What is X-DNA?
X-DNA refers to the DNA found on one of the sex chromosomes, the X chromosome. The other sex chromosome is the Y chromosome. Typically, most people get one X chromosome from their biological mother, and an X or a Y chromosome from their biological father.
In general, if you get an X and a Y chromosome (XY), you're biologically male—and if you end up with two X chromosomes, XX, you're biologically female. There are some rare exceptions, such as a female who receives a single X chromosome from one of her parents and fails to receive a second X (XO).
The X and Y sex chromosomes are different than other chromosomes. The other chromosomes – autosomes – come in nearly identical pairs. But the same is not true for the X and Y chromosome pair found in most biological males: these chromosomes are very different from one another. It's worth noting this is a different situation from most XX folks who do have a pair of nearly identical X chromosomes.
Chromosomes have stretches of DNA called genes that code for proteins, which do the work in our cells to keep us alive. Each of the thousands of proteins in a cell is made in the right amounts to keep a cell healthy and productive.
The amount of protein made by each gene is determined by many factors, but the important one is the number of copies of a gene. Each of us typically has two copies of each gene, one copy coming from each chromosome in a pair. While this is the case for the genes on the X chromosome in XX people, the same is not true for XY people who have a single copy of all of the genes on the X (and the Y) chromosome.
If cells didn't do anything to correct this situation, XX people would make twice as much protein from the genes on the X chromosome compared to XY people. And this would be lethal. Biology has solved this problem (at least in mammals like you) by shutting down most of one of the X chromosomes in XX organisms. This process is called X-inactivation, and it turns the non-activated X chromosome into a small, compact structure called a Barr body.
Just like crumpling a piece of paper makes it so you can't read it anymore, inactivating an X makes it so the cell can't read its genes any more. X-inactivation silences most of the genes and prevents them from being read and translated into proteins. X-inactivation means that everyone pretty much ends up with only one active X chromosome in their cells.
It's important to note that X-inactivation is a random process. So there's no way to tell if the X-DNA that is silenced was from the biological father or the biological mother. Because the process happens individually in each cell at an early stage of embryo development, most mammals (including you) have X-DNA from the mother active in some cells, and X-DNA from the father active in other cells.
One example of how the randomness of X-inactivation affects mammals is seen in female calico cats, which have a mix of black and orange fur. The gene that determines this fur color is found on the X chromosome and it comes in two versions, black and orange. If a female cat has one X chromosome with an orange version of the gene and one with the black version, then she will get a unique pattern of black and orange fur because of X inactivation.
Some of the cells that make hair will have the “orange” X on and make orange hair while the rest of the cells that make hair will have the “black” X on and make black hair. Because of the randomness of X inactivation, each calico cat will have a unique pattern of black and orange. It also helps explain why male calico cats are so rare—they usually only have the one X and so are black or orange.
What Is an X-DNA Test?
An X-DNA test looks at the X chromosome to find out more information about your ancestry. This type of test is more useful for XY males because they know their X chromosome almost always comes from their biological mother. When an XY male finds someone who is a match for their X-DNA, it means they have a shared ancestor with that person on their mother's side.
However, it's rare for an X-DNA test to be done on its own. That's because this test only looks at one chromosome and therefore yields more limited results about your ancestry. Usually, X-DNA is examined when you get another type of DNA test.
Compared to an X-DNA test, an autosomal test is more comprehensive because it examines the DNA not just in your sex chromosomes but also in your autosomes. Autosomes are all the chromosomes except the sex chromosomes.
Of the 23 chromosomes typical in humans, one pair is sex chromosomes and the other 22 pairs are autosomes. An autosomal test thus provides more information about your ancestry because it looks at 22 pairs of chromosomes instead of just one pair of sex chromosomes.