I realize of course that all humans are almost genetically identical; I’m thinking specifically about a rather unusual case, and how many genes these two close relatives would share. I’m just afraid my math isn’t capable of handling this.
Okay, so we have PN, who is the son of GF and LS.
We also have KN, who is the son of KS and SB.
Now, KS is also the father of GF by a woman other than SB. Therefore GF and KN are half-siblings, and KN is PN’s half-uncle.
But here’s where it gets tricky. SB and LS are half-sisters. Therefore their sons, KN and PN respectively, are half-cousins as well as half-uncle and half-nephew. From my own calculations I’ve determined that both GF and LS share 25% of their genes with KN. So would PN keep his parent’s ratio in regard to KN?
This is just to satisfy my intellectual curiosity; obviously, since KN and PN are both male it’s not like they’re going to marry and produce unusually inbred offspring or anything.
They have a consanguinity of 3/16. Regular cousins have a consanguinity of 1/8, and half-cousins have a consanguinity of 1/16. Regular nephew/uncle relationships have a consanguinity of 1/4, and a half-nephew has a consanguinity of 1/8. When two people are related on both sides, you just add the consanguinities.
And, of course, we can’t say how many genes they really share: there is only 50% chance on getting any specific gene from parent. So in theory siblings can have anywhere from 0% to 100% genes identical. Sure, most of them are the same with either copy, but those that are different - are distributed randomly (well, almost randomly, but let’s not complicate things).
They probably have exactly the same genes. It’s the alleles thay share that might be different, and for your purposes, using statistics to work it out will only give you statistical averages, not actual differences between individuals. Actual differences would require genetic analysis of each individual.