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  #1  
Old 04-03-2008, 12:04 PM
diggleblop diggleblop is offline
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Who is genetically closer, father and son or brothers?

I was just wondering for no particular reason. Am I genetically closer to my brother or to my son? Or is it the same?
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  #2  
Old 04-03-2008, 12:15 PM
Dr. Drake Dr. Drake is offline
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In theory, if a parent gives no more than 50% of their chromosomes to a child, isn't it possible (wildly unlikely, but possible) for siblings not to be all that closely related, DNA-wise? Mom could give half her genes to Kid One and the other half to Kid Two --- both are her children, but both have different sets of stuff. Obviously, two brothers would share the same Y-chromosome, of course. Point being that you can share between some-really-low-number (0%?) & 100 (identical twin) % of your DNA with a brother, while your child is a guaranteed 50%, no matter how you slice it.

A related question I've always had: Humans have 46 chromosomes. We have 64 great-great-great-great grandparents. So logically we have at least 18 great-great-great-great grandparents that haven't given us a single chromosome: are we related to them? mtDNA probably clouds this issue.

Last edited by Dr. Drake; 04-03-2008 at 12:16 PM..
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  #3  
Old 04-03-2008, 12:19 PM
Lemur866 Lemur866 is online now
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Chromosomes aren't always passed on as a unit. They cross over, sections from paired chromosomes are exchanged.

http://en.wikipedia.org/wiki/Crossing_over
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  #4  
Old 04-03-2008, 03:13 PM
OldGuy OldGuy is offline
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In general chromosomes do not stay intact in transmission to offspring. The two paired chromosomes swap gene sequences so you likely have some genes from all your great to the n grandparents. And of course some genes are identical across many people so you couldn't tell exactly which ancestor they're from in any case.

The Y chromosome is an exception. It does not swap genes with the X so the father gives his Y chromosome to a son pretty much intact. So a son father and brother have the same Y chromosome pretty much.

The rest of the genes Apart from the X & Y are half from the mother and half from the father so a son has just about exactly half of his genes from his father. He will on average have about half the same genes as his brother but it could in theory vary from absolutely identical (even if not identical twins) to nothing but the Y in common.
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  #5  
Old 04-03-2008, 04:36 PM
BJMoose BJMoose is offline
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Well, that's pretty much been answered, so I'll just do a summary:

So the theoretical overlap between brothers (excluding the Y chromosome) can be anything from 0 to 100 percent, though of course random distribution will hardly ever peg the meter one way or the other. But with the normal crossing over of material between chromosomes the practical range will be somewhat less than that.

The genetic overlap between father and son will always be 50 percent (less the difference in size between X and Y).
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  #6  
Old 04-03-2008, 05:05 PM
Mijin Mijin is offline
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Quote:
Originally Posted by BJMoose
The genetic overlap between father and son will always be 50 percent (less the difference in size between X and Y).
Slight nitpick, but the mother and father will of course share many genes in common, so in terms of genetic similarity (as in the OP), the genetic similarity will be much greater than 50% to either parent.

And further, if we are considering genetic similarity (and not where a particular gene was sourced from), it gets rather more complicated.

e.g. The parents have genotypes AA and BB for a particular gene.

All their offspring will have the genotype AB.

Which means that their children have a 100% chance of sharing genotype and phenotype with their siblings. But they have a 0% chance of sharing genotype with their parents.

Whether they'll share either parent's phenotype depends on which parent has the dominant allele. Furthermore if the alleles are co-dominant, then no child will share geno- or phenotype with either parent.

My intuition is that siblings are more similar, but I can't crunch the numbers.

Nice question, diggleblop!
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  #7  
Old 04-03-2008, 05:11 PM
Mijin Mijin is offline
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Quote:
Originally Posted by Mijin
All their offspring will have the genotype AB.
Actually, I'm wrong about this aren't I? Not all chromosomes cross over. I think.
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  #8  
Old 04-03-2008, 06:11 PM
Smeghead Smeghead is offline
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Think of it this way: one the one extreme are identical twin brothers, who share 100% of their DNA.

