A discovery was made which became a very distracting red herring. To explain, 'abutting segment boundaries' or ASBs are a further type of segment boundary coincidence in which different matching segments abut at a SNP but otherwise don't overlap (we first reported 2 such cases here). The odds of this type of coincidence occurring by chance is the same for a RSBC. Initially we (mistakenly) saw an explanation of ASBs in meiosis, for 4 gametes are produced from one set of parental chromosomes. Complementary gametes show a parent's DNA rejected on one side of a recombination appearing on the other side, thus producing an abutting boundary coincidence which can survive to later generations. With some excitement our presentation 'Fun with Autosomal DNA' used this 'insight' to claim a Baruch Lousada connection across 4 family branches. However Andrew Millard pointed out that the statistics of sperm and egg utilization do not favour our 'insight'. After our first 2 ASBs, 17 new ones were discovered making 19 ASB locations in total. But once we refined our probability estimates of the occurrence of ASBs and RSBCs, we realised that 4-person ASBs occur little more than random while 3-person ASBs are somewhat more common. Of course the meaning of ASBs is not intuitively obvious.

It is possible that they are an artefact of GEDmatch's match-calling methodology. Removal of hard breaks can in effect erroneously insert 'missing' SNPs. But as GEDmatch advised on 24 Oct 2025: 'The boundaries of a segment are practically impossible to get exactly correct. There are alleles in positions which are not SNPs and the crossover is likely not exactly at a SNP. So the mismatch at the front of the segment is before the segment actually starts and the mismatch after the end of the segment is after the segment ends. There is also the possibility that the SNPs after the beginning of the mismatch at the beginning of the segment happen by chance and the segment might actually start after the first aligned SNP following the match. For example if the first SNP inside the segment is AC then it will match what ever is in the other kit but the crossover may be further in. The same is true at the end of the segment. Finally if the two kits are not from the same vendor chip set there are SNPs which do not align and had those SNPs been available the mismatch SNPs which bound the segment may be different'. Consequently it seems likely that ASBs can be considered a random occurrence whose probability can be affected by the number of segment-matches and hence increased (like RSBCs) by the 'pile-up' problem.