Pedigree Collapse

by Pa Rock

Family Historian

A basic assumption in genealogy research is that the number of each person's biological predecessors (forebears) doubles with each generation.  A person will have two biological parents (whether that person knows who the those parents are or not), four biological grandparents, eight great-grandparents, sixteen great-great-grandparents, thirty-two great-great-greats, and so on.  Of course, if you keep projecting those generations back through time, at some point, at about thirty generations or so, they will equal more than the population of the earth at that time.

So at some point our family trees - a.k.a. pedigrees - must begin to collapse.  The human race evolved slowly over millions of years - it did not just spring forth suddenly with countless individuals bathing in the gene pool.

And almost anyone who has traced their family tree back very many generations has stumbled upon the situation where the same individuals occupy different spots on the same tree.  That happens when when people who are already related - usually some degree of cousins - reproduce together.

A pair of first cousins would be individuals who are the children of a pair of siblings and their spouses,  First cousins share one set of grandparents with each other.  If that same pair of first cousins become attracted to each other and produce a child, that child would only have three sets of biological great-grandparents instead of the standard issue four, or, put another way, one set of great-grandparents would hold down two spots on the family tree.  And from there on, more and more ancestors would be repeated in successive generations..  The tree would be experiencing "collapse" - the same number os spaces but occupied by fewer actual individuals.

That collapse kicks in at any level where people who are already related begin to reproduce.  First cousins have a set of grandparents in common.  Second cousins are the next generation, and they have a set of great-grandparents in common.  If two second cousins reproduce, their children will have seven pairs of individual great-great-grandparents instead of eight - and the tree has begun to collapse.

Marriage among cousins has been relatively (pun intended) common throughout history.  People married within their own extended families due to isolation, for protection, or to preserve land or wealth.  Some, such as royalty, used marriage to preserve power - or to acquire more land and power.  It wasn't all blissful romance.

But that was then, right?   Today there are so many people in the gene pool that marrying close relatives has the feel of immorality.  However sensible that might seem, it is not the case in most of the countries of the world - or even here in the US.  Every state in the United States of America allows the marriage of second cousins, and twenty-states and the District of Columbia allow the marriage of first cousins - with six more allowing first-cousin marriages under special circumstances.  

Those states which permit first cousins to marry in the United States include:  Alaska, Hawaii, California, Colorado, New Mexico, Texas, Tennessee, Alabama, Florida, Georgia, South Carolina, North Carolina, Virginia, New York, Vermont, Massachusetts, Rhode Island, Connecticut, New Jersey, and Maryland - - - and the following states allow the marriage of first cousins under special circumstances:  Utah, Arizona, Wisconsin, Illinois, Indiana, and Maine.

And laws like that lead to the collapse of entire forests of family trees!