Pegigree Collapse

John Becker of Toronto, ON wrote an excellent description of "Pedigree Collapse" in OGS Families, Vol. 38. No. 3, 1999, as follows:

"Back in 1985 Alex Shoumatoff wrote an account of kinship and genealogy called The Mountain of Names, A History of the Human Family (A Touchstone Book published by Simon & Schuster, Inc., New York.) He speaks about an amazing paper by Robert C. Gunderson called Tying Your Pedigree into Royal, Noble and Medieval Families. Gunderson invented the term ‘pedigree collapse’. This is a phenomenon which is ubiquitous and makes every genealogist's life a little easier. When one's pedigree collapses, one has a reducing number of ancestors to search for. Here is how it works.

We all are blessed with two parents, four grandparents, eight great grandparents and so on. If the average generation is twenty-five years, in 1200 years (back to 800 AD, the time of Charlemagne) each person has 281.5 trillion grandparents. That's the way geometric progressions work. The number of grandparents doubles every 25 years and in 12OO years or 48 generations, 281.5 trillion names would be on your pedigree.

But hold on, you say! In 800 AD there were not that many people on the whole planet. How could I - or any person - have that many grandparents? The answer is that while you must have this number of grandparents, given the imperatives of human procreation, they are not all different people. Some names on your pedigree appear twice, three times or even hundreds of times in the 1200 years. Cousins have married and, if they were first cousins, their offspring will have only six great grandparents rather than the normal eight. Those offspring will have pedigrees which have "collapsed" from 8 to 6 or 25% in the 4th generation back. That 25 % collapse will be present in each and every one of the 44 generations back to 800 AD. The same phenomenom occurs when 2nd, 3rd, 4th or 5th cousins marry although the percentage ‘collapse’ is not as dramatic. A dramatic collapse occurs when siblings marry as was the norm for Egyptian pharaohs and Inca kings. In those cases there is a 50% collapse (from 4 to 2) at the 3rd generation, the grandparents.

In the 1990s we may not think that this is a common occurrence but the impact on our pedigrees must be significant or the population on the earth in the past must have been much larger than it was. The facts speak for themselves. There could be no other explanation for how the number of one's grandparents can be reconciled with known population figures.

Studies in various parts of the globe have confirmed this and provide us with some case studies:

In 1964 it was found that a third of the marriages in Andhra Pradesh, southern Indian, were between first cousins and that almost 12% of marriages were between uncles and their nieces.

In 1875 in England 7.5 % of Jewish marriages were between 1st cousins, a result of their society being relatively isolated from the main stream community.

King Alfonso XIII of Spain (1886-1941) had only eight different people as his great great grandparents rather than the normal sixteen, a 50% collapse of his pedigree at the 5th generation.

Prince Charles' pedigree has been examined by Gunderson who found that 17 generations back when Charles should have had 65,536 progenitors, he only had about 23,000, a collapse to 35 % of the theoretical.

A great deal is known about the family histories of the Amish who came to North America from Switzerland in the 18th century. It is estimated for one family about whom a very complete genealogy has been compiled that 21.5 % of 627 marriages in this family were between 2nd cousins or closer.

Mr. K. W. Wachtel, a demographer cited by Shoumatoff, built a probability model for a child born in England in 1947. Around the time of King John who reigned from 1199 AD to 1216 AD, this 1947 child would have about two million grandparents in the same generation. This represents about 37% of the progenitors required 30 generations back. This is the first type of pedigree ‘collapse’ that occurs. The child would be descendant from 80% of all the people in England at that time.

But now a ‘collapse’ in the absolute number of progenitors starts to occur for this 1947 child. The actual number of different grandparents would start to decrease at this point - 30 generations back. Theoretically the further one goes back from this point the smaller the number of different grandparents there would be until one reached one's theoretical ‘Adam and Eve’. Put in graphical terms and viewing the child's pedigree from the bottom (now) to the top (early), the number of names creates a diamond with one person at the bottom in 1947, two million people in the generation 700 years earlier and then an ever-decreasing number dwindling to the original ‘Adam and Eve’, say, several thousands of years before that at the very top of the chart.

There are, of course, many genetic, medical and hereditary aspects of kinship. All of these are made more fascinating and complex by the intermarriages within families and small tribes that have been taking place."

So Pedigree Collapse happens when ancestor cousins marry and generate descendant children. 1st Cousins have one set of common grandparents so if a person is descended from their child, the number of ancestors are reduced. In fact it happened to an enormous degree years ago. By definition you double your ancestors every generation, so you will calculate too many humans on the planet back 500 or 1,000 years ago. Pedigree Collapse is the answer!

Every human has two predecessors or parents. Looking at six generations back, you will have 64 (2⁶) ancestors, and if they are all not related, your pedigree will look like this: (you can click the images to enlarge them)

Now, when cousins marry and procreate, each human is generated by two parents as before. However, 1st cousins who marry and have children will have common grandparents. So the actual number of ancestors will be reduced. In the example below, the ancestors have been reduced from 64 to 12 at the sixth generation:

If we have some Double 1st Cousin marriage, that is your 1st cousin spouse's sibling marries your sibling, we will further reduce the actual ancestors. This the following example, there are only 4 ancestors at the 6th generation level:

In the extreme case where siblings marry and procreate, we show that in the 6th generation, only two ancestors exist instead of the theoretical 64 shown in the first chart:

Now let's look at this in a little more depth and detail . . .

Pedigree Collapse
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