X-DNA’s helpful inheritance patterns

X-DNA's helpful inheritance patterns

When you test your autosomal DNA at any of the five genealogy DNA testing companies (ie. Ancestry, 23andMe, MyHeritage, Family Tree DNA, Living DNA), your X-chromosome is also examined and X-DNA is included in your raw data file and your DNA match segment information.

When you share X-DNA with a match, the unique inheritance pattern of X-chromosomes can help limit the search for your common ancestor to particular lines of your ancestry.

X-Matches in autosomal results

Family Tree DNA‘s Family Finder test identifies X-Matches with people you already match via your autosomal DNA, and displays them in your match list (see below).

You can view the X-match segment detail for specific matches in the FTDNA chromosome browser, and download all chromosome browser segment data to Excel or other software or sites for further analysis if desired (eg. GEDmatch).

Your Family Finder default (Detail View) match list shows X-match information in the right-most column, although the quickest and easiest way to identify matches who share X-DNA with you is to switch to the Table View and then click on the X Match column heading to sort so that the highest X-matches appear at the top of the list, as per the example below:

FTDNA Chromosome Browser showing X-match segments

23andMe X-DNA can be viewed in the 23andMe chromosome browser or downloaded, but only for those matches who choose to share genomes with you.  The raw data can be transferred to GEDmatch for comparison with other kits in the GEDmatch database.

AncestryDNA does not provide or show any X-DNA information, so X-chromosome information can only be accessed by transferring a copy of the raw data to FTDNA or GEDmatch for analysis.

MyHeritage DNA does not yet show X-matches, but they announced at their Nov 2018 Live Conference that X-matches would be coming at some stage in future.  You can upload a copy of your MyHeritage DNA raw data to FTDNA or GEDmatch for X analysis.

Living DNA does not yet report on X-matches, but will likely do so in the future when all their matching and features have been fully implemented.  You can upload a copy of your Living DNA data to GEDmatch for X analysis.

GEDmatch

GEDmatch’s One-to-Many report can be used to identify your X-matches.

Select either the One-to-Many Limited Version tool from the Free Tools menu or the One-to-Many Full Version from the Tier 1 menu (for Tier 1 subscribers), enter your GEDmatch kit number, select to Filter by: X (as highlighted in the image below) and click Submit:

The resulting One-to-Many matches report will be sorted to show your highest-sharing X-DNA matches at the top of the list.

You can click any column headings to sort the match list, in ascending or descending order.  Compare the results when you sort the X-DNA column by Total cM vs Largest Segment.  (Note: to save space and make it easier to read, I omitted the Type, Haplogroups and Overlap columns from the image below).

In the example below, you’ll notice there are some matches who share quite large amounts of X-DNA (eg. 43.7cM, 50.1cM) but who share no Autosomal DNA.  This is not uncommon and you may find you have some X-only matches that could be worth exploring.

GEDmatch One-to-Many report filtered by X-DNA matches

X-Chromosomes

Your sex is determined by the X and Y sex chromosomes: chromosome pair number 23.

Males inherit a Y chromosome from their fathers and an X chromosome from their mothers.  So all their X-DNA is inherited from their mother.

Females inherit an X chromosome from their fathers, and an X chromosome from their mothers.

A mother contributes an X chromosome that is usually a recombined mix of both of her X chromosomes (but not always), and fathers contribute their whole X chromosome intact to their daughters.

Because males only inherit an X chromosome from their mothers, if a male has an X-match in his DNA results, the shared ancestor must be on an ancestral line that follows the male X-inheritance pattern, as below (in green):

Because females inherit X chromosomes from both parents, if a female has an X-match in her DNA results, the shared ancestor must be on an ancestral line that follows the female X-inheritance pattern, as below (in green):

X-DNA cannot be passed down through two successive male generations.

Because some X-chromosomes pass down intact (through males) and skip a generation without recombining, and others are recombined (through females), the average expected percentage of shared X-DNA at each generation varies depending on the branch of the pedigree chart.

