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Why would I match someone in a region neither of my parents do?
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I was surprised to find someone show up in FTDNA as a 2nd-3rd cousin to me (168 shared cM, 34 segments with a minimum length of 5 cM) but not among the 17,667 matches of my father or the 18,178 matches of my father.
Fortunately, the person's DNA was also on GEDmatch, where I could compare his DNA to mine and my parents'. The chromosome browser showed me as inheriting most of the segments that my parents had in common with that person; however, on chromosome 14, I had a 3.1 cM match to the person in a region in which neither of my parents matches him. Something similar occurs on chromosome 20, where my match with the unknown person (4.47 cM) is longer than my mother's match to him (3.13 cM) and my father has no match.
Is the likely explanation that my parents have matches too small to show up unless they're combined in me? For example, they could each have a 2 cM stretch:
Mom ------XX-------
Dad --------XX-----
me ------XXXX-----
Or is there a different explanation? In general, what is a reasonable minimum cM threshold?
I am an Ashkenazi Jew. My parents are not closely related to each other.
dna jewish gedmatch.com endogamy
asked Aug 7 at 21:55
Ellen Spertus
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I was surprised to find someone show up in FTDNA as a 2nd-3rd cousin to me (168 shared cM, 34 segments with a minimum length of 5 cM) but not among the 17,667 matches of my father or the 18,178 matches of my father.
Fortunately, the person's DNA was also on GEDmatch, where I could compare his DNA to mine and my parents'. The chromosome browser showed me as inheriting most of the segments that my parents had in common with that person; however, on chromosome 14, I had a 3.1 cM match to the person in a region in which neither of my parents matches him. Something similar occurs on chromosome 20, where my match with the unknown person (4.47 cM) is longer than my mother's match to him (3.13 cM) and my father has no match.
Is the likely explanation that my parents have matches too small to show up unless they're combined in me? For example, they could each have a 2 cM stretch:
Mom ------XX-------
Dad --------XX-----
me ------XXXX-----
Or is there a different explanation? In general, what is a reasonable minimum cM threshold?
I am an Ashkenazi Jew. My parents are not closely related to each other.
dna jewish gedmatch.com endogamy
asked Aug 7 at 21:55
Ellen Spertus
1,061426
add a comment |Â
up vote
3
down vote
favorite
up vote
3
down vote
favorite
I was surprised to find someone show up in FTDNA as a 2nd-3rd cousin to me (168 shared cM, 34 segments with a minimum length of 5 cM) but not among the 17,667 matches of my father or the 18,178 matches of my father.
Fortunately, the person's DNA was also on GEDmatch, where I could compare his DNA to mine and my parents'. The chromosome browser showed me as inheriting most of the segments that my parents had in common with that person; however, on chromosome 14, I had a 3.1 cM match to the person in a region in which neither of my parents matches him. Something similar occurs on chromosome 20, where my match with the unknown person (4.47 cM) is longer than my mother's match to him (3.13 cM) and my father has no match.
Is the likely explanation that my parents have matches too small to show up unless they're combined in me? For example, they could each have a 2 cM stretch:
Mom ------XX-------
Dad --------XX-----
me ------XXXX-----
Or is there a different explanation? In general, what is a reasonable minimum cM threshold?
I am an Ashkenazi Jew. My parents are not closely related to each other.
dna jewish gedmatch.com endogamy
asked Aug 7 at 21:55
Ellen Spertus
1,061426
I was surprised to find someone show up in FTDNA as a 2nd-3rd cousin to me (168 shared cM, 34 segments with a minimum length of 5 cM) but not among the 17,667 matches of my father or the 18,178 matches of my father.
Fortunately, the person's DNA was also on GEDmatch, where I could compare his DNA to mine and my parents'. The chromosome browser showed me as inheriting most of the segments that my parents had in common with that person; however, on chromosome 14, I had a 3.1 cM match to the person in a region in which neither of my parents matches him. Something similar occurs on chromosome 20, where my match with the unknown person (4.47 cM) is longer than my mother's match to him (3.13 cM) and my father has no match.
