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Suppose I have whole genome sequences for two people of the same sex. I know one person is the parent of the other, but I do not know who is the parent and who is the child. Is it possible to determine with certainty the direction of descent using the sequencing information alone? Assume that the individuals were sampled at the same age so that you can't tell parentage from telomeres or other age-related markers.

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    $\begingroup$ @DKNguyen I don't think it is the same, no. I don't care to look if they are relative here, I care to know which one engendered the other, only by looking at their genes. And yes, it is by assuming I have the full knowledge of both genetic codes. I picked 100 k years as an example to remove C14 possibility out of the question, but if you prefer, make it, say, 5000 years ago or even a decade ago (but then please don't cheat using C14 :) ). $\endgroup$ Commented Sep 28, 2021 at 18:23
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    $\begingroup$ What do you mean by 'only looking at the chromosomes'? Staring at them down a microscope? Sequencing them? $\endgroup$
    – user438383
    Commented Sep 28, 2021 at 19:27
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    $\begingroup$ Take a chromosome of individual A. For each gene check if an identical copy is found in individual B. If it is the case then B is the parent of A. $\endgroup$
    – reuns
    Commented Sep 28, 2021 at 23:46
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    $\begingroup$ @rerus if the gene is identical in both, why does that rule out A being the parent of B? $\endgroup$
    – swbarnes2
    Commented Sep 28, 2021 at 23:59
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    $\begingroup$ If one chromosome is identical between A and B, how can that possibly distinguish anything at all between them? If two students in a class turn in identical papers, how is that evidence that A must have copied from B, and that B could not have copied from A? $\endgroup$
    – swbarnes2
    Commented Sep 29, 2021 at 17:10

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If A is XY and B is XX, if A is the parent, A should pass on their X chromosome intact to the offspring. If B is the parent, than the X chromosome A receives would have undergone crossing over with B's other X chromosome before being passed to A.

Otherwise, I don't think you could do it from sequence alone.

I guess if the circumstances were right, you could include haplotpye information, and you could tell; if one person had top to bottom haplotypes only found in China, and the other was half Chinese haplotypes and half African haplotypes, the simplest explanation is that the person with one ethnic origin is the parent, and the person with two is the offspring

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