Compare the percentages across the same generations below.

For example, at the great-great-great-grandparent level, notice how the expected percentage of inherited X-DNA is only 3.1% on the far right direct maternal line, versus 12.5% on the left-most paternal line (and some other lines) at the same generation level.

With no X-DNA inherited from some ancestors, varying amounts of X-DNA inherited from others, the randomness of DNA recombination at each generation, and occasional segments passed down intact over several generations, X-DNA can be quite unpredictable and it can be difficult to identify exactly where it came from.

It is common to share segments of X-DNA with people who share no significant amount of autosomal DNA.

Males generally get far fewer X-matches than females.

X-DNA’s best and most practical use is for isolating matches to particular family lines, even though the amount inherited cannot tell you from whom or how far back it came.

Nor can the absence of any X-DNA disprove your relationship (except for immediate family members – see below).

Focus on larger X-matches, such as 20cM or more, as smaller segments may not be reliable (or may just be too far back).

Excel Surname Template

Click the image to the right to download an Excel spreadsheet surname template to create and print your own X inheritance pattern pedigree charts – create one for each person that you test (separate sheets for male & female charts).

Key takeaways for X-DNA

  • If a male shares X-DNA with a match, then the ancestor in common will be on his mother’s ancestral lines, according to the X inheritance patterns in the ‘Male’ charts above (as males can only inherit X-DNA from their mother).
  • If siblings have tested their autosomal DNA, and a brother has X-matches in common with his sister(s), then the sisters will know that those particular shared segments of X-DNA must have come from their mother, as their brother could only have inherited them from their mother.
  • If brothers share very little X-DNA with each other, one would have inherited most of his X-DNA from his mother’s father and the other would have inherited most of his X-DNA from his mother’s mother, or they inherited the exact opposite of a recombined X.
  • If brothers share most of their X-DNA, they would have inherited it either from the same maternal grandparent, or the same or very similar recombined X from both maternal grandparents.
  • If brothers share no X-DNA at all, they will have inherited a different mix of X-DNA from their mother’s two X-chromosomes.  They could have inherited one of each of their mother’s whole X-chromosomes (ie. one brother inherited their mother’s maternal X-chromosome and the other brother inherited their mother’s paternal X-chromosome) or they could have inherited an opposite mix of recombined X-DNA from their mother’s two X-chromosomes.
  • If maternal half sisters share very little X-DNA with each other, one would have inherited most of her X-DNA from her mother’s father and the other would have inherited most of her X-DNA from her mother’s mother, or they each inherited the exact opposite of a recombined X.
  • If half sisters with the same mother share most of their X-DNA, they would have inherited it either from the same maternal grandparent, or the same or very similar recombined X.
  • A daughter will share a whole (unrecombined) X chromosome with her father, that he received from his mother.
  • Full sisters will always share a whole X-chromosome inherited from their father (and they may share some X-DNA from their mother as well).
  • Paternal half sisters will share a whole matching X chromosome that they inherited from their father.
  • Mothers will share X-DNA with all their children and fathers will share X-DNA with their daughters.
  • You can attribute X segments to particular grandparents by comparing your X-DNA with cousins and other close relatives from each side of your family.
  • Don’t forget to ask your match to look at the charts above and work out which of their lines you could match them on, to narrow down the possible branches from both sides of the family!

One very important point to understand in relation to X-DNA inheritance is that other than (1) paternal sisters (half or full), and (2) mothers and their children, you cannot exclude or rule out close relationships due to the absence of sharing X-DNA.

Full male-female siblings might not share any X-DNA, full brothers (same mother) might not share any maternal X-DNA, and your aunts and uncles and close cousins might not share any X-DNA with you.  That is normal.

If using GEDmatch for X-DNA comparisons, use the Kit Diagnostic Tool to check that both kits being compared include SNP data for the X-chromosome, ie. Chromosome 23 (in case an incomplete or atDNA-only data file was uploaded from FTDNA, instead of the concatenated atDNA+X-DNA file). 

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