#x-dna

Never forget the X factor. X-chromosome DNA - one of the most useful, controversial, and ignored sources of genetic evidence. The X chromosome, a sex chromosome, determines each human’s biological gender. The X chromosome also packs brick-wall-busting power for solving genealogy puzzles. X-DNA can even help adoptees and others with unknown parentage find their biological parents. To discern fact from fiction and make the most of X-DNA’s magical properties, read on.

A Few Simple Rules – On The Basis of Sex

·         X-DNA for Women

o   Women inherit two copies of the X chromosome – one from each parent.

o   Women pass one X-chromosome to each of their children.

o   The X-chromosome a woman gives to each of her children, in most cases, is recombined, a mixture of X-DNA from each of her own parents.

o   A woman contributes an X-chromosome without recombination – an identical copy of one of her own X-chromosomes – to her children about 16% of the time.

·         X-DNA for Men

o   Men inherit one copy of the X chromosome – always from one’s mother.

o   Men only pass their X-chromosome to their daughters.

o   The X-chromosome that a man donates to each of his daughters is an exact replica of his own X-chromosome.

The rules regulating the X chromosome create a unique inheritance pattern that follows a Fibonacci sequence. Only a subgroup of ancestors can contribute to one’s X. Use DNA Painter’s GEDCOM tool to identify one’s potential X-DNA ancestors.

That inheritance pattern makes X-DNA an asset for adoptees and others seeking to identify unknown parents. Analyzing X-DNA can help sort and filter matches. Analyzing X-DNA can also help one build and validate relationship hypotheses. Since men only inherit X chromosome DNA from their mothers, any genetic relative with whom he shares X-chromosome DNA must be a maternal relative. Those maternal relatives may prove useful for identifying one’s mother or even more distant maternal ancestors. A woman seeking to identify her biological father should consider the amount of X-DNA shared with close relatives and other details (age difference, amount of shared autosomal DNA, etc.). While a woman’s grandmother, aunt, and half-sister will all share, on average, 25% of their autosomal DNA with her, only the paternal half-sister is guaranteed to share a completely identical copy of Chromosome X (approximately 196.1 cM of X-DNA). That paternal half-sister might prove useful for naming one’s biological father.

At AncestryDNA, genetic relatives sharing about 25% of their autosomal DNA appear in the “Close Relative-1st cousin” category. Many AncestryDNA users misinterpret the matches labelled as “Close Relative-1st cousin” as being first cousins. AncestryDNA’s “Close Relative-1st cousin” never refers to first cousins. These matches are grandparents and grandchildren. They are aunts and uncles. They are nieces. They are nephews. Half-siblings. Even double first cousins. Never expect or accept that these matches have a first cousin relationship.  On multiple occasions, I have had to inform clients that their “first cousin” was an unexpected half-sibling. Adoptees and others with unknown parentage ought to closely examine the family trees of “Close Relative-1st cousin” matches and consider the X-DNA shared among them. Since AncestryDNA neither reports nor visualizes shared X-DNA, one must employ other genetic genealogy websites and recruit matches to do the same.

 Optimize Opportunities

While 23andMe and Family Tree DNA’s Family Finder use X-chromosome DNA in addition to autosomal DNA (atDNA) to generate relative matches, AncestryDNA and MyHeritage DNA still do not. However, raw DNA data files from AncestryDNA and MyHeritage DNA include X-chromosome data. Transfer raw DNA data files from AncestryDNA and MyHeritage to GEDmatch.com and Family Tree DNA’s Family Finder to view X-DNA matches.

AncestryDNA’s raw DNA data files contain at least 28,892 X-chromosome markers (SNPs) – more than data files from any of the other commercial DNA tests. If one has the choice, choose to transfer the AncestryDNA raw data files for higher-quality X-DNA matching. Wherever you transfer your DNA data, transfer your GEDCOM there as well.

Beware of The X Chromosome Myth

“If someone matches you on the X chromosome and only on the X chromosome, then that someone should not be considered a true match.” Variations of this claim rotate in genetic genealogy circles – along with “4th cousins are really hard,” “DNA testing is just not useful for non-Europeans right now,” and “chromosome browsers are only good for ethnicity research” – and all are bogus.

Consider this example:

My paternal grandfather has two DNA matches (predicted 4th cousins) with whom he shares both significant autosomal DNA and X-DNA: Jason (18.5 cM atDNA and 37 cM X-DNA) and Sarah (17.2 cM atDNA and 30.2 cM X-DNA).

Grandpa Christmas, Jason, and Sarah all share the same segment of X-DNA.

Since my grandfather and Jason are men, they each only have one copy of the X chromosome; both inherited that one copy of the X chromosome from their mothers. The X-DNA segment that my grandfather and Jason share must have come from a common maternal ancestor. Comparing Grandpa Christmas’s family tree to Jason’s pedigree shows a pair of common maternal ancestors: Philemon Hawkins (1767-1856) and Mary Christmas (1775-1822) of Warren County, North Carolina and Franklin County, North Carolina. My grandfather and Jason descend from two of Philemon and Mary’s sons, Adam Hawkins and Brehon Hawkins respectively. The Hawkins brothers must have inherited this X-DNA from their mother, Mary Christmas. Remember: Men only inherit X-DNA from their mothers. Mary Christmas is the most recent common ancestor for this X-DNA segment.

Now, Sarah’s family tree shows that among her X-DNA ancestors are William Duke and Mary Green of Purchase Patent Plantation, Warren County, North Carolina.

William Duke and Mary Green were Mary Christmas’s maternal grandparents – and her X-DNA ancestors. William Duke and/or Mary Green passed this X-DNA segment to their twin daughters, Ann Duke (mother of Mary Christmas, maternal ancestor of Jason and Grandpa Christmas) and Thamar Duke (ancestor of Sarah). See the chart below.

Sarah and Jason share only this single X-DNA segment; GEDmatch found that Sarah and Jason share no autosomal DNA.

Despite only sharing X-DNA, Jason and Sarah have a verifiable relationship in a genealogical timeframe. Jason and Sarah likely inherited their shared X-DNA from their common ancestors, William Duke and Mary Green.

This example not only disproves the X Chromosome myth, but also demonstrates how chromosome browsers add value beyond ethnicity research and how 4th cousins are not “really hard.” In fact, repeating these steps - pooling our DNA segment data, comparing pedigrees, and using GEDmatch’s Tier 1 tools - enabled my family to reconstruct more than half of one 4x great-grandfather’s X chromosome, laying the groundwork for future research.

Collaborating with one’s DNA matches to map the X chromosome offers several benefits. Identifying the ancestors who passed down one’s X-DNA can help determine how other unknown genetic relatives sharing this same DNA are related. Mapping DNA segments can also reveal traits that one’s ancestors inherited and passed down. Among other things, Chromosome X contains genes associated with hereditary baldness and color blindness.

Use DNA Painter’s chromosome mapping tool to track which of your ancestors passed those genes to you.

To get the most out of your X-DNA:

·         Transfer your raw DNA data file and your GEDCOM file from AncestryDNA and MyHeritage DNA to GEDmatch and Family Tree DNA’s Family Finder to access X-DNA matches. AncestryDNA raw DNA is the best transferable raw DNA data file.

·         Upload your GEDCOM to DNA Painter to identify your X-DNA ancestors.

·         Map your X-DNA and associated traits on DNA Painter.