Ancestry - number of dna matches

[Pheno]

I wrote the following in response to a post by LizzieL earlier in the week:

'My sister and I are full siblings, I have 13,467 matches and she has 24,479 (data from shared DNA tab).  There has always been this magnitude of difference'

and it has set me thinking.  Does this seem right - that there would be such a discrepancy in the number of matches we have.  I know we each get 50% of dna from each of our parents but it really seems to suggest that we might have almost entirely different strings of dna from each other.

I know ethnicity is nothing to write home about but according to Ancestry they are not much different:

                                           Me                                                          Her
England & NW Europe            45%                                                       47%
Scotland                               29%                                                       26%
Wales                                     9%                                                         9%
Ireland                                   6%                                                         8%
Germanic Europe                    8%                                                          -
Sweden                                 3%                                                          10%

Is it likely that these small differences, mainly in Germanic Europe and Swedish account for the discrepancy in the number of matches?

For numerical comparison (I know they will not have the same dna/matches as me) my husband has 18,716, my daughter has 15,366 and my brother in law has 18,096 matches.

My sister and I are at extreme ends of our small scale, and yes we are full siblings.

Does anybody else have a similar pattern or is a discrepancy of this magnitude unusual?

Pheno

[Gadget]

Have you compared your  shared DNA  segments in My Heritage or Gedmatch?

Add -

https://www.dataminingdna.com/four-ways-to-use-a-chromosome-browser-with-ancestry-dna/

[Pheno]

No I haven't Gadget.  Is this something I should do?  Will it contribute to understanding the anomaly?

Pheno

PS Just noticed you have added a link - will take  look at that as it may answer some of these questions.

[Gadget]

Here's an example of using the My Heritage Chromosome Browser showing the shared segments by each chromosome between my niece and me.  There would be a closer match if it were possible to use her mother but, sadly, she's not with us.

My Heritage and others also have the facility to compare your segments with a selection of matches. If you are able to have enough shared matches on one of the sites that offer a chromosome browser. you'd be able to see the differences between your sister's and your matches.

Gadget

[Gadget]

I've just realised that I have access to some sibling data that you might be able to use for comparison with your sibling data.  This shows how DNA inheritance changes quite significantly between siblings and generations:

[AnnieBard]

Hi Pheno

Those matches look fine to me, full siblings generally only share about half of their DNA with one another. That’s the result of genetic recombination, and the more diverse your recent ancestry the more pronounced the effects of that can be.

Another factor is points of comparison within the databases used. E.g. there will almost certainly be less data available for Sweden than for Scotland say, so you may both have similar amounts of Swedish DNA (an odd way to phrase it, I know) but yours won’t be identical to your sister’s and there may simply have been more matches for hers than for yours in the database.

Hope that makes sense.

[whoamitoday]

If it helps, I have 14% more matches than my sister on Ancestry. We share 2637cm and are definitely full siblings but our ethnicity breakdown is very different - mostly the same 4 places (plus I have a little bit of Norwegian and she has a little bit of Germanic) but in a totally different proportions e.g. I'm supposedly 34% of Welsh origin while my sister is only listed as 16% Welsh. I don't have Germanic on my breakdown but match with more people on our German ancestors line than she does and have more shared cm with the cousins we both match with on that branch.

I really don't take the stats on ethnicity seriously but what does puzzle me is that all of my matches have a single percentage figure of shared DNA next to them, except for my sister, where it says 47%-54%. Surely a 2637cm match is more like 37%? Maybe it's a glitch that will correct itself at some point, but it seems pretty odd. Is it clearer and more accurate on your results between you and your sister?

[Pheno]

Hi Whoamitoday, our cM match is 2706cM which is calcuated as 48% - 55%.  If our average shared cM % is about 50% then it just seems odd that she has so many more matches.

Pheno

[whoamitoday]

Hi Whoamitoday, our cM match is 2706cM which is calcuated as 48% - 55%.  If our average shared cM % is about 50% then it just seems odd that she has so many more matches.

Pheno

You have the same quirk in your results that I do, in that Ancestry doesn't specify the exact percentage of DNA that you share with your sister. I know the theory is that we share approx. 50% with a sibling and 50% with each parent. One of my parents has tested and both my sister and I share 3489cm with our parent which Ancestry classify as 50% shared DNA. A grandparent has also tested and our parent shares 3489cm/50% with their parent. Incidentally, I got 27% of this grandparent's DNA and my sister got 25%.

What's odd is that if 3489cm is 50%, why won't they be more specific about the percentage between the siblings? Using their base line of around 7000cm being 100%, then I actually only share around 37% of my DNA with my sister. You and yours share only slightly more, a lot less than 50% in both cases. They are well within the accepted range for full sibling relationships. I'm not questioning the relationships, only Ancestry's ambiguity. If I'm right and the amount of similarity is actually smaller then it follows that the differences are larger and therefore might explain why you and your sister don't match with many of the same people.

It's unfortunate and probably quite random that you don't have more matches that don't also match with her. Maybe as more people test you will, and the gap will close.