DNA Research #DNA RE: False negatives in Family Finder test #dna

Sarah L Meyer

The answer to the probability question is in part given in the FAQ's (What
is the probability that my relative and I share enough DNA for Family Finder
to detect?
The FAQ's have chance of detection
2nd cousin or closer >.99
3rd cousins >.90
4th cousins >.5
5th cousin >.1
6th and more distant <.02

I had a second cousin once removed, test as a distant cousin, so my
interpretation is that that these percentages are the chances of detecting
the correct relationship (and not just any relationship). Unless FTDNA is
saying that those probabilities are the probability of detection at any
level- I have to assume that they are at the stated level. So the
probability of being missed is 1- the probability of being detected. So for
second cousins or closer there is less than a 1% chance of being missed and
for third cousins is <10%, fourth cousins <50 percent etc. (BTW Gedmatch had
him at 3.5 generations to MRCA).
Ignoring the less than or greater than signs (since they did not give us
more places past the decimal), in my case without considering the
possibility of a second cousin being considered a 3rd cousin etc with TWO
people being missed, the chances of missing both were (less than) .0001 or 1
in 10,000, which is why if there was no error in the DNA processing, I was
going to look for "the milkman". And if I were to take into consideration -
a match at *any* level, the chances would have been much smaller.
In this case, I think we need to look at this as if they were second
cousins (they are actually closer) and the chances that they did not find
them as second cousins is less than 1%, the chance that they will be found
as second cousins or third cousins (but not second) is .99+.01*.9=.999 so
the chance of missing them as both a second cousin and a third cousin would
be .001 etc. A calculations gives that the probability of detection at any
level up through 5th cousins is .99955 which would say the probability
assuming that each of those > signs was an equal sign and that the match did
not occur at any level is .00045. This does not even take into account the
probability of a 6th or greater cousin match. That is why at this level of
known relationship, I suggested that GEDMATCH.com be used to see if the
location of the error could be determined.
Sarah L M Christiansen
Georgetown TX 78633

From: Judy Simon <heyjude0701@gmail.com>
Date: Sun, 22 Jul 2012 09:15:22 -0400

There is a lot of randomness in the process of passing autosomal
chromosome segments to the next generation; by the luck of the draw,
it is possible for a woman and her sister's grandson to have no
segments in common in the Family Finder test, yielding a false negative
test. I don't think this this situation is common, but I don't have
enough knowledge and experience to put a probability on it.
There are others on the SIG who know more than I do, so hopefully someone
will come up with an estimate.

Judy Simon
Stony Brook, NY

On Sat, Jul 21, 2012 at 6:12 PM, Gary Mokotoff <mokotoff@earthlink.net>

It is my understanding that the Family Finder test of Family Tree DNA
can produce false positives, that is, two people may show close
kinship but actually may not be that closely related.

My question is whether the Family Finder test can also produce false
negatives. Specifically, I tested a woman and a man that hopefully was
her grandnephew (her sister's son's son). There was no Family Finder
match. Does this unconditionally conclude that they are not related?
If false negatives do occur, does anyone know at what rate given the
kinship described above?

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