system
Given the following post, respond as if you're commenting on Reddit:
human
If two siblings have a kid, as a product of incest, is that kid genetically a son of the grandparents as well? Is it also genetically a sibling of its parents?
gpt
While on average, the grandchild would share 50% of its genes with each grandparent, the distribution of those genes would look quite different from the grandchildren than for the children.

Let's refer to the genes from the male grandparent as M and from the female grandparent as F.

For the children, all the genes will be (MF) -- one copy from M and one copy from F.

For the grandchildren, something different will happen.  The siblings are combining, at each gene spot, an (MF) and an (MF).  Assuming the pairings are randomly distributed, this means that the genes in the grandchildren will be distributed as follows: 1/4 of them will be (MM), 1/4 of them will be (FF), and 1/2 of them will be (MF)  [coming from either an MF or an FM pairing].  Also, when you have something like MM or FF, it doesn't necessarily mean both genes are the same -- just that both come from the same grandparent.  (So if the grandmother has a spot with genes {Aa}, and that spot is (FF) in the grandchild, it could be {Aa}, {aA}, {AA}, or {aa}.)

Thus, based on overall percentage of DNA, both the children and grandchildren will be 50% M and 50% F, but the way that is achieved is quite different.

Note, of course, that we are talking only about that piece of the genome that varies; the vast bulk of the human genome is the same from person to person.  Also, this leaves out the mitochondrial DNA; here, everyone but the male grandfather will have the same mitochondrial DNA, since that is inherited exclusively from the mother.