Percentage of genetic material I share with my cousin?

Question

A distant cousin asked me what percentage of our genetic material we share, and I tried to figure it out. For simplicity's sake, let's say he was my 2nd cousin. Since we have 2 of our 8 great-grandparents in common, that would be 1/4 or 25%. But after thinking about it, it wouldn't be accurate to say that 25% of our genes are the same, would it? Because even siblings don't have identical genes. So I guess it would be better to say that 25% of our genes came from the same source, even though they might not be the same. Is that right? Or am I just confusing myself and I should go to bed?

Answer 1

Full siblings have exactly the same ancestry, so they do carry 100% the same. Which of those genes would be dominant (so to speak) is a variable.

1st cousins share 2 ancestors of 4.  (grandparents from the fathers side may be the same, and different from mom's side). That would be 50%.

2nd cousins, as you said share 2 out of 8, working out to 25% of shared ancestry.

The percentage of material is the same.. how it affects a person is unique.

Sleep tight.

Answer 2

Check books such as "Trace Your Roots with DNA" Using Genetic Tests to Explore Your Family Tree" by Megan Smolenyak Smolenyak and Ann Turner.

On page 120, they state that a first cousin has 12.5% the same DNA; 1st cousin once removed, 6.25%; second cousin is 3.125%, 1st cousin twice removed is 3.125%; 2nd cousin once removed is 1.563%; second cousin twice removed is 0.781%.

Answer 3

I say 1/32.  

I seem to be at variance with the others, so maybe I am no longer the top math whiz (lack of brain use recently).

That is only if all marriages are external from any related people.

Siblings "average" 50% the same DNA.  This is because they each picked some DNA, while the other picked DNA as well, but differently.  They get 50% the same coincidentally, average.  Unless one from a pair of chromosomes has predominant power to get passed on?  (actually wouldn't cystic fibrosis or other genetic diseases, cause this to happen?)

You also don't get the same quarter as your cousin got.  

You can be lucky and end up with more coincidences in which chromosomes you got, or unlucky and get less.  The middle (and peak) possibility is 1/32, or a sliver higher or different or whatever for those small science technicalities if there be any.   1/4  X 1/2 X 1/4  =1/32.

This is my opinion honestly.

They aren't completely random throughout your DNA which would (occasionally) put two chromosomes for the same chromosome number; they should all be on different chromosome numbers.

1/32 computes to 1 or 2 chromosomes in common.  Chance allows the possibility that 0 or 3+ are shared between normal second cousins.

Answer 4

You're right but to double-check, try working backwards:

Assume the cousin is through the maternal line.

You have 50% of your mothers DNA. She has 50% of her mother, her grandmother (hence you have 50% X 50%, 25% of her DNA). Similarly, you have 12.5% of your great-grandmother's DNA.

The sums work the same for your 2nd cousin - he also has 12.5% of your mutual great-grandmother's DNA.

Finally, you also got the same amount, 12.5% from your shared great-grandfather, meaning 25% DNA is shared.

Ignoring mutations, all the DNA is transmitted so you can say that the 25% is identical.