On the other extreme, think of two brothers. Brother A got all of his odd numbered chromosomes from his mother and all of his even numbered chromosomes from his father (or any other unit instead of chromosomes to avoid the crossing-over issue). The other brother just happened to get all of his even numbered chromosomes from his mother and all of his odd numbered chromosomes from his father, just through random chance. They share the same sex chromosomes - Y from their father and X from their mother - but otherwise they don't share anything.
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  #9  
Old 04-03-2008, 07:11 PM
Mijin Mijin is offline
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Quote:
Originally Posted by Smeghead
Brother A got all of his odd numbered chromosomes from his mother and all of his even numbered chromosomes from his father.
But this is quite an extreme scenario you're proposing.
I'm not absolutely sure, and I'm waiting for a biologist to join this thread, but AFAIK, crossover is the rule, not the exception.
You inherit one allele from both parents for each gene. So, as I've already pointed out, if your parent's genotypes are AA and BB, then the offspring will all have the same geno- and phenotype as their siblings but none will have the same genotype as their parents, and it's quite possible that none will even have the same phenotype of either of the parents.

This confounds the "common sense" analysis that tells us that a given child is 0-100% the same as its siblings and 50% the same as its parents.

Of course it depends what the OP meant by "genetically closer". I'd argue that defining it as a greater proportion of identical genotypes or phenotypes (how similar two people are genetically or biologically, respectively), makes a lot more sense than where the alleles were sourced from (which guarantees nothing and seems irrelevant).
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  #10  
Old 04-03-2008, 07:19 PM
postcards postcards is offline
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I recall reading somewhere that one's siblings are ones closest relatives, since, with the exception of each other, they have the exact same relatives. (i.e., same parents, brothers, sisters, cousins, aunts, uncles, grandparents, etc.)

Don't know whether that helps the OP, but I thought I'd throw it in.
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  #11  
Old 04-03-2008, 07:24 PM
ultrafilter ultrafilter is offline
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Quote:
Originally Posted by Smeghead
They share the same sex chromosomes - Y from their father and X from their mother - but otherwise they don't share anything.
It's also possible that they have different X chromosomes, as their mother has two.
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  #12  
Old 04-03-2008, 07:28 PM
chrisk chrisk is online now
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So, on the average your brother shares about as many genes with you as your father and your mother. However, there's much more of a possibility of variance with a sibling.

Does that sum it up pretty well?
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  #13  
Old 04-04-2008, 03:56 PM
Mijin Mijin is offline
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Yes, that sums up how genetics works in Imaginary Land very well.

I know I'm being crabby, but it seems no-one is listening to the point I'm trying to make.


Let's attempt to crunch through some of the numbers.

Imagine there's a gene that has just two alleles: A and B. Forget dominance and all that, because we'll look at the chance of the offspring sharing genotype with their parents only.

'pParent' = % chance same genotype as a random parent
'pSibling' = % chance same genotype as a random sibling

AA x AA = AA (pParent = 100, pSibling = 100)
AA x BB = AB (pParent = 0, pSibling = 100)
AB x AB = AA AB BB (pParent = 50, pSibling = 37.5)
AA x AB = AA AB (pParent = 50, pSibling = 50)

...and from this set of combinations it appears that a similarity to siblings is slightly more likely.
Of course it gets more complicated if we throw in extra alleles, codominance and so forth, but these extra cases almost certainly favour similarity to siblings anyway.

e.g.

AB x CD = AC AD BC BD (pParent = 0, pSibling = 25)

Last edited by Mijin; 04-04-2008 at 04:00 PM..
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  #14  
Old 04-05-2008, 12:15 AM
Ponderoid Ponderoid is offline
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I just asked a very similar question to this one: DNA test for familial relationship. The answer I got there was that full siblings are just as closely related to each other as they are to their parents: 50%.

*** Ponder
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  #15  
Old 04-05-2008, 04:10 AM
Mijin Mijin is offline
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Quote:
Originally Posted by Ponderoid
The answer I got there was that full siblings are just as closely related to each other as they are to their parents: 50%.
What a shame....there was a geneticist on that thread, but he/she didn't weigh in on the genetic similarity question.

