The current technology for personal genome testing cannot tell you which of the two chromosomes, maternal or paternal, in a pair that an allele comes from. It can tell you that there is an AG at a specific position and a CT at the next position but not whether the A came from your mother or your father. This leads to much confusion about DNA segment matching.
The matches that these testing companies find are for stretches of DNA that are half identical regions (HIRs). This is due to the fact that a relative who shares a DNA segment from a common ancestor with you will match you along the chromosome you got from the parent who is descended from that ancestor. Thus your new relative will match you for half the alleles in those positions. Only a sibling will share fully identical regions of DNA. Click here for a page that has a picture of the DNA I share with my brother Shipley.
For example, if my Dad gave me AAAAAAAAAAA and my Mom gave me CCCCCCCCCCC then I would seem to match absolutely everyone on that segment because every position has both an A and a C. So an ACACCAACCAC or a CCAACCCACA looks like a match, but only those with an AAAAAAAAAAA or a CCCCCCCCCC would be real matches. This is simplistic and the segment runs used for matching are much longer than this to try to avoid that sort of false matching. Also note than when your testing company shows an AC it is really an AT and a CG but just one of the known pairing is shown for brevity.
The term for a real match is IBD, which is an abbreviation for Identical By Descent. The term IBS means Identical by State which would apply to any false match. So in our example, the CCAACCCACAA match would be considered IBS.
However IBS is used interchangeably for false matches and matches that might be IBD. In other words, this term is used for all matches not proven to be IBD. So IBS is also used for matches that are from so long ago that we are unlikely to find the ancestor. I find this confusing. We had a discussion in one of the facebook groups about genetic genealogy where we came up with a new term, IBC, for identical by chance, to be used for matches that are known not to be real. In other words matches that are proven to be a mix of alleles from the chromosomes from each parent.
You may ask how a match can be proven to be IBC. Well if both your parents are tested, you can prove this by seeing that you have a match that neither parent has. If all three kits are uploaded to GEDmatch you can look at this in more detail by lowering thresholds.
Another way to prove an IBC match is when you have a number of people who match you on a segment but none of them match each other there. Then it is a false match like our example above. You can compare people to each other at GEDmatch or at 23andme if you are sharing with them. At family tree DNA you have to get one of them to check the match with the other in order to see if they match each other at a specific location. The ICW function cannot tell you where they match.
When we have a DNA segment that matches another person we cannot be sure that it is a real match unless it is also a match to a third person who matches both of us at that spot. This is called triangulation. Having a parent or child tested can also help a great deal with finding IBD matches. When my Dad and I or my Dad and my brother have the same match at the same location, I can be confident it is IBD since it is phased. Parent-child phasing occurs when my Dad and I both match a third person at the same spot. I know that for us it is a true match since they are matching DNA I got from my Dad. Often the match will be a little smaller when passed along to us or larger due to fuzzy boundaries.
When only Dad has a match, I try comparing to various cousins to see if they have that match too, in order to confirm that it is IBD. I have found that there are certain locations where he has many IBC matches. Comparing notes to others, I find that these spots vary from person to person.
Additional things to be aware of in segment matching. DNA tends to stay together in chunks, thus the larger the segment match, the more recent the common ancestor (CA) may be. More than one matching segment of at least 8cM is usually indicative of a close relative (4th cousin or better) unless there is more than one common ancestor or you are both from an endogamous group like Mennonites, Polynesians, or Ashkenazi Jews.
John Walden has done some statistical analysis of the sizes of segments that are likely to be IBD versus IBC, summarized at this link on the ISOGG wiki. His results show that in most population groups, segments of 10cM or larger are almost always IBD, 8-10cM segments are good about half the time, 6-8cM more likely IBC, and smaller segments more and more likely to be IBC.
So I hope this clarifies DNA segment matching and the terms IBD and IBS and I really hope some of you start to use the term IBC as well.