A member of the Transitional Genealogists Forum email list, Kathie, posted an excellent question this morning, which I’ve paraphrased below:
What does it mean when I don’t share autosomal DNA with an individual, but do share an X-DNA segment?
A basic understanding of human genetics is pivotal to interpreting DNA test results. A chromosome “is an organized package of DNA found in the nucleus of the cell. […] Humans have 23 pairs of chromosomes—22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes.”1 “During the formation of egg and sperm cells, also known as meiosis, paired chromosomes from each parent align so that similar DNA sequences from the paired chromosomes cross over one another. Crossing over results in a shuffling of genetic material and is an important cause of the genetic variation seen among offspring.”2
This crossing-over of segments is known as recombination. As a result of meiosis, the “newly formed chromosome in the egg will be a patchwork of contributions from the maternal grandparents, and the newly formed chromosome in the sperm will be a patchwork of contributions from the paternal grandparents, but there is still one copy of every gene. Some eggs and sperm will retain a complete copy of one grandparent’s chromosome without recombination.”3
The ISOGG wiki includes a concise explanation of how X-DNA is inherited:
“Females have two X chromosomes, whereas males have one X and one Y chromosome. Both males and females retain one of their mother’s X chromosomes, and females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother (father’s side), and one X chromosome from her mother.”4
Visual learners might take advantage of X-DNA inheritance charts created by Debbie Parker Wayne.5 As a result of this unique inheritance pattern, the individual with whom Kathie shares an X-DNA segment may share a common ancestor on the X-DNA line of Kathie’s mother or paternal grandmother. It stands to reason that they did not inherit shared autosomal DNA from their common ancestor(s) due to recombination.
As the number of generations that separates two related individuals increases, the probability of shared DNA segments decreases.6 However, since the X chromosome does not recombine when passed from father to daughter, matching X-DNA segments may be indicative of a common ancestor further back in time than autosomal segments of comparable length (in cM).7
On a related note—it is important to consider recombination when interpreting ethnicity estimates provided by many DTC testing companies. An admixture (ethnicity) analysis is only a reflection of the select ancestors from whom you inherited genetic material.
Citing this post:
Thomas Greve, “Shared X-DNA Segment Without Shared Autosomal DNA,” blog post, 8 November 2017 (http://www.grevegen.com/2017/11/08/shared-x-dna-segment-without-shared-autosomal-dna/ : accessed 25 February 2018).
- National Human Genome Research Institute, Talking Glossary of Genetic Terms (https://www.genome.gov/glossary : accessed 7 November 2017), see “Chromosome.”
- Ibid., see “Crossing Over.”
- International Society of Genetic Genealogy, ISOGG Wiki (https://isogg.org/wiki : accessed 8 November 2017), “Recombination,” updated 21 February 2017.
- Ibid., “X-chromosome,” updated 31 January 2017.
- Debbie Parker Wayne, “X-DNA Inheritance Charts,” posted 25 October 2013, Deb’s Delvings in Genealogy (http://debsdelvings.blogspot.com : accessed 8 November 2017).
- Blaine Bettinger, “Q&A: Everyone Has Two Family Trees – A Genealogical Tree and a Genetic Tree,” posted 10 November 2009, The Genetic Genealogist: Adding DNA to the Genealogist’s Toolbox (https://thegeneticgenealogist.com : accessed 8 November 2017), para. 5.
- Kitty Cooper, “What does shared X DNA really mean?,” posted 6 January 2014, Kitty Cooper’s Blog: Musings On Genealogy, Genetics, and Gardening (http://blog.kittycooper.com : accessed 8 November 2017), para. 3.