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).