The current technology for personal DNA testing shows us the pair of values (A,T, G or C) from each of our parents at every tested position on a chromosome but cannot tell us what we got from which parent. If we could separate the DNA that we inherited from one parent, called phasing, and use that for DNA comparisons then perhaps there would be fewer false matches.
X chromosome diagram – http://ghr.nlm.nih.gov/chromosome/X
Phasing might bring matching segments smaller than 5cM into play. There have been many recent online posts and discussions among leading genetic genealogists about whether those small segment matches are real (IBD) or pseudo matches (IBC). Links to some of those articles are at the end of this article.
The X chromosome is particularly interesting for small segment comparisons because a male only has one of them to go with his Y, so we know all his X DNA is from his mother. Thus it is ‘phased’ to his mother’s results already. Perhaps then smaller matching X segments are more often real for men.
Dr. Kathy Johnston is a retired dermatologist who has been doing genealogical research for over 25 years and genetic genealogy for 10 years. She has been researching the X chromosome since 2008. She recently posted on facebook that small X segments can be IBD so I asked her for a guest post on the subject.
Read on to see what she has to say on this subject.
My family’s eyes glazed over when I mentioned enthusiastically at Thanksgiving dinner that I was reading a great book about the history of doing family history. So I did not go on to tell them all the ways 
, which reads like a mystery novel.
