… is defined by the ISOGG as “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.”
For the mathematic take in simple terms(mostly) Murray Bourne explains it here.
And then we have the Royals..those cousin’ lovin’,can’t let the money out of the family,gotta keep the lines pure Royals.
The best or worst example of this is probably Charles II of Spain(Carlos II; 6 November 1661 – 1 November 1700),last of the Habsburg Dynasty. My family is from South Eastern Kentucky,North Carolina and other small pocket communities,not much bothers me genealogy wise. This did. If the chart is confusing,let us just say,they didn’t leave the house much to have the next successor.
My own personal example.Maternal side from SE Ky.Not quite as bad as Chucky there.