Assistance Interpreting FamilyFinder results

[SmittyInTheSky]

Hello group,

I'm starting in this basic forum to make sure I'm not missing something fundamental about how I'm interpreting my results.

I recently found a match, we'll call them ALG, who matches me on my Cr4 at around 9cM. We share a total of 36.64cM, making them a somewhat distant match.

The match has a number of their family members, two at the parental level (a father and uncle) all the way to grandchild level (the son of a daughter) in the FTDNA database, and, as expected, I match all of them in the same place as I match ALG.

In trying to determine which side of my family the match comes through, I've found that my father's FamilyFinder match list includes ALG's daughter and shows that she matches my father at the exact same place as myself, Cr4 at around 9cM. The confusing part is now, when looking at my father's results, he only matches ALG's daughter and not ALG or any of their other family members. Additionally, I am shown to have more SNP's in common with ALG's daughter than my father is shown to have. I find this odd. I have seen the FTDNA information that suggests the random recombination can contribute to false identification of people who aren't related, but I highly doubt this is the case in this situation, and in general, highly improbable, but not impossible.

What I'm having a hard time understanding is the following:

If I match ALG and ALG's relatives, why does my dad, who matches ALG's daughter not match the other relatives who all match at the same place?

I ran this scenario through GEDmatch and have seen the results I expected to, my father being related to ALG and all of her family members.

I wish to suggest that this scenario shows that FamilyTreeDNA's matching algorithm is greatly flawed.

Thank you,

Chris Smith

[prairielad]

This most like has to do with FTDNA's matching algorithm.
A person needs to share at least 20cM of DNA in total to be declared a match.

We have two of each chromosomes, a maternal and a paternal. Matching algorithm does not know which value is to which, so it looks for matching segments using both values. Some matching segments will be IBS due to this, it is picking a few values from maternal chromosome and a few from paternal chromosome. These are usually segments under 5cM.

You probably only have this one segment of 9cM with ALG (maybe a few smaller ones)which are IBD (Identical by descent), but your IBS segments push you over the matching threshold of 20cM.

For your father on the other hand, the algorithm does not find enough IBS segments to push him over the matching threshold to match this family other then the daughter even though this 9cM segment maybe present.

EDIT

FTDNA's matching Algorithm seems to need a minimum of 20cM of shared DNA, 1 longest block of at least 7.7cM with a minimum of 500SNPs, additional segments all have to be at least 500SNPs

If you look at Gedmatch and compare your father to this family, How many of the segments are under 500 SNP's. These are all probably false segments (IBS).

At FTDNA most of the smaller segments between your father and daughter(ALG) as well as your smaller segment to ALGs family are more then likely to be IBS. You just have enough IBS segments to push you over FTDNAs minimum 20cM total needed. While your father does not have enough when comparing him to ALGs other family members