Pedigree Collapse

My family tree has married cousins.

If you trace your family back far enough it is normal and even required to have marriages of cousins in your family tree. This is known as Pedigree Collapse because eventually any pedigree chart will start collapsing in on itself. Starting with yourself, imagine your pedigree chart, add your 2 parents, double this and you have your 4 grandparents, double this again as you go to the generation of your 8 great grandparents. Each generation that you go back has double the places of the previous generation, until you reach that first couple who are cousins to some degree and then the pedigree chart is no longer doubling with every new generation.

If we could completely fill a pedigree chart going back to fifteen generations or more then we need to start seeing duplication of names. For most people this starts to appear around the tenth generation and the intermarriage of families located within close proximity to each other is easily identified. The further back in time we go, a pedigree chart which doubles the number of people in each new generation would quickly require more people then the total human population on the planet, particularly when you consider that the world population was smaller. At about the fortieth generation, if you had no duplication of people in your pedigree chart then your ancestors would have to include every living person on the planet. Considering large populations on different continents didn’t interact with each other or know of each others existence then that seems highly unlikely.

It is well known that around the world Royal and noble families consistently married into the same families and were often related by more then one line of descent and their pedigree charts would collapse inwards. Looking at lines of descent from royalty has prompted mathematicians to make calculations which show that it is probable that every person of European descent are most likely related to Charles the Great. Some genealogists have argued that this would be unlikely due to royal intermarriages, regardless of how much intermarriage occurred there were also external marriages and a large number of lesser known younger siblings who were not set to inherit and married outside of the royal families.

During most of human history, people have lived in small, isolated communities with a population of between three and five hundred people, without immigration this means that after six generations or so there are only third cousins or closer to marry. Parish registers in nineteenth-century rural England show that the radius for the average pool of potential spouses, was about the distance a man could comfortably walk twice on his day off, then the bicycle arrived and parish registers show that this tripled the distance. Then came the motor car and the gene pool was diversified with longer distances travelled easily.

Instead of an ever growing triangle it would be better to imagine your pedigree chart as a large diamond. Locating records of our families going that far back is mostly impossible but it hasn’t stopped me hunting for treasure.

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