What's hidden in your chromosomes?

DNA & Genealogy

X-DNA's helpful inheritance patterns

When you test your autosomal DNA, your X-DNA is also examined and included in your raw data &/or match results.  When you match someone on your autosomal DNA and also share segments on the X-chromosome, the unique inheritance pattern of X-chromosomes can help isolate 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 clearly displays them in your match list (as below).

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

 

X matches on FTDNA's Family Finder

 

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.

 

X-Chromosomes

Your gender 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.

 

X Chromosome Inheritance

(Click on image above to open in a new tab, then right-click to save a copy if desired)

 

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):

 

X-chromosome Inheritance Chart - Male

 (Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired)

 

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-chromosome Inheritance Chart - Female

(Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired)

 

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.

 

X-chromosome percentage inheritance chart - Male

(Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired)

 

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.

 

X-chromosome percentage inheritance chart - Female

(Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired) 

 

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

 

Download link for Excel spreadsheet for X inheritance pattern male & female surnamesExcel Surname Template

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

 

 

 

Practical uses of 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.
  • 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 X-matches 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 from his mother's father and the other would have inherited most of his X 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 half sisters with the same mother share very little X-DNA with each other, one would have inherited most of her X from her mother's father and the other would have inherited most of her X 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 X chromosome with her father.
  • Full sisters will share a whole X chromosome, from their father.
  • Half sisters will share a whole X chromosome if they have the same father.
  • 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!

 

BACK TO DNA HOME

 

Useful Links

 

 

Link to Facebook Group - Using DNA for Genealogy - Australia & NZ

Facebook group - DNA for Genealogy UK

 

 

23 Pairs of Chromosomes. 1 Incredible You. Get your DNA story at 23andMe.com.

 

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