Friday, February 10, 2012
So what? In earlier times when transportation was lacking and communities were often isolated, young men sought wives five miles from home. Soon everyone in an area was related and it was important to know just what those relationships were from a genetic standpoint. We did wish to avoid the consequences of being "inbred hillbillies." While that's no longer an issue, the habit was well-formed and genealogy has become a hobby for many.
So just what do terms such as "double third cousins" or "second cousins once removed" actually mean? Here's a primer that will help explain:
When counting cousins, the number of generations between the people of concern and their nearest common ancestor is the degree of relationship. For example, the children of siblings are first cousins because there is only one generation (their parents) between them and the nearest common ancestors (their grandparents). Likewise, the children of first cousins are second cousins because there are now two generations between them and the nearest common ancestors (their great grandparents). The two ladies above are double third cousins because there are three generations between them and common great-great grandparents in two family lines. You see, their respective great grandfathers were brothers who took wives that were sisters, giving them two sets of shared great-great grandparents.
The expression once removed adjusts for differences in the number of generations between the two people of concern and their nearest common ancestors. For example, the relationship between one sibling's child and another sibling's grandchild would be first cousins once removed. The degree of relationship is always the smaller of the number of generations being considered. Thus, it would be possible to speak of first cousins thrice removed, although the utility of it might be questioned.
It's really all quite simple. Just write the genealogies down side-by-side and count the intervening generations. And as for our two ladies, although they share 4 great-great grandparents, there are another 12 great-great grandparents between them that are not shared.