Q re shared match with father and son

[neelawson]

Why would a match's son share 5cM more and 1 segment more with me than his father? Does it mean the mother is also a relative?

[Glen in Tinsel Kni]

As most tests only test part of the genome and different companies use different SNP's it's impossible to do a like for like comparison.
When comparing my Ancestry result to the MH test by Dan and Alfie (Dan is my 1c1r, Alfie is his son), the start and end points of each segment often are somewhat different. 

On 12,14 and 21 I share 22cM more with Alfie than I do with his father but there is no link to Alfie's mum.


Ancestry-MH results;
Dan 319.6‎ cM across 16 segments
Alfie  286.6‎ cM across 12

They have also tested with Ancestry with the following results;
Dan  321 cM across 15 segments
Alfie 260 cM across 12 segments

[neelawson]

The results in question are all with MH not different companies

[Glen in Tinsel Kni]

What I omitted from my initial comment and to avoid any issues with modifying it after the event the testing process uses imputation to reconstruct the dna and calculate results, it's not as precise as some would believe.
Going back to one point I did mention earlier, MH do not test the entire genome so even a like for like company comparison is still only a partial comparison.

[TreeDigger]

Shared DNA can be sorted into two groups; Identical by descent (IBD) and Identical by state (IBS).

IBD means you've inherited that DNA from a direct ancestor, which means that segment came from a parent. If the parent also took a DNA test, said segment should show up (either identical or a bit larger than the one you inherited). These segments are usually larger than 7-10cM with at least 1,000 SNP for them to be considered IBD, though smaller IBD segments are possible.

IBS is DNA shared among a population or ethnic group, meaning this DNA is shared by many people who usually originate from the same geographic region. There is a common ancestor somewhere, but usually thousands of years ago.

It is quite possible for you but not your parent to share an IBS DNA segment with a match, which then gets a so-called 'false positive' result. Usually these segments are smaller than 7cM. It may even happen that you seem to share a larger DNA segment with a match than your parent, in which case the segment is combined of both the true (IBD) segment and a false (IBS) segment.

Hopefully my explanation makes sense.

TD