100% match with a parent?

[dermotb]

This has to be a bug

Our family did three Family Finder tests
* mother
* son
* daughter

The matches of the mother against EACH of her son and daughter show as 3384.31 cM, which the internet tells me is a 100% identical match - this is obviously wrong. The son and daughter match with 2700 cM, which is also much higher than I would expect, with 5 chromosomes showing a 100% match.
In case this was a display error, I ran the chromosome browser chart which showed a 100% match too, and downloading a list of the matches in Excel confirmed it again.

There is also a strange result in the son's ancestral origin chart, which doesn't fit with the known family history.

Finally, neither the son nor daughter's raw data will download, producing a browser error.

What on earth is going on?

[DaveInGreece]

A full match (which only identical twins have) is around 7400 cM.

What you can see between mother and children on chromosome browser is that the children are identical to their mother all the way along one copy of each chromosome. This is "half identical" and accounts for around 3400 cM. Each child also has a copy of each chromosome from their father, totalling another 3400 cM.

Full siblings (who aren't twins) share about half of their DNA (closer to 3400 cM in total). But siblings can share DNA on their maternal side, paternal side, both sides or neither. The segments where they match both parents are "fully identical". But FTDNA only counts the basic number of cM, without counting them double when it's a full IBD match, so siblings always appear as less than 3400 cM.

FTDNA should really report siblings as being something like "2700 cM half IBD or greater".

[dermotb]

Thank you for that, I'll have to absorb it

I thought I understood this stuff *sigh*

[prairielad]

In regards to Downloading of Raw Data, if tests have just complete, it takes a few days before Raw Data is available.

[mkdexter]

This has to be a bug

Our family did three Family Finder tests
* mother
* son
* daughter

The matches of the mother against EACH of her son and daughter show as 3384.31 cM, which the internet tells me is a 100% identical match - this is obviously wrong.

Expanding on what DaveInGreece said, Family Finder is a test of 22 pairs of chromosomes plus the X. One half of each chromosome pair is paternal and one half is maternal, thus 50% is 22 chromosomes matching one parent and 50% is the other 22 matching the other parent; plus either an X or a Y depending upon gender, to equal a total of 23 out of 46 chromosomes (50%).

This is called a half identical by descent comparison test, so only 22 are ever shown as a match to the other 22 (100% match) but behind the scenes, there are two sides (44 not 22 autosomes total), not one side.

Matt.