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In genealogy, pedigree collapse describes how reproduction between two individuals who knowingly or unknowingly share an ancestor causes the family tree of their offspring to be smaller than it would otherwise be. The term was coined by Robert C. Gunderson, first supervisor of the Genealogical Society of Utah's Royalty Identification Unit. Pedigree collapse is also known by the German term Ahnenschwund which roughly translates as "loss of lineage".
How it works
Without pedigree collapse, a person's ancestor tree is a binary tree, formed by the person, the parents, grandparents, and so on. However, the number of individuals in such a tree grows exponentially and will eventually become impossibly high. For example, a single individual alive today would, over 30 generations going back to the High Middle Ages, have roughly a billion ancestors, more than the total world population at the time. This apparent paradox occurs because the individuals in the binary tree are not distinct: instead, a single individual may occupy multiple places in the binary tree. This typically happens when the parents of an ancestor are cousins (sometimes unbeknownst to themselves). For example, the offspring of two first cousins has at most only six great-grandparents instead of the normal eight. This reduction in the number of ancestors is pedigree collapse. It collapses the binary tree into a directed acyclic graph with two different, directed paths starting from the ancestor who in the binary tree would occupy two places.
In some cultures, cousins were encouraged or required to marry to keep kin bonds, wealth and property within a family (endogamy). Among royalty, the frequent requirement to only marry other royals resulted in a reduced gene pool in which most individuals were the result of extensive pedigree collapse. Alfonso XII of Spain, for example, had only four great-grandparents instead of the usual eight. Furthermore, two of these great-grandparents, Charles IV of Spain and Maria Luisa of Parma, were parents of another twice great-grandmother, Maria Isabella of Spain. More generally, in many cultures intermarriage may frequently occur within a small village, limiting the available gene pool.
Most historians consider the House of Habsburg as an example of genetically-induced disease as the direct result of pedigree collapse. The last Habsburg King of Spain, Charles II, makes an instructive case. In anyone's family tree, there are seven unions in the most recent three generations. In Charles' case, there were three uncle-niece marriages among those seven unions. His father and two of his great-grandfathers married their nieces. His paternal grandparents were first cousins, once removed, but they comprised two of the seven marriages because they were also parents to his maternal grandmother. His maternal grandparents' marriage and the final marriage of great-grandparents was between first cousins. Like most people, the family tree of Elizabeth II to six generations has 64 different people in the 64 different positions. The family tree of Charles II had only 32 different persons in the 64 positions. Going back two more generations, he had only 82 different people in 254 positions. Charles II was born with extensive physical, intellectual and emotional problems and was incapable of producing an heir, a fact which resulted in the War of Spanish Succession. His lineage was so intermarried that he had a higher inbreeding ratio than if he had been born to a brother-sister couple.
The maximum pedigree collapse of 50% within a single generation is caused by procreation between full siblings. Such children have only two different grandparents, instead of the maximum four. If a child and parent were to procreate, their offspring would have four grandparents, although one of these would also be a parent and therefore introduce no additional genes – thus procreation between parents and children would result in less pedigree collapse than procreation between full siblings. If two half-siblings procreate, their children have three grandparents instead of four. If a person procreates with a full sibling of one of their parents, the offspring have four different persons as grandparents, and eight great-grandparents, but again some of these contribute no additional genes.
Small, isolated populations such as those of remote islands represent extreme examples of pedigree collapse, but the common historical tendency to marry those within walking distance, due to the relative immobility of the population before modern transport, meant that most marriage partners were at least distantly related. Even in America around the 19th century, the tendency of immigrants to marry among their ethnic, language or cultural group produced many cousin marriages.
If one considers as a function of time t the number of a given individual's ancestors who were alive at time t, it is likely that for most individuals this function has a maximum at around 1200 AD. Some geneticists believe that everybody on Earth is at least 50th cousin to everybody else.
- Common ancestors of all humans A website created by Mark Humphreys
- Cousin couples A website with information on cousin marriages
- How many genetic ancestors do I have? by Bob Jenkins. Includes tables based on simulations showing the number of genetic ancestors for both males and females going back for 100 generations.
- Where did your genetic ancestors come from? by Graham Coop, The Coop Lab, 19 December 2017.
- Your ancestors lived all over the world by Graham Coop, The Coop Lab, 28 November 2017.
- Our vast shared family tree by Graham Coop, The Coop Lab, 20 November 2017.
- Family fortunes: Adam Rutherford on how we are all related to royalty. Waterstone's Blog, 14 September 2017.
- Average British person has 193,000 living cousins says new research. Daily Mirror, 17 June 2015.
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- Henry Louis Gates and Julie Granka. The interconnectedness of the human family. Ancestry blog, 8 April 2015.
