# Shared DNA (simple mathematical question) **URL:** \ **Category:** Factual Questions\ **Created:** [February 12, 2022, 4:50pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372 "2022-02-12T16:50:06Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Roger\_That](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/roger_that/32/8505_2.png) [@Roger\_That](https://boards.straightdope.com/u/Roger_That)\ **Post date:** [February 12, 2022, 4:50pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/1 "2022-02-12T16:50:06Z")
A first cousin once removed just notified me that she has become a grandparent, and I thought of writing back to her that it was good to know that x% of my DNA will live on. I thought it would be simple arithmetic to figure out what x is, but it was beyond my weak arithmetic abilities to do so. Is there a simple answer? Is “first cousin once removed” sufficient information? ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 12, 2022, 5:05pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/2 "2022-02-12T17:05:48Z")
6.25% > **[Coefficient of relationship](https://en.m.wikipedia.org/wiki/Coefficient_of_relationship)** > > Pages for logged out editors learn more > > The coefficient of relationship is a measure of the degree of consanguinity (or biological relationship) between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of the coefficient of inbreeding of 1921. The measure is most commonly used in genetics and genealogy. A coefficient of inbreeding can be calculated for an individual, and is typically one-half the coefficient... ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 12, 2022, 5:34pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/3 "2022-02-12T17:34:43Z")
The number you’re looking for is 1/64. Siblings have a consanguinity of 1/2, first cousins 1/8, first cousins once removed 1/16, and the grandchild of a first cousin once removed (either a first cousin thrice removed, or a second cousin once removed) is 1/64. Of course, that really just means 1/64 of your distinctive DNA, since the vast majority of DNA is common between all humans, and in fact a great deal of it is common to all living things on the planet. ---
**Author:** ![Roger\_That](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/roger_that/32/8505_2.png) [@Roger\_That](https://boards.straightdope.com/u/Roger_That)\ **Post date:** [February 12, 2022, 5:45pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/4 "2022-02-12T17:45:00Z")
The two answers I’ve gotten so far (“1/64th” and “6.25 %”) seem to need reconciliation, no? This is more or less where my own attempts to find the answer broke down, btw. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [February 12, 2022, 5:48pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/5 "2022-02-12T17:48:20Z")
> [@Roger\_That](#): > > The two answers I’ve gotten so far (“1/64th” and “6.25 %”) seem to need reconciliation, no? The 6.25% (1/16) is for the first cousin once removed, @Andy_L overlooked the fact that you were considering the grandchild. ---
**Author:** ![Roger\_That](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/roger_that/32/8505_2.png) [@Roger\_That](https://boards.straightdope.com/u/Roger_That)\ **Post date:** [February 12, 2022, 5:55pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/6 "2022-02-12T17:55:37Z")
Ah. So the answer in percentage terms would be something like 1.625%? When you put it like that, it hardly seems worth remarking upon, or even celebrating at all. More like, “I’m getting obliterated before my own eyes here.” ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [February 12, 2022, 5:57pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/7 "2022-02-12T17:57:18Z")
Assuming that’s a typo for 1.5625%, then yes - 1/64 as @Chronos said. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 12, 2022, 5:59pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/8 "2022-02-12T17:59:08Z")
Yes - i did overlook that. Sorry. I thought FC-OR was your relationship with the child to be. Two more generaions makes it 1/64. ---
**Author:** ![Roger\_That](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/roger_that/32/8505_2.png) [@Roger\_That](https://boards.straightdope.com/u/Roger_That)\ **Post date:** [February 12, 2022, 5:59pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/9 "2022-02-12T17:59:33Z")
Not a typo as much as a mark of what goes wrong when I do division in my head. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [February 12, 2022, 6:04pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/10 "2022-02-12T18:04:14Z")
> [@Roger\_That](#): > > I’m getting obliterated before my own eyes here Well, except that (a) you share the great majority of your DNA with all humans, much of it with many other species, and some of it with all other species; (b) most sites where there is variation _at all_ within the human population are neutral - differences is DNA sequence that have no phenotypic effect; (c) since we all share more distant ancestry, any more interesting segments of DNA that you do carry are almost certainly carried by _some_ other humans. I suppose I’m just telling you that you weren’t all that special in the first place… The only thing that would be lost completely to the human population would be _de novo_ mutations in your germline DNA. If any of those gave you superpowers… we’ll have to wait until the same mutation happens (by chance) once again in the future. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 12, 2022, 6:08pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/11 "2022-02-12T18:08:29Z")
Or to look at it yet another way, every last bit of your DNA is an A, a C, a T, or a G, and those will all survive as long as there’s any life on Earth. It’s not the pieces themselves that matter; it’s the combinations of those pieces. But then we have to ask how big of a combination counts as part of “your DNA”. The usual cutoff is at the gene level, but even that’s not entirely clear-cut, since you can have a pair of genes that have an effect in combination that’s unrelated to the effect of either one alone, and you can also have two different genes (i.e., with different genetic sequences) that nonetheless have the exact same effect, due to the large amount of redundancy in the genetic code. ---
**Author:** ![duality72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duality72/32/3910_2.png) [@duality72](https://boards.straightdope.com/u/duality72)\ **Post date:** [February 12, 2022, 6:09pm UTC](https://boards.straightdope.com/t/shared-dna-simple-mathematical-question/959372/12 "2022-02-12T18:09:13Z")
And the award for motivational speaker of the year goes to…