In any case, even while we're using the simplistic and wrong way of looking at genetics, it's clear that you must mean 50% of the genes that vary among living humans.
If you're just comparing the whole genetic code then I'd suppose the similarity of any two humans is above 99% (even if we include the Y chromosome).

Last edited by Mijin; 04-05-2008 at 04:12 AM..
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  #16  
Old 04-05-2008, 08:29 AM
Mijin Mijin is offline
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Oh, and I realised that my table of genotypes didn't include the fact that certain pairings e.g. AB x AB might happen more frequently than others e.g. AA AA.

So assuming the alleles A and B are equally distributed in the population.

From the set of permutations (not included) we see that:

The pairing AA x AA (or BB x BB) is 12.5% of couples
The pairing AA x AB (or BB x AB) is 50% of couples
The pairing AB x AB is 25% of couples
The pairing AA x BB is 12.5% of couples

So multiplying the probabilities of similarity to parent and sibling (from the table in my previous post), by the probability of that particular pairing, gives:

AA x AA = (pParent = 100 x .125 = 12.5) (pSibling = 100 x .125 = 12.5)
AA x AB = (pParent = 50 x .5 = 25) (pSibling = 50 x .5 = 25)
AB x AB = (pParent = 50 x .25 = 12.5) (pSibling = 37.5 x .25 = 9.375)
AA x BB = (pParent = 0 x .125 = 0) (pSibling = 100 x .125 = 12.5)

So for this particular gene, the chance of sharing the same genotype with a random parent = 12.5 + 25 + 12.5 + 0 = 50%

The chance of sharing the same genotype with a random sibling = 12.5 + 25 + 9.375 + 12.5 = 59.375%

Not a huge difference, but if all the genes favour similarity to sibling slightly*, then the chance of your entire genepool being more similar to a random parent than a random sibling is extremely, bet-your-life-on-it, low.

* And as I've already said, other situations such as more than 2 alleles, almost certainly favour sibling similarity, although there are too many permutations to list out.

Last edited by Mijin; 04-05-2008 at 08:31 AM..
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  #17  
Old 04-05-2008, 08:46 AM
Alive At Both Ends Alive At Both Ends is offline
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Another related question

Quote:
Originally Posted by Dr. Drake
Humans have 46 chromosomes. We have 64 great-great-great-great grandparents. So logically we have at least 18 great-great-great-great grandparents that haven't given us a single chromosome: are we related to them?
As has been pointed out, the units being passed on are genes not whole chromosomes, so my question is, how many genes are there? And how many generations apart do you have to get before there is a greater than 50% chance that you have no genes inherited from your ancestor?
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  #18  
Old 04-05-2008, 11:28 AM
Dr. Drake Dr. Drake is offline
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Thanks for the education in this thread. I can (sort of) narrow down my own question now.

The Human Genome Project suggests fewer than 30,000 protein-coding genes.

Going back fifteen generations, you have 32,768 great(x)-grandparents. So, logically, they can't have all given you genes.

Except that you're bound to have a considerable number of repeat ancestors among those 32K.

And of course the point mentioned above about which of those genes are unique to humans.
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  #19  
Old 04-05-2008, 02:40 PM
Bob55 Bob55 is offline
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Quote:
Originally Posted by Dr. Drake
Thanks for the education in this thread. I can (sort of) narrow down my own question now.

The Human Genome Project suggests fewer than 30,000 protein-coding genes.

Going back fifteen generations, you have 32,768 great(x)-grandparents. So, logically, they can't have all given you genes.

Except that you're bound to have a considerable number of repeat ancestors among those 32K.

And of course the point mentioned above about which of those genes are unique to humans.
Well, even though less than 5% of the genome codes for proteins, we're now finding microRNAs have a huge contribution to protein expression. They call this branch the "Conductome".

