Sharing Genes with Brothers and Strangers
Suppose a person has X genes. He will share, on average X/2 of those with his
brother. He will share exactly X/2 with his father. There is a 50% chance he
will share less than X/2 with his brother. There is a tiny chance, even, that he
shares 0 genes with his brother, because his brother got the complementary X/2
from their father and the complementary X/2 from their mother.
How many genes will our person share with the average person in the population?
Not zero, but maybe a fraction of 1. It depends on how big X is, and how many
people are in the population. What is interesting is the probability that
there is someone out there in the population who shares X/2 genes with our
person. And what is the probability there is someone with all X genes? It is not
zero.
If the population is big enough, N’, there is over a 50% chance that someone
exists with X/2 genes in common with him. The size of N’ depends on X. We will
assume an even distribution of genes– no matching of male and female by genes.