From Half to Whole

 

It would seem natural to expect, in using DNA for family tree charting purposes, to see closer matches in our full cousins rather than in half-cousins. In the DNA cousins I've been examining this week, all descendants of my fifth great-grandfather, John Carter of colonial Virginia, each of the lines of descent had one specific detail in common: these DNA matches of mine all descended from John Carter and his first wife. The catch? I descend from John Carter's second wife.

Naturally, when I moved from analyzing those half-cousin DNA results to looking at my full-cousin relatives, I thought I'd see larger amounts for the genetic material we share in common. For instance, the three half-relationships I looked at this month were the descendants of John's son William and his two daughters, Margaret and Sarah. For William's two descendants in my DNA matches, the closest relative shared ten centiMorgans with me—a puny genetic amount, bordering on chance similarities, though the paper trail points to the connection. For Margaret, the numbers were slightly better, with the highest shared centiMorgan count at nineteen. Sarah's descendants shared about the same with me, about eighteen centiMorgans. What can we expect from a half-fifth cousin once removed?

When we move to those DNA matches descending from John and his second wife, Hannah Chew, I was surprised to see the numbers for my best matches were generally no higher than those half-matches. The best in one line shared seventeen centiMorgans with me, the other boasted eighteen. (For those descendants of my own fourth great-grandmother, John's daughter Margaret Chew Carter, the centiMorgan count was a much higher sixty two, but that is owing to the fact that those relationships, though tracing back to John Carter, are actually closer cousins to me who simply share that same ancestry.)

Next week, we'll take a look at the three daughters among my DNA matches who descend from John Carter and his second wife: the lines of Judith Carter, Mary Beverley Carter, and finally my fourth great-grandmother, Margaret Chew Carter.