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 Unit. Pedigree Collapse is also known by the German term AHNENSCHWUND which roughly translated as "loss of lineage"
How it works.
Without pedigree collapse, a person's ancestor tree is a binary tree, formed by the person, parents, grandparents and so on. However, the number of individuals grows exponentially and will eventually become impossibly high. For example, a single ins the binary tree individuals alive today would, over 30 Generations going back to the 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 parent's of an ancestor are cousins, (/wiki/Cousin) (some times unbeknown 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 to or required to marry to keep kin bonds, wealth and property within a family (endogamy) (/wiki/Endogamy). Among royalty, the frequent requirement to only marry other royal's resulted in a reduced gene pool.
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