Is the likely explanation that my parents have matches too small to show up unless they're combined in me? For example, they could each have a 2 cM stretch:
Mom ------XX-------
Dad --------XX-----
me ------XXXX-----
Or is there a different explanation? In general, what is a reasonable minimum cM threshold?
I am an Ashkenazi Jew. My parents are not closely related to each other.
dna jewish gedmatch.com endogamy
asked Aug 7 at 21:55
Ellen Spertus
1,061426
asked Aug 7 at 21:55
Ellen Spertus
1,061426
asked Aug 7 at 21:55
Ellen Spertus
1,061426
asked Aug 7 at 21:55
Ellen Spertus
1,061426
1,061426
add a comment |Â
add a comment |Â
1 Answer
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What I see often said is that any matching segments under 5 cM, if not under 7 cM, are more likely to be identical by chance rather than by descent, and should usually be disregarded as noise.
A Small Segment Round-Up (Blaine Bettinger posted 29 December 2017 on his blog The Genetic Genealogist)
Sharing Large Segments With a Match Does Not Validate Small Segments Shared With That Match (Blaine Bettinger posted 3 September 2017 on his blog The Genetic Genealogist)
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
answered Aug 7 at 22:02
Leah Worster
6367
2
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
2
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
6
down vote
What I see often said is that any matching segments under 5 cM, if not under 7 cM, are more likely to be identical by chance rather than by descent, and should usually be disregarded as noise.
A Small Segment Round-Up (Blaine Bettinger posted 29 December 2017 on his blog The Genetic Genealogist)
Sharing Large Segments With a Match Does Not Validate Small Segments Shared With That Match (Blaine Bettinger posted 3 September 2017 on his blog The Genetic Genealogist)
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
answered Aug 7 at 22:02
Leah Worster
6367
2
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
2
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
add a comment |Â
up vote
6
down vote
What I see often said is that any matching segments under 5 cM, if not under 7 cM, are more likely to be identical by chance rather than by descent, and should usually be disregarded as noise.
A Small Segment Round-Up (Blaine Bettinger posted 29 December 2017 on his blog The Genetic Genealogist)
Sharing Large Segments With a Match Does Not Validate Small Segments Shared With That Match (Blaine Bettinger posted 3 September 2017 on his blog The Genetic Genealogist)
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
answered Aug 7 at 22:02
Leah Worster
6367
2
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
2
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
add a comment |Â
up vote
6
down vote
up vote
6
down vote
What I see often said is that any matching segments under 5 cM, if not under 7 cM, are more likely to be identical by chance rather than by descent, and should usually be disregarded as noise.
A Small Segment Round-Up (Blaine Bettinger posted 29 December 2017 on his blog The Genetic Genealogist)
Sharing Large Segments With a Match Does Not Validate Small Segments Shared With That Match (Blaine Bettinger posted 3 September 2017 on his blog The Genetic Genealogist)
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
answered Aug 7 at 22:02
Leah Worster
6367
What I see often said is that any matching segments under 5 cM, if not under 7 cM, are more likely to be identical by chance rather than by descent, and should usually be disregarded as noise.
A Small Segment Round-Up (Blaine Bettinger posted 29 December 2017 on his blog The Genetic Genealogist)
Sharing Large Segments With a Match Does Not Validate Small Segments Shared With That Match (Blaine Bettinger posted 3 September 2017 on his blog The Genetic Genealogist)
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
answered Aug 7 at 22:02
Leah Worster
6367
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
edited Aug 8 at 4:03
Jan Murphy♦
17.4k33694
17.4k33694
answered Aug 7 at 22:02
Leah Worster
6367
answered Aug 7 at 22:02
Leah Worster
6367
answered Aug 7 at 22:02
Leah Worster
6367
6367
2
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
2
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
add a comment |Â
2
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
2
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
2
2
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down.
– Jan Murphy♦
Aug 8 at 3:58
2
2
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
@JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself.
– Leah Worster
Aug 8 at 18:10
add a comment |Â
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