I'm not sure how my partly true response relates to the rest of your email. I will admit, however, that what I often say is partly true. I prefer to say it's mostly true;>j. DNA is random, so many outcomes are possible. For your discussion about siblings - let's say the atDNA from a parent can be aggregated into 4 groups: a, b, c and d; so a+b+c+d=1. You got say a+b=.5; your sibling got b+c=.5; you both got b, neither got d. Do the algebra and a=c and b=d. So the amounts in each category can be determined. Note a and b are rarely equal, although over a large enough sample the average would be. This aggregate analysis is not the same as a Triangulation analysis of segments among cousins. When you have segments among you and two Matches, on the same area of a chromosome, you have Triangulation (Y=A=B=Y), the segments are IBD - the chances that any of these segments are IBS are slim (almost 0). It's hard to make a concrete statement and waffle it just a little to allow for that random possibility. Triangulation analysis is not usually done among siblings and very close relatives - for two reasons: 1. The relationships and Common Ancestors are known 2. A large "segment" between siblings and close relatives will involve multiple ancestors. You get a whole chromosome - a very large "segment" - from a parent, even a grandparent. Clearly there are multiple ancestors in that segment when it's broken up by matches with distant cousins. To a smaller extent this is true with large shared "segments" with aunts/uncles or close cousins. In fact this is the case with many matching segments - they come from multiple ancestors - but the smaller segments are below thresholds and we'll never see any cousin Matches on those more distant ancestors. The point is to be prepared to break large segments into smaller ones. Jim - Sent from my iPhone - FaceTime! On Nov 16, 2013, at 8:59 AM, jlerch1@lighttube.net wrote: > Jim's statement about the chances being slim that 3 have an IBS is only > partly true. My sister has recently gotten her data, and as expected > at 1/2 of my matches at the 5th cousin level do I match her. But of > that 1/2, 1/2 of that is at places where our parents each gave each of > us the exact same DNA, so at that 1/4th of our genomes we are like > identical twins. So we only count as 1 person at that 1/4th of our > genomes. (It actually is a slight bit of corroboration since my data > was V2 and hers is V3 chip.) You need to use the Family Traits app on > 23 to see if you are Full IBD there instead of HIBD. >