# How do Cousins Share 1/8 of genes?

#### Dr.Bajaber

Hello
I was reading this in Kaplan Biochemistry Lecture Notes but I tried to understand it and I could not. I really feel so frustrated. It does not make any sense to me
I share quarter of my genes with my grandfather and my cousin share quarter of his genes with my grandfather too.How come I do not share quarter of my genes with my cousin?
Can anyone explain?
thanks

Last edited:

#### DrPicard

2+ Year Member
I vaguely remember understanding this as follows:

So with first degree relatives you share 1/2 your genes.

For every additional degree of relation, multiply by 1/2.

So for your grandfather, 1/2 x 1/2 = 1/4.
Uncle = 1/2 x 1/2 = 1/4.
First cousin = 1/2 x 1/2 x 12 = 1/8.

Dr.Bajaber
OP
D

#### Dr.Bajaber

I vaguely remember understanding this as follows:

So with first degree relatives you share 1/2 your genes.

For every additional degree of relation, multiply by 1/2.

So for your grandfather, 1/2 x 1/2 = 1/4.
Uncle = 1/2 x 1/2 = 1/4.
First cousin = 1/2 x 1/2 x 12 = 1/8.
Hey
thanks I understood the rule but I do not know why it is like this
maybe I will memorise the rule if no one replies

#### DrPicard

2+ Year Member

Your mother gets 1/2 her genes from her father, half from her mother.

This means that of the 1/2 genes you got from your mother, 1/4 came from your grandfather, 1/4 from your grandmother.

(this is a simplified explanation but its basically probability math)

#### mw18

5+ Year Member
Hey
thanks I understood the rule but I do not know why it is like this
maybe I will memorise the rule if no one replies
Is your question why brothers and sisters are half genetically? Because that is what I think you're missing. Your mom and your grandfather share half of their DNA (because it came from him directly). Your uncle and your grandfather share half of their DNA (because it came from the grandfather directly). Your uncle and your mom share half their DNA (from their mom and dad), but it's not the same half of the DNA that they share with their father. It's a different combination. Which is why your cousin wouldn't have the same quarter of the DNA you have. Because they got half of a different combination.

OP
D