Good Question 13 Jan 2016

Someone told me what customer service at Ancestry.com told them, and asked me if it was true:

A lady found out from her dying mom, that dear old dad might not have been bio dad. She followed some clues and came up with a good candidate, who's since died, and she met his sons. She wants to test her DNA against theirs, but the customer service person at Ancestry.com said "it wouldn't prove anything"!!! WTF?!!!!

No! NO! No!

There are hundreds of half siblings on GEDmatch, you can tell who they are because they are an average of 1.5 gens from each other.  When 2 people are that close (as half siblings) there's no way you can miss having a DNA relationship, purely by chance.

Would DNA testing miss a 1st cousin?  No.

When you get out to 3rd - 4th cousin,  that's when its possible to be related (genealogically) but not share enough DNA to me considered a match.  One of my 4th cousins and I only share 4.5 cM.  Another only shares  3 cM, and at that level, everyone one in our ethnic group shares it.  I call those 3 cM pieces "ethnic DNA".