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My father and I have a DNA match to a cousin but my son does not. How is this possible?

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    What degree of cousin is involved? Can you share cM values for each relationship? – ColeValleyGirl Jan 26 at 17:05
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    Also, are you sure this is Y-DNA. And what does 'my son does not always show that he is also related' mean. Where have you all tested? – ColeValleyGirl Jan 26 at 17:15
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    I did a massive edit on this question to try to make sense of it. I also changed the type of DNA since Y-DNA is not relevant here. – Cyn Jan 27 at 1:44
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    Let's be careful about editing questions based on our assumptions of people's gender which is based on their usernames. 'Kim' may be used more often as a girl's name in the US at the moment, but it has a long history of being a boy's name, and is in use in other parts of the world as a man's nickname. en.wikipedia.org/wiki/Kim_(given_name) In other words, not unlike my own name, which can be used for both males or females, depending on locality of origin. Also, let's be inclusive and not assume every visitor is cisgender or using a username which matches their gender in real life. – Jan Murphy Jan 27 at 2:13
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    @JanMurphy I did not make assumptions about gender. Original title was "Grandfather and mother reflect relation to cousin but not grandson" then the OP said "my father" & "my son." Clearly, she is the mother. It's also clear that the OP was talking about autosomal DNA & not Y-DNA because Y-DNA doesn't have "matches" in that sense. Instead, someone talking about Y-DNA would have said "my father, my cousin, & I all have the same Y-DNA halpogroup, but my son's is different." The fact that the original title said "mother" also is a clue that Y-DNA was not used. Someone trans would have clarified. – Cyn Jan 27 at 20:39
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You inherit 50% of your DNA from your mom and 50% from your dad. On average that means you'll inherit 25% from each of your 4 grandparents. But the reality is that which DNA you get from each parent is completely random. You always get the same number and length of chromosomes, but you don't know which side (DNA being double stranded) you'll get.

So if your father's match with the cousin is A cM, then your match might be half of A, and your son's match might be 1/4 of A. But human beings are not averages. Maybe your match is small and your son didn't get any of the relevant segments (not ones long enough to count anyway).

If this is a very close cousin, then that's different. If your son is a full second cousin to this person then there should be a DNA match. If it's a more distant cousin, then often the match is 0.

  • It is theoretically possible for full siblings to share no DNA. With cousins, especially second cousins or further, the possibility becomes more than theoretical. – JPmiaou Jan 28 at 23:21
  • It is not possible for full siblings to share no DNA. It has never happened. Show me one case. Cite your sources. @JPmiaou – Cyn Jan 28 at 23:42
  • I said it's theoretically possible. That doesn't mean it actually ever happens; the probability is infinitesimal. (You'd either need the "other half" of the specific egg meeting up with the "other half" of the specific sperm, or you'd need the chromosomes to split identically twice.) The probability of second cousins not sharing any DNA is also very low, but well within the realm of "actually possible". – JPmiaou Jan 29 at 17:41
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    @JPmiaou It's not possible period. By mentioning it in this context you are confusing or giving false hope to people (maybe my sibling really is my blood sibling). Anyone who is 2nd cousin or closer will show a DNA match. "There has NEVER been a single demonstrated case of second cousins or closer who fail to share DNA....There are claims out there, but they are completely unsupported at the current time." thegeneticgenealogist.com/2016/10/03/… – Cyn Jan 29 at 19:50
  • Can we please not use X as a mathematical variable in DNA questions and answers? I suggest that if you want to talk about a quantity of shared DNA, express it as "n cMs" to remove ambiguity. – Jan Murphy Jan 30 at 1:11

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