I don’t understand why the origin of grandparents should matter.
To the best of my knowledge, the main problem with incest is recessive alleles. For example, if the grandfather’s genotype is ”aA” (where “a” is a very rare recessive allele) and his children (parents’ generation) mate with each other, then there is a relatively high chance (1/16) that the grandchildren would be of “aa” genotype (which might be extremely deleterious or even lethal). Having another grandparent from a different continent should not change this.
Why wouldn't having grandparents from different continents make rare alleles less likely to be reinforced?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.