Hi, nice to hear from you. Okay, so you are sure your Reiswigs are "Germans from Russia" and the only way Denmark comes into it, is through this DNA analysis (autosomal?) I guess your dad is the Reiswig? By a specific search being needed, I meant a search of one person's ancestry. So you'd trace your dad's ancestry (and he may have done much of this, sounds like).
Probably your ancestors here, 1895 census in Marion county, Kansas:
http://www.kshs.org/genealogy/genealogy_censuses/search/surn...Find-a-Grave memorial to one of the couples mentioned on above link, Leonhard Reiswig (1812-1902) and Johanna Philippine Neumann (1819-1896):
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=36...I didn't link to Gary Reiswig because you might be related; I was giving examples of Denmark, German, and Russian Reiswigs and noticed that he had commissioned a study of the surname Reiswig among Germans from Russia, which might be of interest to you (depending whether or not there are holes in your dad's research).
Don't have a fix on the geography of your Reiswigs. You mention the Baltic Sea and mention the Odessa region of Russia, but Odessa is in the Ukraine on the Black Sea - isn't it? Hmmm. Well you don't need to explain it to me (and I'm sure you could!), as I can't continue on with you in this journey (not my area!) If you know your Reiswigs lived in Russia on the Baltic Sea, they lived near St Petersburg and were also close to Finland. But I kinda guess maybe you meant Black Sea. Also some parts of Poland are within 50 miles of Denmark (island of Bornholm, for example). Yes it will be interesting for you to trace this, if your dad has not. DNA is not equally inherited from all ancestors equally, as I understand it (I'm no expert admittedly) so the percentage of this or that within you doesn't necessarily reflect the percentage of ancestors who lived here or there (not well said but hope you see what I mean).
Instead of adding to comments here, though, after checking out your dad's genealogy, you might consider posting a new query on the board most likely to have expertise as needed. This is the "Scandinavia General" board, so this ain't it. What a blessing that your dad prepared that history for you. I'm sure he's missed, but you honor his interests and ancestry by pursing this now.
Good luck and warm best wishes!
http://en.wikipedia.org/wiki/Volga-Baltic_Waterway