DNA differences between two half cousins

[Mart 'n' Al]

My grandparents split up soon after my father was born, and eventually had families with new spouses. I had my DNA matched against two half cousins, one a descendant via my grandmother and her new husband's child, and one via my grandfather and his new wife's child. Both of them matched me to about 470cM, but one of them matched me with 10 large groups while the other matched with 20 smaller groups, all above 7cM in both cases. What does that indicate if anything? I've read a lot about genetic genealogy recently but have been unable to find an answer for this.

Martin

[BushInn1746]

https://isogg.org/wiki/CentiMorgan

I'm still on the search for a document / paper trail, Mark

[Mart 'n' Al]

Mark, thank you.  I am an ISOGG member, and I've read most of the Wiki, but it still hasn't answered my query.  Both my half-cousins used AncestryDNA for their tests.  I need more than just "Segments which share a large number of centiMorgans in common are more likely to be of significance and to indicate a common ancestor within a genealogical timeframe."  I have a common ancestor with each half-cousin just two generations back.

Martin

[BushInn1746]

Hello Martin

I thought you might have seen it. I should like to come across one of those Scientists who works in a criminal testing Laboratory and get their opinion on all these DNA Tests for family history purposes.

Mark

[Mart 'n' Al]

Mark, I know what you mean. They do make it look so easy. They just slide two sheets of paper in opposite directions towards each other until the bars line-up and then they go out and arrest the baddie. I do think it should be made a lot clearer that ethnicity estimates are just estimates based on developing technology. I do thoroughly believe that autosomal tests are accurate. Many of the videos on YouTube are quite misleading comma where you see people opening their results on camera and being totally shocked by what they find when it's just people that are ill informed. I am impressed that so many Media organisations have done things like getting twins and identical triplets to do tests, to try and disprove autosomal matching, but as yet nobody has managed to do this. My main concern is that somebody with a bad attitude on a Friday afternoon just before changing jobs does something a little bit unethical. I am still surprised to have found out last week that I have 8% Greek ethnicity, something which isn't reflected in my papertrail research. All I can assume is that one of my female ancestors gave into the Temptation of a visiting sailor.

Martin

[Mart 'n' Al]

After doing a lot of research myself, before starting this thread, I can now say that I have found the following on genie1.com.

"Regarding the 'largest cM', generally, the larger the biggest individual segment, the closer the relationship.".

The problem is that my two half cousins in question are both equally closely connected to our most recent common ancestor. My original question still stands.

Martin

[hdw]

I've mentioned before in a previous post my MyHeritage close match who I have worked out must be the grandson of one of my uncles, hence my 1st cousin once removed. What surprises me is how many segments of DNA we have in common, given that he is a only a relative on my father's side. When I tot up the segments I share with him there isn't much left over for my (unrelated) mother's side.

This chap and I match on chromosomes 1, 2, 3, 5, 7, 8, 9, 11, 12, 16, 17 and 22 and in most cases it's between 2 and 4 segments on each chromosome.

Harry

[davidft]

After doing a lot of research myself, before starting this thread, I can now say that I have found the following on genie1.com.

"Regarding the 'largest cM', generally, the larger the biggest individual segment, the closer the relationship.".

The problem is that my two half cousins in question are both equally closely connected to our most recent common ancestor. My original question still stands.

Martin

The problem there, IMO, is that genie1 is paraphrasing and has altered the true sense of the meaning.

If you look at the link given above it says

Segments which share a large number of centiMorgans in common are more likely to be of significance and to indicate a common ancestor within a genealogical timeframe.


i.e it is not giving it as an absolute just a more likely than not. In the case of your two cousins the reason why there is a difference in the sizes of the matches to you is that DNA is inherited at random, if it was not the matches would be of the same size.

(oh and I am no expert on this so if anyone wants to disagree feel free  )

[Mart 'n' Al]

Thanks again David, I feel like one of the five blind men of the Fable each feeling a different part of the Elephant and describing it to the others.

Martin