Explaining unshared DNA segments with full sibling?

Question

Recently, a full-sibling had her DNA tested on ftDNA. It was exciting to compare our DNA. But there are a number of things that I found confusing. One was that when I viewed and compared our DNA via the chromosome browser, there were segments that were "black" -- or we didn't share any DNA in that location. I expected to see that we would share DNA from at least one parent on every one of the 22 chromosome pairs.

Even more confusing was the fact that, when I compared my sibling's DNA to mine we shared DNA in different segments then when my sibling compared her DNA to mine. For instance, when my sibling viewed me in the ftDNA chromosome browser, we shared no DNA on chromosome 17, yet when I compared my DNA to this same sibling, we shared about 50% of the DNA on the same chromosome (over 3 segments).

Answer 1

Each sibling gets half their DNA from each parent. At each pair of chromosomes, one chromosome is from their father and one is from their mother.

But the siblings don't get the same parts from each parent. Their grandparents' DNA crosses over and which grandparent they get is random. There is a 1/2 chance the siblings get the same section of DNA from the same grandparent.

So there is a 1/2 chance on any segment that the siblings will match on one parent, a 1/4 chance that they will match on both parents, and a 1/4 chance that they will not match on either parent.

The chromosome browser at Family Tree DNA does not show matches by parent. It only shows if either parent matches. So either single matches or double matches will appear as a match. Therefore, about 75% of the DNA segments of siblings should appear to match, not the 100% that you thought.

Regarding the second part of your question, the matches at Family Tree DNA between any two people, siblings or not, should appear identical from either person's chromosome browser. If they are not for you and your sibling, you should contact Family Tree DNA and ask what's going on.