...how you you expect to find the comparative DNA for the descendants or this grandchild?
Sketch out a chart---start with two sisters (Betty & Ruth)---then the son of Betty and the daughter of Ruth.
Simon is the father of these two sisters.
Betty has a son, Ron.
Ruth has daughter, Linda. Ron married Linda. first cousin
Willard is the son of Ron and Linda.
Willard has a son, Steve, and Steve has a son, Paul.
So, when Paul researches his family he discovers that instead of 32 individuals that he can call
3rd great grandparents, he only has 31. Because Simon fills the role of the grandfather of Ron AND the grandfather of Linda.
Now, consider that Paul takes a DNA test and compares his findings with a descendant of Willard. Because of the pedigree collapse, would the relationship be overstated? Because Paul and his first cousin have the same pedigree collapse in their common pedigree, would their degree of relationship be overstated?
The probability of Simon passing on a DNA shadow to Paul is slight---but is the probability twice the chance because Paul descends from Simon in two paths?
Consider further that the pedigree of Simon include four cousin marriages in six generations of ancestors. Making a further collapse of the pedigree. Paul would discover that he descends from the ancestors of Simon in 8 different paths! Does that mean he would have more of that DNA? How much more?
I realize that too much of intermarriage can result in a family that has deformed jaw and is unable to chew, as in the family of Carlos II. But what is the tipping point?
I would be appreciative of any referrals to published papers on this subject.