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## December 27, 2004

### 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.

Posted by erasmuse at December 27, 2004 12:38 PM

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