# Common genes in families
**URL:** https://boards.straightdope.com/t/common-genes-in-families/543290
**Category:** Factual Questions
**Created:** [June 15, 2010, 1:27pm UTC](https://boards.straightdope.com/t/common-genes-in-families/543290 "2010-06-15T13:27:33Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@AllFreedomUnlessDefyingScience](https://boards.straightdope.com/u/AllFreedomUnlessDefyingScience)
#### Post date: [June 15, 2010, 1:27pm UTC](https://boards.straightdope.com/t/common-genes-in-families/543290/1 "2010-06-15T13:27:33Z")
If I have a brother, we both gain half our genes from each parent, however statistically we should have about 1/2 our genes in common, as we don’t get exactly the same genes from each parent (assuming we aren’t identical twins). My question is how strong is the bias towards our genes being exactly 50% commonality. My impression is considering the number of genes being selected by a process that seems to be strongly random would result in the the bias being very high. Am I correct? If I am, how strongly biased are later generation towards maintaining this bias?
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### Author:  [@Shmendrik](https://boards.straightdope.com/u/Shmendrik)
#### Post date: [June 15, 2010, 2:49pm UTC](https://boards.straightdope.com/t/common-genes-in-families/543290/2 "2010-06-15T14:49:36Z")
> [@AllFreedomUnlessDefyingScience](#):
>
> If I have a brother, we both gain half our genes from each parent, however statistically we should have about 1/2 our genes in common, as we don’t get exactly the same genes from each parent (assuming we aren’t identical twins). My question is how strong is the bias towards our genes being exactly 50% commonality. My impression is considering the number of genes being selected by a process that seems to be strongly random would result in the the bias being very high. Am I correct? If I am, how strongly biased are later generation towards maintaining this bias?
IIRC, the genome is large enough, and the number of crossing over events high enough, to ensure that siblings will share very close to 50% of their variable genetic material.
ETA: I did find a numerical answer to this question a few years ago; I think the standard deviation was 0.1% or so, but I can’t find the source right now.
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### Author:  [@AllFreedomUnlessDefyingScience](https://boards.straightdope.com/u/AllFreedomUnlessDefyingScience)
#### Post date: [June 15, 2010, 3:48pm UTC](https://boards.straightdope.com/t/common-genes-in-families/543290/3 "2010-06-15T15:48:09Z")
> [@Shmendrik](#):
>
> IIRC, the genome is large enough, and the number of crossing over events high enough, to ensure that siblings will share very close to 50% of their variable genetic material.
>
> ETA: I did find a numerical answer to this question a few years ago; I think the standard deviation was 0.1% or so, but I can’t find the source right now.
Thank you for the answer, if anyone else has anything pertinent to add, please do however.
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### Author:  [@Shmendrik](https://boards.straightdope.com/u/Shmendrik)
#### Post date: [June 15, 2010, 4:08pm UTC](https://boards.straightdope.com/t/common-genes-in-families/543290/4 "2010-06-15T16:08:18Z")
> [@AllFreedomUnlessDefyingScience](#):
>
> Thank you for the answer, if anyone else has anything pertinent to add, please do however.
Sorry, it looks like I was kind of far off on that standard deviation:
[http://www.ncbi.nlm.nih.gov/pubmed/16121355](http://www.ncbi.nlm.nih.gov/pubmed/16121355)
> [@](#):
>
> Analyzing highly polymorphic genetic variations from the Centre d’études du polymorphisme humain (CEPH) database, we report the first genome-wide estimation of the mean and variation in IBD sharing among siblings. From 1,522 microsatellite markers spaced at an average of 2.3 cM on 498 sibling pairs, we estimated a mean of 0.4994 and a standard deviation of 0.0395.
So 95% of sibling pairs will share 42 to 58 percent of “highly polymorphic genetic variations”.
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### Author:  [@AllFreedomUnlessDefyingScience](https://boards.straightdope.com/u/AllFreedomUnlessDefyingScience)
#### Post date: [June 15, 2010, 4:31pm UTC](https://boards.straightdope.com/t/common-genes-in-families/543290/5 "2010-06-15T16:31:02Z")
> [@Shmendrik](#):
>
> Sorry, it looks like I was kind of far off on that standard deviation:
>
> [Genome-wide identity-by-descent sharing among CEPH siblings - PubMed](http://www.ncbi.nlm.nih.gov/pubmed/16121355)
> So 95% of sibling pairs will share 42 to 58 percent of “highly polymorphic genetic variations”.
Thank you for the correction, another beautiful theory slain by an ugly fact.