In a Nutshell – 23andme

Sources and Tips for the Genetic Genealogist

(see also:

The following is based on my experience with 23andme however much should be relevant when using other DNA testing sites/companies.

Download your Aggregate Data regularly (from the bottom of your DNA Relatives page) and keep the older versions as the list of cousins will change over time.

If you know how to use spreadsheets (Libre Office provide a good, free spreadsheet software) – sort your aggregate data by Chromosome Number, Chromosome Start Point and Chromosome End Point (columns c,d &e). You’ll see cousins that share less than 5cM* of DNA with each other which may give some useful clues about who’s related to who that don’t show up on the website.

In order to find segments of your DNA are from which geographic regions, go to Ancestry Composition for a summary and at the bottom of the Scientific Details section you can download raw data showing which chromosomes are from which region.

*Centimorgan (cM)
“In genetics, a centimorgan or map unit (m.u.) is a unit for measuring genetic linkage. It is defined as the distance between chromosome positions (also termed loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer distance along a chromosome. However, it is not a true physical distance” ~ wikipedia
The X Chromosome (Chr X)
For a general explanation of X Chromosome inheritance see

If you don’t share the same maternal haplogroup* as a Chr X cousin you can rule out the possibility of being related to them through your (and their) mothers mothers mothers etc line.

*A haplogroup is a genetic population group of people who share a common ancestor on the patriline or the matriline. Haplogroups are assigned letters of the alphabet, and refinements consist of additional number and letter combinations. see also

For male cousins the potential shared ancestors list is shorter due to inheritance patterns.
“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” ~

Anywhere in a family tree where you have two males in succession, one after the other, the X Chromosome can’t be passed along or inherited through that line.

Draw a rough pedigree chart of your ancestry then highlight the relevant names from which you would have inherited your X Chromosome.

Due to the inheritance patterns of Chr X, a problem occurs with a DNA segment match between cousins in that the segment could have been passed from a shared ancestor twice as far back as the usual predictions for other chromosomes. This makes it very difficult to find who your shared Chr X ancestor is.

see also,
Useful Sources for further information
Social networking websites are a good place to look for DNA help groups. – International Society of Genetic Genealogy – The Coop lab.
A wealth of info. here’s one example about autosomal* DNA segments persisting for many generations.
Identification of genomic regions shared between distant relatives

* “Autosomal DNA is a term used in genetic genealogy to describe DNA which is inherited from the autosomal chromosomes. An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes” ~

Gedmatch and Gedmatch Genesis
These free tools match cousins who have obtained their DNA from various testing sites (23andme, ancestry, ftdna etc).
Depending when you had your 23andme DNA test you may only need to sign up and upload to the main gedmatch site, once signed up, you can then follow the link to the beta (genesis) site and upload again.
There are a number of useful tools here including the following recommended Admixture Calculators for European ancestry:
Eurogenes EUtest
Eurogenes EUtest V2 K15
Eurogenes K13
Dodecad World 9
PuntDNAL K12 modern
PuntDNAL K12 ancient
see the following, excellent blog, for more info. – DNA Painter
Their shared cM calculator gives you all the possibilities for your relationship to someone based on cMs or shared DNA percentage.
Tips, Questions and Answers
Q. What’s a snip?
A. SNPs (pronounced ‘snips’) are single-nucleotide polymorphisms.

“SNPs are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA building block, called a nucleotide. For example, a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA.

SNPs occur normally throughout a person’s DNA. They occur once in every 300 nucleotides on average, which means there are roughly 10 million SNPs in the human genome. Most commonly, these variations are found in the DNA between genes. They can act as biological markers, helping scientists locate genes that are associated with disease. When SNPs occur within a gene or in a regulatory region near a gene, they may play a more direct role in disease by affecting the gene’s function.

Most SNPs have no effect on health or development. Some of these genetic differences, however, have proven to be very important in the study of human health. Researchers have found SNPs that may help predict an individual’s response to certain drugs, susceptibility to environmental factors such as toxins, and risk of developing particular diseases. SNPs can also be used to track the inheritance of disease genes within families. Future studies will work to identify SNPs associated with complex diseases such as heart disease, diabetes, and cancer.” ~ U.S. National Library of Medicine

Q. Do SNPs provide useful clues?
A. The greater the number of SNPs in a DNA segment you share with a cousin (half identical region or HIR*), is better as proof of being related. Remember the X Chromosome has special rules depending on whether the person is a male or female, but more is better.

*Half-identical region
A half-identical region (HIR) is a region of two paired chromosomes where at least one of the two alleles from one person’s pair of chromosomes matches at least one of the two alleles from a different person’s pair of chromosomes throughout the entire region. A half-identical region may be either identical by descent (IBD) or identical by state (IBS). ~ ISOGG Wiki,

Q. If, for example, I share 3 segments of DNA with someone, could it mean I share 3 different ancestors with that person?
A. Yes. It is possible to share multiple segments with someone and those segments each be from a different shared ancestor.

Search through the country of origin lists to find cousins reporting their grandparents were born in countries other than those of your ancestors, it may help you prove that some of your ancestry is from those countries too.

Cousin relationship predictions on 23andme are a bit out, i.e. predicted 4th cousins can be as much as 5th once removed or 6th, possibly more. I don’t know if this is due to inbreeding somewhere in a tree or the algorithms used.

If you have a distant cousin you want to stay in touch with, even if she or he is already open sharing, you should send them a sharing request (and have it accepted) to prevent them from disappearing when newer, closer matches appear.

Remember, the DNA databases used by DNA testing companies are based on DNA from people unrelated to each other, in modern populations, who have ancestry in that region.




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