DNA Inheritance Patterns
The human genome is composed of Twenty-three (23) pairs of chromosomes. Each chromosome comes in a pair, with one copy coming from each parent. Twenty-two (22) of these chromosome pairs are autosomal, while the 23rd is the sex chromosome - it is the one that determines a person's genetic sex.
For all autosomal DNA except the sex chromosome, the copy that you inherit from each parent is a mixture of the two copies they have - the ones they got from their father and mother. For example: the copy of chromosome 1 that you receive from your mother is a mixture of the two copies of chromosome 1 that she received from her parents. The slight differences in these copies are what we use to track relatedness between two people. You can read more about random recombination and autosomal relatedness here. X-DNA is a little different, however.
X-DNA Inheritance Patterns
Woman have two copies of the X chromosomes, so the copy they pass down to their children is a mixture of these two copies, just like the other chromosomes they pass down.
Males only have one copy of the X chromosome however, the one they got from their mother. They receive a Y chromosome from their father instead of an X chromosome. This Y chromosome is what determines the child's genetic sex. Because fathers only possess one copy of the X chromosome, there is nothing for that chromosome to mix with in their children's DNA. Instead hey pass their lone X chromosome to their daughters exactly as they received it from their mothers. They do not pass it on at all to their sons at all.
This means the X chromosome has an inheritance pattern that is different than the rest of your autosomal DNA. The X chromosome is included in our autosomal DNA test - the Family Finder - and its unique inheritance pattern can help shed light on your matches.
Paternal X matches
If you are a male, you have not inherited an X chromosome from your father. This means that anyone who matches your X-DNA must be related to you through your mother's side. The portion of the X-DNA that you share with your match could come from your mother's mother's side, or your mother's father's side, but it cannot be from any of your father's ancestors. This pattern is repeated in each generation as illustrated by the diagram below.
If you are male, your X-DNA match could share an ancestor from any of the ancestors highlighted below If your match is male, they could share a common ancestor with you through any of their ancestors as highlighted below:
Note: Even if you are female, if your match is male, they share a common ancestor with you through one of their ancestors as highlighted above.
Maternal X matches
If you are female, your match could share an ancestor from any of the ancestors highlighted below. If your match is female, they must share a common ancestor with you through one of their ancestors as highlighted blow.
Note: Even if you are female, if your match is male, they share a common ancestor with you through one of their ancestors as highlighted in the Paternal X Matches section above.
Measuring X matches
Because X DNA is not inherited in the same way as other chromosomes, and we tend to see larger blocks from ancestors, the number of shared centimorgans is weighted differently. For example, a 16 cM block on the X chromosome represents the same probable number of recombination events (shuffles) as a 7 cM block on any other chromosome. This is because we know there are fewer shuffles involved in every generation on the X. For this reason, cM on the X chromosome do not contribute to overall shared cM. This can lead to confusing results where the number of cM on the X Match column is greater than the amount of Shared DNA.
In addition, unless a match meets the minimum number of shared cM (not counting X DNA), they will not appear on your match list.
X-DNA vs. mtDNA
X-DNA and mtDNA are often confused, so it is important to understand the distinction. They are similar in that they are both inherited by mothers, but their inheritance patterns and what they can tell you are very different. mtDNA, or mitochondrial DNA, is not in the nucleus like the rest of DNA, including X-DNA. Instead, it is found in tiny organelles in the cell called mitochondria. Mitochondria are found in human eggs, so only the mother’s mitochondria are passed on to the embryo.
Everyone has mitochondria, and therefore mtDNA, but both men and women only inherit it from their mothers. They inherit it from their mothers and so on. Because of this it represents an unbroken, exclusively maternal line, called a Direct maternal Line. X-DNA has limited ancestral paths that it can descend from, but mtDNA only has one. This can help trace deep ancestry of your mother's line.