This project is essentially an extension of the Shared cM Project, but it specifically focuses on more complex familial relationships and genetic phenomena.
Here’s a breakdown of the key components:

• Pedigree Collapse: In an idealized family tree, you’d expect an exponential increase in the number of ancestors as you go back each generation. However, due to intermarriage among relatives—often seen in small or isolated populations—the same ancestors can appear in multiple branches. This “collapse” results in fewer unique ancestors than expected and can affect how much DNA is shared between individuals, often increasing the shared centimorgans (cM) beyond standard relationship estimates.
• Double/Multiple Cousin Relationships: These occur when individuals are related through more than one ancestral line. For example, double cousins form when two siblings from one family marry two siblings from another family, resulting in cousins who share multiple sets of grandparents. This overlapping ancestry usually means they share more DNA than typical first cousins. This project collects data on such relationships to help interpret shared DNA amounts in these complex scenarios.
• ROH (Runs of Homozygosity): ROH are long stretches of the genome where the DNA sequence is identical on both chromosomes. They often indicate inbreeding or a history of endogamy—situations where a population interbreeds over many generations. ROH can affect the total amount of shared DNA between individuals, and by including ROH data, the project aims to refine relationship estimates and better understand the genetic impact of inbreeding or endogamy.

In summary, The Pedigree Collapse, Double/Multiple Cousin, and ROH Shared cM Project is a collaborative effort to gather and analyze genetic data from these complex family relationships. Its goal is to improve our understanding of how overlapping ancestry and inbreeding influence the amount of DNA shared between individuals, thereby helping genetic genealogists interpret DNA test results more accurately in non-standard scenarios.

This project is being compiled by Blaine T. Bettinger with submissions to the project collecting the DNA cM matches on this linked form https://forms.gle/9U8SVsYQXLsoVwLo6