DNA tests are fun. They can give you a hint on your origins (though the results depend a lot on the data sets from the company providing them), get you in touch with cousins (or even closer relatives) you didn't know about, confirm genealogy hypothesis, and much more...

One thing that is interesting to do with DNA results is to trace known DNA segments to a known ancestor. This is not always possible, and usually requires several triangulated matches on that segment to ensure where it comes from.

X chromosome inheritance

In my DNA results, I have a rather large match on my X chromosome. This match is about 40cM long (for a total length of about 196cM at FamilyTreeDNA).

Where could this segment have come from? Is there a way to tell?

In autosomal tests, the X chromosome is often included, but it plays by
its own rules. This is because X is transmitted in a funny way, due to
the fact that males only have one X chromosome (and one Y chromosome),
while females have two (and no Y chromosome).

Here's an illustration from Wikipedia explaining how this affects the inheritance rules of that special chromosome:

As you can see, the consequence is pretty simple:

• men get their only X chromosome from their mother (I'm leaving aside the very rare XXY cases)
• women get one X chromosome from each of their parents.

As a result, a segment of an X chromosome can travel through both men and women (unlike segments on the Y chromosome, which travel only through men), but can never be carried by two men in a row.

In other words, since I am a man, I cannot have gotten this segment from:

• my father (or his side ancestors, obviously)
• my grand-father's father
• my grand-mother's father\'s father
• or any other relationship involving a father and his son

Now I'm quite lucky, as I happen to know the person I match on 40cM on that X chromosome, even though the match is quite distant:

• First off, that person is Jewish, an ethnic group that matches my mother's side exclusively. Check ✅
• Then, this person comes from a very special family branch from Egypt, which happens to be on my grand-father's mother's side. Check again ✅
• I don't know any other possible (close enough) relation with that person that could explain this X match, and we both have quite developed family trees ✅

So, unless a closer path can be found in the future (which is unlikely), the inheritance hypothesis checks with both sides of the tree, confirming the chain of ancestors:

You can see how that X segment was transmitted from my 4th great-grandmother Luna Mondolfo (born around 1790) all the way to myself (on the left) and my DNA match (on the right).

Once this is confirmed, it also allows to consider that any other match triangulating on this X segment is more likely linked to this side of the family, known and verified until the 18th century.

Keeping track of segments

A great way of tracing known DNA segments is to use DNA Painter, a free website where you can gather segment information from various companies (FTDNA, Ancestry, GedMatch, MyHeritage, etc.) to "paint" your chromosomes.

Here's an example showing my maternal labels, with chromosome X at the bottom:

Did you make interesting finds in DNA tests as well?