-chromosome mixing that I’m skipping over. It really isn’t a single gene 50% independent chance for everything.)
There’s also more variance between siblings. That is to say: A child is guaranteed to share exactly half of their varying genes with each parent (assuming that the parents shared no genes), but that proportion can range between 0 and 100% between siblings (though those extremes would of course be exceedingly rare). On average, though, it’s still 50%.
Continuing for other relatives, there’s 1/4 consanguinuity between a grandparent and grandchild, a niece/nephew and aunt/uncle, or half-siblings, 1/8 between great-grandparent and great-grandchild or first cousins, and in general, for nth cousins k times removed, the consanguinuity is 1 in (2*4[sup]n[/sup]2[sup]k[/sup]) and for half-nth-cousins k times removed, it’s 1 in (44[sup]n[/sup]*2[sup]k[/sup]). Siblings can bee considered zeroth cousins, aunts and uncles of degree k can be considered zeroth cousins k times removed, and descendants or ancestors of degree k can be considered negative oneth cousins k times removed. That should be enough to cover all blood relations without inbreeding; with inbreeding things get too complicated to sum up in a single quick formula.