We each have two parents, four grandparents, eight great-grandparents, and so on; the numbers double with each generation. In ten generations we have 1024 great great … grandparents.
In twenty generations we would have 1,048,576 18th great grandparents. While this is possible in theory, it is more likely that we have far fewer, simply because our forebears married someone related to them. This is called ‘pedigree collapse’: we have fewer forebears, with multiple lines of descent from the same person.
Recently I have started to record more of my genealogy on Wikitree. I like the idea of collaborating with cousins on a single family tree. Wikitree strives for historical accuracy by requesting that facts are supported with citations. Collaboration means I can see my connections to forebears further back in time. Wikitree is free and contributors sign an honor code of shared of ethics and principles.
There are some interesting applications to help one analyse one’s family tree on Wikitree and one of these is “Ancestor Explorer” developed by Chase Ashley https://apps.wikitree.com/apps/ashley1950/ancestorexplorer/. The app allows you to see a sortable list of all ancestors of a particular person (Descendant) for up to 20 generations back. That list shows the number of lines to the same ancestor if there are multiple lines of descent; an indication of pedigree collapse.
Amongst my forebears the first cousin marriage I have come across is Charles Gordon (1670 – 1702) who married his first cousin Elizabeth Lyon (aft 1662 – 1739). They were first cousins, the grandchildren of Elizabeth Maule (abt 1620 – 1659) and John Lyon (1596 – 1646). Elizabeth Maule and John Lyon are my tenth great grandparents.
The tool tells me that I have 3030 unique ancestors and 20,016 duplicate ancestors (additional lines of descent from a unique ancestor) within 20 generations. That is a lot of cousin marriages leading to a lot of duplicates.
It appears, for example, that I am descended in 1035 different ways from Marjorie Carrick (abt 1252 – 1292). By one line of descent she is my 19th great grandmother and by another line of descent she is my 30th great grandmother. As far as I can see, all the 1035 lines of descent are either through Helen (Kinnaird) Dana (abt 1746 – 1795), one of my fifth great grandmothers or through Sophia Henrietta (Duff) Mainwaring (abt. 1790 – 1824), one of my fourth great grandmothers on a totally different branch. Both women have long Scottish lines of descent that have been well documented on Wikitree.
I have recently added to Sophia’s pedigree, for I discovered her father William Duff (1754 – 1795) was the illegitimate son of James Duff, the second Earl of Fife (1729 – 1809). A link to the nobility of course adds enormously to the family tree as the pedigrees are well documented. William is recorded in the Duff House Mausoleum in Banff, Aberdeenshire. The mausoleum was built in 1793 by his father, James Duff, second Earl of Fife, but included tombs of people not related to James Duff who was attempting to prove descent from an ancient lineage.
My tree on Wikitree is by no means complete. I have recorded only 20 of my 32 possible great grandparents, 28 of my possible 64 great grandparents, and 30 of my possible 128 5th great grandparents. When looking at my personal family tree I have 26 of my 32 possible great grandparents, 36 of my possible 64 great grandparents, and 49 of my possible 128 5th great grandparents; there are 6+8+19 = 33 more ancestors to add to Wikitree.
My personal research tree has only 15 people at the 14th great grandparent level and there are only 13 people who appear more than once. My Wikitree pedigree has 341 14th great grandparents and 177 people who appear more than once.
I intend to work on increasing my tree completeness at Wikitree and collaborating with cousins on distant genealogy.