It's kind of like the old belief that we only use 10% of our brain (which is incorrect). We use more than 5% of our genome, and it's now thought that it might be a very high percentage. So you probably do have important regulatory components from all of those great grandparents, even if it's not a protein.
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  #20  
Old 04-06-2008, 03:20 PM
Mijin Mijin is offline
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Ok, my last word on the subject. I promise

I think I can sum up my argument without recourse to a table of probabilities.

Like so:

Question: Who is genetically closer, father and son or brothers?

Answer: Almost-certainly brothers, though not by much.

Explanation: For every gene, each parent has two alleles, and each parent contributes one allele to form the pair that will determine the child's genotype.
Because of this mechanism, the probability of brothers having the same genotype can never be less than 25%, regardless of the parents' genotypes.
However, the probability of sharing the father's genotype could be as low as 0%; if the father and mother have no alleles in common for that gene (e.g. Father has two A alleles, mother has two B alleles: all the children will have the genotype AB, which is different to either parent).
It's this fundamental difference that skews the probabilities in favour of sibling similarity.
Greater similarity to the father is theoretically possible, but very unlikely, because if individual genes favour sibling similarity then it is overwhelmingly likely that the set of thousands of genes will also.
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  #21  
Old 04-06-2008, 11:06 PM
Smeghead Smeghead is offline
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I don't have the time now to address this at length, but I think where we're getting tangled up is in the difference between genetically similar and related. You could, theoretically, have two people who are genetically identical yet completely unrelated through random chance. My understanding is that the OP was asking about relatedness. My interpretation of that was looking at where the genetic information came from, rather than what it was. In other words, for this question, we're treating a paternal chromosome as a completely different thing from a maternal chromosome, although of course we know they aren't.
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  #22  
Old 04-07-2008, 12:03 AM
Bob55 Bob55 is offline
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I don't think this question is answerable without specific sequencing of each genome, but I know the answer would rely in SNPs, or single nucleotide polymorphisms. A SNP would be the mother having an A on one chromosome, maybe an A on the allele, and the father having Ts at the same spot on his two alleles. Of course, the father could be A/T or A/A as well.

There are ~3 million SNPs in the genome out of 3 billion base pairs, which means at maximum 0.1% of the genome is different between individuals. Granted, there are other variations like VNTRs, but they should work in the same manner, and SNPs account for 90% of the differences. So you need to specifically look at each person at each SNP to determine the relationship. Also, I do not believe there is a single SNP (or at least one has not been found) that has more than 2 nucleotides at a position.

For example, lets look at a single SNP and how it relates to the son and sibling:

There are 9 possible combinations of mother/father at a single nucleotide (considering 2 alleles):

Number 1...2...3..4...5..6...7..8...9...
Mother AA AA AA AT AT AT TT TT TT
Father TT AT AA TT AT AA TT AT AA


Son/sibling possibilities for each scenario:
1. AT (100% chance brothers are identical)
2. AA or AT (50%)
3. AA (100%)
4. AT or TT (50%)
5. AA, AT, or TT (33%)
6. AA or AT (50%)
7. TT (100%)
8. AT or TT (50%)
9. AT (100%)


1. 0%
2. 50%
3. 100%
4. 50%
5. 33%
6. 50%
7. 100%
8. 50%
9. 0%

So the only difference is at scenario #1 and #9 where the father/son are 0% identical for the SNP. But what confuses me is the "identical" vs. "similar". The father still has 1 copy of the nucleotide of the son in both of those scenarios, so they are still 50% similar, correct (even though it's a 0% identical)? Brothers may have a 50% chance to have the same SNP AA or AT, but still brother #1 has 2 copies of A, and brother #2 has 1 copy of A and 1 of T...does this make them 50% identical, or 75% identical?

I really need someone with a probability background to analyze my data.

Overall, considering most people tend to marry people with similar ethnic backgrounds (and therefore similar SNPs), I will guess that brothers will be overall slightly more similar than fathers/sons.
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  #23  
Old 04-07-2008, 04:11 AM
mr. jp mr. jp is offline
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Ok, Bob55 I think you have the right idea. You made one error though, AT x AT gives (AA,AT,AT,TT), so the chance of father-son similarity is 50% not 33%. Also as you note, we should look at the similarity, not at the chance that they are identical.


