I think you misunderstood me. I didn't mean that there was a Celtic expansion into the Germanic heartland, I meant that their cultural history is closely related, as they are both come from IE origins. IE, of course, is quite young in the grand scheme of things. And I brought up shared non-patrilineal lines to help explain why their autosomal DNA would be similar. Regardless, many modern Welsh, Irish, etc. are clearly Celts. Genetics wouldn't exclude that even if we found that they were all 99% Germanic in origin... I find that genetics helps us understand how they came to be Celtic, and how they relate to other historically Celtic peoples and non-Celtic peoples. It doesn't determine whether or not they are Celts.