FTDNA X FAQs
Below is from the FAQs concerning X-Chromosomes at Familytreedna.com
X-DNA Comparisons
Q. I have a close match on the X-DNA chart, but my testing company doesn’t show us as matches. Why?
A. First, you have to understand that the inheritance patterns for X-DNA are very different than Autosomal matches. X-DNA is not passed from father to son. So, when you compare with somebody else where there is a father-son generation between you, your score will be zero. Otherwise, the genetic contribution is halved for each generation EXCEPT for mother to son, where it stays the same. Compare that to Autosomal DNA where the contribution by each parent is pretty much half for each generation – comparatively simple to understand.
If a son was comparing his X-DNA to his mother, he would get a perfect score, about 196 cM. After that, the shared contribution goes down as you go further back in time – within the constraints described above. If a son was comparing his X-DNA to his father, he would get a zero score. A daughter would get a perfect score with either parent, since she gets a copy of X from each of them.
Be careful when comparing 2 scores. The scores might be the same for 2 cases, but have very different implications. An ancestry which is through the straight maternal line for 3 generations might have about the same score as an ancestry which alternates male / female for 6 generations (see rules of inheritance in the first paragraph of this answer).
Q. What is a significant result for X-DNA comparisons?
A. X-DNA should not be used to determine the degree of a match, for the reasons stated in the previous Q/A. Having said that, a good rule of thumb for determining how significant the X-value is might be: “Any X-DNA comparison which results in a total cM value of at least 1 cM and also at least 3.5% of the total cM value produced by the autosomal comparison.”
Q. What’s the best way to use X-DNA results?
A. Use autosomal results to determine how closely related you are, and then use X-DNA results to help you narrow down the ancestral path. For instance, autosomal may indicate that you are 3rd cousins. X-DNA could help you determine if it’s straight maternal, alternating male/female, or something in between.