Jim, I am having a problem with how first cousins overlap each other on average by only 880cM. If I do a Tops down from grandparents each grandparent passes 3,600cM in one chromosome to their child (lets call him parent 1) who is the parent of a grandchild (grandchild 1). Each of their children (Parent1 and Parent 2) have 50% overlap so have common DNA, on average about 1800cM from each grandparent. On GEDMATCH this would be recorded on average as 2,600cM on average rather than 3,600cM for both grandparents due to the fact that it does not account for the places where the two siblings have double identical segments across both chromosomes in a pair. The children of the siblings would have their parents DNA diluted by 50% from their parent’s spouses DNA. That means that the common DNA between the two parent siblings will be reduced by 50% so move from 1800cm to 900cm from each grandparent. So on average their should be 1800cM collective DNA from both grandparents shared between two first cousins. If you account for double identical positions this may look like 1300cM. So why do we only see 880cM as an average for Cousins as a complete overlap of both grandparent’s DNA. Can you see where I am going wrong ?