Can someone explain this please

[AndrewJE]

I have been using DNA painter and Chromosome browsers to map my matches. I have solved one brickwall and thought I understood how it all worked until the following result

I have a match on MyHeritage of 21.7 cmgs  on chromosome 16 and a match on Ancestry with her daughter of 22cmgs and  eight other shared matches ranging between 33cm and 17 cm  on Ancestry, six with trees have the same common Ancestors. Ancestry assigning them to my maternal side
So far so good

I then get contacted by the first matches first cousin(DFH) on Ancestry. There we do not match, but on GEDMATCH we have a 7.1 cmg match on Chromsome 15. This segment on my maternal side  is a subset of a known match(SH) that I have all ready painted. So I am thinking right I know what line this group must fit into my tree

But when I run a 1:1 on GEDMATCH between DFH and SH they don't match, unless I drop the minimum segment cm limit, when there is a 5.3 cm match on Chromosome 17 nothing on 15

DFH has the same common ancestors as the others in her tree.

The only thing I can think of is that she matches me  on my paternal side not maternal( hence the duplicate segment with SH is irrelevant), and that either she hasn't inherited the  DNA that I have  from the common Ancestors or there is an NPE and her tree is wrong

What do people think?

Andrew

[Glen in Tinsel Kni]

Ancestry kits uploaded to MH produce a lot of false positives due to the imputation methods MH use to fit their chip. MH often create/report a single segment where other sites choose to see two segments with a less populated (by SNPs) area in between. 

[melba_schmelba]

Ancestry kits uploaded to MH produce a lot of false positives due to the imputation methods MH use to fit their chip. MH often create/report a single segment where other sites choose to see two segments with a less populated (by SNPs) area in between.

Actually I would say, in my experience, Ancestry probably produce less false positives than uploading 23andme or FTDNA kits, or somewhat bizarrely with their own kit. That is because I do not think they changed their database parameters from the old FTDNA SNPs - (MH used to use FTDNA to do their tests), which is the same problem with GEDMATCH. And actually Ancestry has a better overlap to the original FTDNA/MyHeritage SNPs than the newer GSA chipped MyHeritage/FTDNA since 2019 or 23andme (since 2017). The only company I think expanded their database parameters were FTDNA, so it might be better to upload there if you have a 23andme or MH kit.

OP, from what company and date of test or chip if you know it did you upload to GEDMATCH? That can have a big effect on false positives as I explained above.