Dear Barton, It is certainly reasonable to believe that both of these HIRs (presumably IBD segments) can be traced back to the known shared ancestors at the 5th cousin level of relationship. I have an example in my family of two fifth cousins sharing 33 cMs in two segments. However, you should always keep in mind the possibility that one of the two segments comes from a different shared ancestor than the known shared ancestors at the 5th cousin level of relationship. My mom recently had a match at 23andMe that shares two HIRs with her. I had previously mapped one of those segments to my mom's father and the other to my mom's maternal grandfather using data from first and second cousins. When one is dealing with people from endogamous populations then one has to be particularly alert for situations such as this where one shared segment came from one shared ancestor and the other shared segment came from a different shared ancestor. This is where your chromosome map is crucial. If you have tested a fair number of first, second, and third cousins then you will have a good idea whether two segments you share with a 5th cousin may have came from the same shared ancestor if you have previously mapped the segments in question. The location of shared segments within the genome is entirely random. You can certainly have crossovers that split a shared segment such as what appears to be the case in this situation. I recently had a similar situation in my family's results at 23andMe. However, you should also keep in mind that one or more genotyping errors in the section between 30027010 and 33868089 could be preventing the company from calling this particular section an HIR. In such cases, the entire HIR (from 25354798 to 47167537) is actually a shared IBD segment. Sincerely, Tim Janzen -----Original Message----- From: autosomal-dna-bounces@rootsweb.com [mailto:autosomal-dna-bounces@rootsweb.com] On Behalf Of Barton Lewis Sent: Tuesday, October 08, 2013 7:51 PM To: autosomal-dna@rootsweb.com Subject: [AUTOSOMAL-DNA] Same or different segment? My aunt matches her fifth cousin on chr 17 as follows: 25354798 30027010 7.84 1249 33868089 47167537 12.67 2900 Is it likely these 2 sections are from the same ancestor? How close do segments have to be to make it likely they are from the same ancestor? What accounts for a section being "broken up" in this way? What's a good layman's way of understanding "how big is big" when talking about these kinds of break up on a chromosome? Barton