I will give a difference of one nucleotide the value 1. If there is 50% there is a 1 nucleotide difference, that will get the value 0.5. A difference of 2 nucleotides will get the value x.


Case - father/son - brother/brother

1.............1..............0
2..........................
3.............0..............0
4..........................
5..................... + 1/8x
6..........................
7.............0..............0
8..........................
9.............1..............0


So in order to look at the difference, we can focus on case 1,5 and 9.

Now I assume that the population is in Hardy-Weinberg equilibrium.
Thus:

A has the frequence a
T has the frequence t

AA has the frequency a^2
AT has the frequency 2*a*t
TT has the frequency t^2


Case 1 will happen if AA meets TT, which happens with w frequency of a^2*t^2

Case 5, AT meets AT: 2*a*t * 2*a*t = 4*a^2*t^2

case 9, TT meets AA: a^2*t^2


So, to total up. the extra difference for father son is:
(case 1 + case 9) * 1 = 2*a^2*t^2


The extra difference for brother/brother is:
(case 5) * 1/8x = 4*a^2*t^2*1/8*x =*a^2*t^2*x


So it all boils down too: How different are two people who have both nucleotides different? I personally have no idea. If we assume that they are twice as different as two people with one nucleotide different, then x = 2, and we have:

father/son diff:
2*a^2*t^2

brother/brother diff:
*a^2*t^2*2 = a^2*t^2

So in that case, fathers and sons are more different overall, by a small degree.



(I hope this is possible to understand. I'm too lazy to explain thoroughly.)
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  #24  
Old 04-07-2008, 05:41 AM
Mijin Mijin is offline
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Quote:
Originally Posted by Smeghead
My understanding is that the OP was asking about relatedness. My interpretation of that was looking at where the genetic information came from, rather than what it was.
And I disagree.
He said "genetically closer" in both the title and content of the OP.

Say a mother and father with brown eyes produce a child with blue eyes, because they were both carrying a recessive blue eye gene. It seems to me utterly misleading and meaningless to say this gene is 50% the same as the father's and 50% the same as the mother's.

More importantly though, this interpretation of relatedness seems to imply that any two siblings are 0% related, since neither sibling obtained any genetic material from their sibling.
To interpret sibling similarity in any other way would be inconsistent.
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  #25  
Old 04-07-2008, 09:09 AM
Quercus Quercus is offline
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Hey, this isn't nearly complicated enough yet. Let me add a couple of things.
First, you need to account for the fact that the X chromosome is larger than the Y, so the boys are actually more related to their mother than their father. Girls will, at this level, be equally related to both.

But hold on, you need to also account for mitochondrial DNA, of which there is a single copy, inherited from the mother. So even the girls are slightly more related to their mother than their father. In a theoretical sense anyway. There isn't a whole lot of variation in m-DNA, and I don't think it's ever been shown to create any real difference in organisms.
[Finally, there's a possibility that non-DNA material inherited in the egg may have an effect on the child. I don't know if this has been found anywhere yet. It would mean slightly more relatedness to the mother again]
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  #26  
Old 04-07-2008, 10:50 AM
mr. jp mr. jp is offline
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Quote:
Originally Posted by Quercus
Hey, this isn't nearly complicated enough yet. Let me add a couple of things.
First, you need to account for the fact that the X chromosome is larger than the Y, so the boys are actually more related to their mother than their father. Girls will, at this level, be equally related to both.

But hold on, you need to also account for mitochondrial DNA, of which there is a single copy, inherited from the mother. So even the girls are slightly more related to their mother than their father. In a theoretical sense anyway. There isn't a whole lot of variation in m-DNA, and I don't think it's ever been shown to create any real difference in organisms.
[Finally, there's a possibility that non-DNA material inherited in the egg may have an effect on the child. I don't know if this has been found anywhere yet. It would mean slightly more relatedness to the mother again]
Those are valid points. The first one doesn't matter in the father/son vs brother/brother comparison though. But the second one does. Brothers will have the same mitochondria while the father will never have the same as his sons. (Unless they happen to be identical while not inherited of course.)
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