- Your family: past, present and future by Tim Urgan, Wait But why, 28 January 2014.
- How many genetic ancestors do I have? by Graham Coop, The Coop Lab blog, 11 November 2013.
- Steve Jones. Why we're having less sex with our genetic relatives. The Telegraph, 2 April 2013.
- Stephen Lewis. Counting your ancestors. The Wild Peak, 4 March 2012
- Christopher Landauer Pedigree collapse. Border-Wars blog, 20 October 2011.
- Andrew Millard. Probability of descending from Edward III. Andrew Millard's Genealogy, 18 July 2008, updated 14 August 2010.
- Luke Jostins. How many ancestors share my DNA? by Luke Jostins, Genetic Inference blog, 11 November 2009.
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- John Becker. Pedigree collapse. OGS Families, Vol. 38. No. 3, 1999. (Internet Archive)
- Brian Pears. Our ancestors, conceptions, misconceptions and a paradox. Article written in 1985 for UK readers and last updated on 13 October 2012.
- Brian Pears. The ancestor paradox revisited Article first written in 1985. Updated in 2006.
- Brian Pears The ancestor paradox yet again Article first written in 1985. Updated in 2006.
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- Alex Shoumatoff. Reporter at Large: The Mountain of Names. Dispatches from the Vanishing World. Web version of an article first published in the New Yorker, 13 May 1985.
- Information charts showing the number of your ancestors and the number of boxes to Complete Your Ancestor Chart
- How many ancestors do I have? From Family Tree DNA's Family Finder FAQs.
- Learnforever Learn B F Lyons' Exploring Family Trees site allows a user to upload a GEDCOM file to produce a visualisation of pedigree collapse. For further details see the review article The “Exploring Family Trees” Tool by Leah Larkin (The DNA Geek blog, 3 January 2016).
- Felix Immanuel's Pedigree Collapse Calculator
- Joseph Chang (1999). Recent common ancestors of all present-day individuals. Advances in Applied Probability, Volume 31, Number 4 (1999), 1002-1026.
- Bernard Derrida, Susanna C Manrubia and Damiad N H Zanette (2000). On the genealogy of a population of biparental individuals Journal of Theoretical Biology 2000 203, 303-315.
- Agnar Helgason, Birgir Hrafnkelsson, Jeffrey R. Gulcher, Ryk Ward, and Kári Stefánsson (2003). A populationwide coalescent analysis of Icelandic matrilineal and patrilineal genealogies: evidence for a faster evolutionary rate of mtDNA lineages than Y chromosomes American Journal of Human Genetics. 2003 June; 72(6): 1370–1388.
- Douglas L. T. Rohde (2003). On the common ancestors of all living humans. Unpublished paper dated 11 November 2003
- Douglas L. T. Rohde, Steve Olson & Joseph T. Chang (2004). Modelling the recent common ancestry of all living humans Nature, Volume 431, 30 September 2004, pp562-565.
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- Ralph P, Coop G (2013). The geography of recent genetic ancestry across Europe. PLoS Biol 11(5).
- European genealogy FAQ Peter Ralph and Graham Coop answer some of the questions raised by their paper
- We are the authors of a recent paper on genetic genealogy and relatedness among the people of Europe. Ask us anything about our paper! by Peter Ralph and Graham Coop, AskScience AMA.
- Identification of genomic regions shared between distant relatives by Peter Ralph and Graham Coop, The Coop Lab blog, 10 May 2013
- Ralph P, Coop G. Our paper: The geography of recent genetic ancestry across Europe. Haldane's Sieve. 5 October 2012.
- Most Europeans share recent common ancestors by Ewen Callaway. Nature News and Comment, 7 May 2013.
- Charlemagne's DNA and our universal royalty by Carl Zimmer. National Geographic: The Loom, 7 May 2013.
- We are all princes, paupers and part of the human family Veronique Greenwood, Nautilus blog, 17 May 2013.
- Gazal, S. et al (2015). High level of inbreeding in final phase of 1000 Genomes Project. Scientific Reports 5: 17453.
Are we all related? A video from It's OK To Be Smart:
How many ancestors do you have? A video from RootsandRoutes:
- Richard Dawkins. "All Africa and her progenies". In: River Out of Eden. Basic Books, New York, 1995.
- John E. Pattison (2001), A New Method of Estimating Inbreeding in Large Semi-isolated Populations with Application to Historic Britain, HOMO: Journal of Comparative Human Biology 52(2):117-134..
- John E. Pattison (2007), Estimating Inbreeding in Large Semi-isolated Populations: Effects of Varying Generation Length and of Migration, American Journal of Human Biology 19(4):495-510.
- Adams C. 2, 4, 8, 16 ... how can you always have MORE ancestors as you go back in time? The Straight Dope, 21 August 1987.
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