“Pedigree collapse” occurs in genealogy when an individual’s ancestors appear in their family tree more than once, typically because the ancestors are related to each other, either distantly or closely. This phenomenon is also known as inbreeding or consanguinity when referring to closer relationships. In simple terms, it happens when the same ancestors are found multiple times across different branches of a family tree due to the marriages between relatives (whether knowingly or unknowingly).

Effects on DNA Matches and cM Results:

1. Increased cM Totals: Pedigree collapse can result in DNA matches sharing more centimorgans (cMs) than expected for their apparent genealogical relationship. This is because they inherit multiple copies of the same DNA from the common ancestors who appear more than once in the tree.

2. Misleading Predictions: The genetic relationships deduced from shared DNA segments (measured in centimorgans) might suggest a closer relationship than the true genealogical relationship. For instance, second cousins might share the amount of DNA more typical of first cousins.

3. Complexity in Relationship Estimation: For individuals with pedigree collapse in their family trees, it can be challenging to accurately estimate the degree of relationship using standard DNA cM tools and charts. These tools generally assume that each ancestor contributes DNA to the descendant only once, without duplication.

4. Impact on Genetic Genealogy: Pedigree collapse complicates the use of DNA testing for constructing family trees and identifying how exactly matches are related. It requires a more nuanced analysis of shared DNA, often needing detailed genealogical records to understand the shared ancestry and repeated genetic contributions.

Strategies for Dealing with Pedigree Collapse:

– Detailed Family Records: Keeping thorough genealogical records can help clarify the relationships and the potential for repeated ancestry in the tree.
– Advanced DNA Tools: Utilizing tools that can analyze complex DNA matches, like those considering segments from specific regions or offering more refined relationship predictions, might be necessary.
– Expert Consultation: Sometimes consulting with a genetic genealogist can help make sense of complicated match patterns resulting from pedigree collapse.

Understanding and accounting for pedigree collapse is crucial for anyone deeply involved in genetic genealogy, as it ensures a more accurate interpretation of DNA results and the relationships depicted in family trees.