Is this my grand daughter? Avuncular DNA
July 19, 2010 9:34 AM Subscribe
How dependable are avuncular DNA test for determining paternity.
There is a shadow of a doubt regarding the paternity of my grand daughter. This situation, such as it is, is one where I don't want to involve any more family members than necessary. How dependable is a DNA test using samples only from the grand daughter and the grand father?
Second, Googling brings up multiple potential labs. Recommendations regarding any dependable lab will also be appreciated.
There is a shadow of a doubt regarding the paternity of my grand daughter. This situation, such as it is, is one where I don't want to involve any more family members than necessary. How dependable is a DNA test using samples only from the grand daughter and the grand father?
Second, Googling brings up multiple potential labs. Recommendations regarding any dependable lab will also be appreciated.
I was under the impression that avuncular DNA required an aunt or uncle's DNA sample.
The grandparent-grandchild relationship has the same degree of genetic similarity as the aunt/uncle-niece/nephew relationship. In theory, it should be possible to get a definitive negative (e.g. non-related) result from such a test, but a positive (related) result will have some degree of uncertainty.
posted by mr_roboto at 10:38 AM on July 19, 2010
The grandparent-grandchild relationship has the same degree of genetic similarity as the aunt/uncle-niece/nephew relationship. In theory, it should be possible to get a definitive negative (e.g. non-related) result from such a test, but a positive (related) result will have some degree of uncertainty.
posted by mr_roboto at 10:38 AM on July 19, 2010
OK, let's see how well I remember high school biology. Men have an X and a Y chromosome; Women have two X chromosomes. Let's say you have a son:
Grandmother (X1 X2) + Grandfather (X3 Y1) -> Son (X4 Y1)
Son (X4 Y1) + Step-daughter (X5 X6) -> Granddaughter (X4 X8)
In this case, we know that X4 is either X1 or X2; and we know that X8 is either X5 or X6. So there;s are no shared chromosome between the grandfather and the granddaughter - and no paternity test without the grandmother or the son being involved.
On the other hand, say you've got a daughter (though doubting a daughter's paternity seems unlikely):
Grandmother (X1 X2) + Grandfather (X3 Y1) -> Daughter (X4 X3)
Daughter (X4 X3) + Step-son (X5 Y2) -> Granddaughter (X7 X5)
In this case, X7 is either X4 or X3, while X4 is either X1 or X2. So there's a 50% chance that there's no shared chromosome between the grandfather and the granddaughter. So a 'no shared chromosome' result is inconclusive. For a conclusive result, you'd need either the grandmother or the daughter involved.
Of course, it's possible I'm mis-remembering this stuff; hopefully if I'm wrong, someone can correct me!
posted by Mike1024 at 10:57 AM on July 19, 2010
Grandmother (X1 X2) + Grandfather (X3 Y1) -> Son (X4 Y1)
Son (X4 Y1) + Step-daughter (X5 X6) -> Granddaughter (X4 X8)
In this case, we know that X4 is either X1 or X2; and we know that X8 is either X5 or X6. So there;s are no shared chromosome between the grandfather and the granddaughter - and no paternity test without the grandmother or the son being involved.
On the other hand, say you've got a daughter (though doubting a daughter's paternity seems unlikely):
Grandmother (X1 X2) + Grandfather (X3 Y1) -> Daughter (X4 X3)
Daughter (X4 X3) + Step-son (X5 Y2) -> Granddaughter (X7 X5)
In this case, X7 is either X4 or X3, while X4 is either X1 or X2. So there's a 50% chance that there's no shared chromosome between the grandfather and the granddaughter. So a 'no shared chromosome' result is inconclusive. For a conclusive result, you'd need either the grandmother or the daughter involved.
Of course, it's possible I'm mis-remembering this stuff; hopefully if I'm wrong, someone can correct me!
posted by Mike1024 at 10:57 AM on July 19, 2010
Of course, it's possible I'm mis-remembering this stuff...
Modern genetic testing looks at more than 2 alleles. I believe that most tests for relatedness look at 13 loci, none of which are on the sex chromosomes. The standard technique is STR analysis; your memories of high-school biology are neither here nor there.
posted by mr_roboto at 11:10 AM on July 19, 2010 [1 favorite]
Modern genetic testing looks at more than 2 alleles. I believe that most tests for relatedness look at 13 loci, none of which are on the sex chromosomes. The standard technique is STR analysis; your memories of high-school biology are neither here nor there.
posted by mr_roboto at 11:10 AM on July 19, 2010 [1 favorite]
Best answer: The results would be better if both grandmother and grandfather DNA is analyzed and compared with grandchild. From the grandparents (generation I) the possible permutations of one parent (generation II) are determined, and from that the analysis with the granddaughter (generation III) is performed.
Locus 1: generation I a,b,c,d alleles (grandfather is ab, grandmother is cd)
Locus 2: generation I g,h alleles (in this case grandfather is gh, grandmother is gh)
Locus 3: generation I k,l,m alleles (in this case grandfather is kl, grandmother is mm)
Generation II possibilities:
Locus 1: ac,ad,bc,bd
Locus 2: gg,gh,hh
Locus 3: km, lm
Generation III results:
Locus 1: ef
Locus 2: hi
Locus 3: mm
In this case paternity is excluded with 99% certainty because generation II could not have contributed either e or f for Locus 1. Locus 2 and 3 do not tell you anything. There is a good possibility of the answer being "Not determinable" even with four loci analyzed.
Without both the grandparents the situation is more difficult, with a high possibility of the answer being "not determinable".
posted by francesca too at 12:16 PM on July 19, 2010
Locus 1: generation I a,b,c,d alleles (grandfather is ab, grandmother is cd)
Locus 2: generation I g,h alleles (in this case grandfather is gh, grandmother is gh)
Locus 3: generation I k,l,m alleles (in this case grandfather is kl, grandmother is mm)
Generation II possibilities:
Locus 1: ac,ad,bc,bd
Locus 2: gg,gh,hh
Locus 3: km, lm
Generation III results:
Locus 1: ef
Locus 2: hi
Locus 3: mm
In this case paternity is excluded with 99% certainty because generation II could not have contributed either e or f for Locus 1. Locus 2 and 3 do not tell you anything. There is a good possibility of the answer being "Not determinable" even with four loci analyzed.
Without both the grandparents the situation is more difficult, with a high possibility of the answer being "not determinable".
posted by francesca too at 12:16 PM on July 19, 2010
Response by poster: That "breach of trust" thing - I get the concept but not my question.
posted by TurdBlossom at 4:30 PM on July 20, 2010
posted by TurdBlossom at 4:30 PM on July 20, 2010
This thread is closed to new comments.
Also, as an aside, are you doing this without the permission of the child's parents? If not, it is a huge, massive breach of their trust. What do you expect to gain from the results of this DNA test?
posted by kpht at 10:02 AM on July 19, 2010 [1 favorite]