With regard to: " This friend is manic-depressive and her doctor here in Michigan has made a 20 year study of the connection between Celts with scandinavian blood and manic-depression! He says the combination of the two heiritages produced a mutant gene that causes manic-depression....I'm worried...for my children!"": 1.Spontaneous mutations in human genes are very common (some occur every generation), but are largely kept under control through mixing of genes through normal reproductive processes. This is why sexual reproduction is common in higher organisms; constant asexual reproduction -cloning- would repeat an increasing number of mutations (i.e. errors) and the species would die out. 2. It is thus generally accepted in the scientific community that a larger gene pool is more beneficial than a smaller gene pool because there is a then a less likelihood of 'bad' recessive genes becoming dominant. I.e. less inbreeding is better than more inbreeding. 3. My point in #2 above is that "mixing" Scandinavian Norse populations with Island Celt populations would tend to be beneficial in that the gene pool would be more robust (larger.) 4. While there is not much question that different populations carry different physical traits (e.g. Africans as a group tend to have different skin color than Scandinavians as a group) there is very little information on psychological traits being carried in large populations. It is theoretically possible that IF two populations were each more predisposed towards, say, depression, then a combination of the two could maybe create a greater predisposition towards depression. But these are all very very big IFs and virtually no large scale studies have been done on the subject. The rest is speculation. In any event it is *certainly* not a question of "creating a mutant gene". 5. Please also keep in mind that: a) the Norse invasions of occurred quite some time ago. IF the issue you raise were a problem (and I don't believe it is), there's been an awful lot of other mixing since then, customs and laws on incest being what they are. b) American Scots-Irish (or Scotch-Irish or Ulster Scots) refer to a relatively small group of people whose ancestors immigrated to Ulster from Scotland, and then some of whose descendents left Ulster for America. There would have been some intermarrying with (other then residing) Irish, but the thinking is that there wasn't much. 6. You may want to ask your friend to get you a copy of the scientific study that her doctor has done, and see if it meets rigorous research standards. 7. If you are worried about your children, I'd highly recommend giving them lots of love and listening to them when they speak, vs. listening to your friend. Cheers, Doug Doug